servo/components/script/dom/webglrenderingcontext.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#[cfg(feature = "webgl_backtrace")]
use backtrace::Backtrace;
use byteorder::{ByteOrder, NativeEndian, WriteBytesExt};
use canvas_traits::canvas::{byte_swap, multiply_u8_pixel};
use canvas_traits::webgl::{DOMToTextureCommand, Parameter, WebGLCommandBacktrace};
use canvas_traits::webgl::{TexParameter, WebGLCommand, WebGLContextShareMode, WebGLError};
use canvas_traits::webgl::{WebGLFramebufferBindingRequest, WebGLMsg, WebGLMsgSender};
use canvas_traits::webgl::{WebGLProgramId, WebGLResult, WebGLSLVersion, WebGLSender};
use canvas_traits::webgl::{WebGLVersion, WebVRCommand, webgl_channel};
use canvas_traits::webgl::WebGLError::*;
use dom::bindings::codegen::Bindings::ANGLEInstancedArraysBinding::ANGLEInstancedArraysConstants;
use dom::bindings::codegen::Bindings::EXTBlendMinmaxBinding::EXTBlendMinmaxConstants;
use dom::bindings::codegen::Bindings::OESVertexArrayObjectBinding::OESVertexArrayObjectConstants;
use dom::bindings::codegen::Bindings::WebGLRenderingContextBinding;
use dom::bindings::codegen::Bindings::WebGLRenderingContextBinding::TexImageSource;
use dom::bindings::codegen::Bindings::WebGLRenderingContextBinding::WebGLContextAttributes;
use dom::bindings::codegen::Bindings::WebGLRenderingContextBinding::WebGLRenderingContextConstants as constants;
use dom::bindings::codegen::Bindings::WebGLRenderingContextBinding::WebGLRenderingContextMethods;
use dom::bindings::codegen::UnionTypes::ArrayBufferViewOrArrayBuffer;
use dom::bindings::codegen::UnionTypes::Float32ArrayOrUnrestrictedFloatSequence;
use dom::bindings::codegen::UnionTypes::Int32ArrayOrLongSequence;
use dom::bindings::conversions::{DerivedFrom, ToJSValConvertible};
use dom::bindings::error::{Error, ErrorResult, Fallible};
use dom::bindings::inheritance::Castable;
use dom::bindings::reflector::{DomObject, Reflector, reflect_dom_object};
use dom::bindings::root::{Dom, DomOnceCell, DomRoot, LayoutDom, MutNullableDom};
use dom::bindings::str::DOMString;
use dom::event::{Event, EventBubbles, EventCancelable};
use dom::htmlcanvaselement::HTMLCanvasElement;
use dom::htmlcanvaselement::utils as canvas_utils;
use dom::htmliframeelement::HTMLIFrameElement;
use dom::node::{Node, NodeDamage, document_from_node, window_from_node};
use dom::webgl_extensions::WebGLExtensions;
use dom::webgl_validations::WebGLValidator;
use dom::webgl_validations::tex_image_2d::{CommonTexImage2DValidator, CommonTexImage2DValidatorResult};
use dom::webgl_validations::tex_image_2d::{TexImage2DValidator, TexImage2DValidatorResult};
use dom::webgl_validations::types::{TexDataType, TexFormat, TexImageTarget};
use dom::webglactiveinfo::WebGLActiveInfo;
use dom::webglbuffer::WebGLBuffer;
use dom::webglcontextevent::WebGLContextEvent;
use dom::webglframebuffer::{WebGLFramebuffer, WebGLFramebufferAttachmentRoot, CompleteForRendering};
use dom::webglobject::WebGLObject;
use dom::webglprogram::WebGLProgram;
use dom::webglrenderbuffer::WebGLRenderbuffer;
use dom::webglshader::WebGLShader;
use dom::webglshaderprecisionformat::WebGLShaderPrecisionFormat;
use dom::webgltexture::{TexParameterValue, WebGLTexture};
use dom::webgluniformlocation::WebGLUniformLocation;
use dom::webglvertexarrayobjectoes::WebGLVertexArrayObjectOES;
use dom::window::Window;
use dom_struct::dom_struct;
use euclid::Size2D;
use half::f16;
use ipc_channel::ipc;
use js::jsapi::{JSContext, JSObject, Type};
use js::jsval::{BooleanValue, DoubleValue, Int32Value, UInt32Value, JSVal};
use js::jsval::{ObjectValue, NullValue, UndefinedValue};
use js::rust::CustomAutoRooterGuard;
use js::typedarray::{ArrayBufferView, CreateWith, Float32, Float32Array, Int32, Int32Array, Uint32Array};
use js::typedarray::{TypedArray, TypedArrayElementCreator};
use net_traits::image::base::PixelFormat;
use net_traits::image_cache::ImageResponse;
use offscreen_gl_context::{GLContextAttributes, GLLimits};
use script_layout_interface::HTMLCanvasDataSource;
use serde::{Deserialize, Serialize};
use servo_config::prefs::PREFS;
use std::cell::Cell;
use std::cmp;
use std::ptr::{self, NonNull};
use webrender_api;

pub fn is_gles() -> bool {
    // TODO: align this with the actual kind of graphics context in use, rather than
    // making assumptions based on platform
    cfg!(any(target_os = "android", target_os = "ios"))
}

// From the GLES 2.0.25 spec, page 85:
//
//     "If a texture that is currently bound to one of the targets
//      TEXTURE_2D, or TEXTURE_CUBE_MAP is deleted, it is as though
//      BindTexture had been executed with the same target and texture
//      zero."
//
// and similar text occurs for other object types.
macro_rules! handle_object_deletion {
    ($self_:expr, $binding:expr, $object:ident, $unbind_command:expr) => {
        if let Some(bound_object) = $binding.get() {
            if bound_object.id() == $object.id() {
                $binding.set(None);
                if let Some(command) = $unbind_command {
                    $self_.send_command(command);
                }
            }
        }
    };
}

macro_rules! optional_root_object_to_js_or_null {
    ($cx: expr, $binding:expr) => {{
        rooted!(in($cx) let mut rval = NullValue());
        if let Some(object) = $binding {
            object.to_jsval($cx, rval.handle_mut());
        }
        rval.get()
    }};
}

fn has_invalid_blend_constants(arg1: u32, arg2: u32) -> bool {
    match (arg1, arg2) {
        (constants::CONSTANT_COLOR, constants::CONSTANT_ALPHA) => true,
        (constants::ONE_MINUS_CONSTANT_COLOR, constants::ONE_MINUS_CONSTANT_ALPHA) => true,
        (constants::ONE_MINUS_CONSTANT_COLOR, constants::CONSTANT_ALPHA) => true,
        (constants::CONSTANT_COLOR, constants::ONE_MINUS_CONSTANT_ALPHA) => true,
        (_, _) => false,
    }
}

/// Set of bitflags for texture unpacking (texImage2d, etc...)
bitflags! {
    #[derive(JSTraceable, MallocSizeOf)]
    struct TextureUnpacking: u8 {
        const FLIP_Y_AXIS = 0x01;
        const PREMULTIPLY_ALPHA = 0x02;
        const CONVERT_COLORSPACE = 0x04;
    }
}

#[dom_struct]
pub struct WebGLRenderingContext {
    reflector_: Reflector,
    #[ignore_malloc_size_of = "Channels are hard"]
    webgl_sender: WebGLMsgSender,
    #[ignore_malloc_size_of = "Defined in webrender"]
    webrender_image: Cell<Option<webrender_api::ImageKey>>,
    share_mode: WebGLContextShareMode,
    webgl_version: WebGLVersion,
    glsl_version: WebGLSLVersion,
    #[ignore_malloc_size_of = "Defined in offscreen_gl_context"]
    limits: GLLimits,
    canvas: Dom<HTMLCanvasElement>,
    #[ignore_malloc_size_of = "Defined in canvas_traits"]
    last_error: Cell<Option<WebGLError>>,
    texture_packing_alignment: Cell<u8>,
    texture_unpacking_settings: Cell<TextureUnpacking>,
    // TODO(nox): Should be Cell<u8>.
    texture_unpacking_alignment: Cell<u32>,
    bound_framebuffer: MutNullableDom<WebGLFramebuffer>,
    bound_renderbuffer: MutNullableDom<WebGLRenderbuffer>,
    bound_buffer_array: MutNullableDom<WebGLBuffer>,
    current_program: MutNullableDom<WebGLProgram>,
    /// https://www.khronos.org/webgl/wiki/WebGL_and_OpenGL_Differences#Vertex_Attribute_0
    #[ignore_malloc_size_of = "Because it's small"]
    current_vertex_attrib_0: Cell<(f32, f32, f32, f32)>,
    #[ignore_malloc_size_of = "Because it's small"]
    current_scissor: Cell<(i32, i32, u32, u32)>,
    #[ignore_malloc_size_of = "Because it's small"]
    current_clear_color: Cell<(f32, f32, f32, f32)>,
    extension_manager: WebGLExtensions,
    capabilities: Capabilities,
    default_vao: DomOnceCell<WebGLVertexArrayObjectOES>,
    current_vao: MutNullableDom<WebGLVertexArrayObjectOES>,
    textures: Textures,
}

impl WebGLRenderingContext {
    pub fn new_inherited(
        window: &Window,
        canvas: &HTMLCanvasElement,
        webgl_version: WebGLVersion,
        size: Size2D<u32>,
        attrs: GLContextAttributes,
    ) -> Result<WebGLRenderingContext, String> {
        if let Some(true) = PREFS
            .get("webgl.testing.context_creation_error")
            .as_boolean()
        {
            return Err("WebGL context creation error forced by pref `webgl.testing.context_creation_error`".into());
        }

        let webgl_chan = match window.webgl_chan() {
            Some(chan) => chan,
            None => return Err("WebGL initialization failed early on".into()),
        };

        let (sender, receiver) = webgl_channel().unwrap();
        webgl_chan
            .send(WebGLMsg::CreateContext(webgl_version, size, attrs, sender))
            .unwrap();
        let result = receiver.recv().unwrap();

        result.map(|ctx_data| {
            let max_combined_texture_image_units = ctx_data.limits.max_combined_texture_image_units;
            Self {
                reflector_: Reflector::new(),
                webgl_sender: ctx_data.sender,
                webrender_image: Cell::new(None),
                share_mode: ctx_data.share_mode,
                webgl_version,
                glsl_version: ctx_data.glsl_version,
                limits: ctx_data.limits,
                canvas: Dom::from_ref(canvas),
                last_error: Cell::new(None),
                texture_packing_alignment: Cell::new(4),
                texture_unpacking_settings: Cell::new(TextureUnpacking::CONVERT_COLORSPACE),
                texture_unpacking_alignment: Cell::new(4),
                bound_framebuffer: MutNullableDom::new(None),
                bound_buffer_array: MutNullableDom::new(None),
                bound_renderbuffer: MutNullableDom::new(None),
                current_program: MutNullableDom::new(None),
                current_vertex_attrib_0: Cell::new((0f32, 0f32, 0f32, 1f32)),
                current_scissor: Cell::new((0, 0, size.width, size.height)),
                current_clear_color: Cell::new((0.0, 0.0, 0.0, 0.0)),
                extension_manager: WebGLExtensions::new(webgl_version),
                capabilities: Default::default(),
                default_vao: Default::default(),
                current_vao: Default::default(),
                textures: Textures::new(max_combined_texture_image_units),
            }
        })
    }

    #[allow(unrooted_must_root)]
    pub fn new(
        window: &Window,
        canvas: &HTMLCanvasElement,
        webgl_version: WebGLVersion,
        size: Size2D<u32>,
        attrs: GLContextAttributes,
    ) -> Option<DomRoot<WebGLRenderingContext>> {
        match WebGLRenderingContext::new_inherited(window, canvas, webgl_version, size, attrs) {
            Ok(ctx) => Some(reflect_dom_object(
                Box::new(ctx),
                window,
                WebGLRenderingContextBinding::Wrap,
            )),
            Err(msg) => {
                error!("Couldn't create WebGLRenderingContext: {}", msg);
                let event = WebGLContextEvent::new(
                    window,
                    atom!("webglcontextcreationerror"),
                    EventBubbles::DoesNotBubble,
                    EventCancelable::Cancelable,
                    DOMString::from(msg),
                );
                event.upcast::<Event>().fire(canvas.upcast());
                None
            },
        }
    }

    pub fn limits(&self) -> &GLLimits {
        &self.limits
    }

    fn current_vao(&self) -> DomRoot<WebGLVertexArrayObjectOES> {
        self.current_vao.or_init(|| {
            DomRoot::from_ref(
                self.default_vao
                    .init_once(|| WebGLVertexArrayObjectOES::new(self, None)),
            )
        })
    }

    pub fn recreate(&self, size: Size2D<u32>) {
        let (sender, receiver) = webgl_channel().unwrap();
        self.webgl_sender.send_resize(size, sender).unwrap();

        if let Err(msg) = receiver.recv().unwrap() {
            error!("Error resizing WebGLContext: {}", msg);
            return;
        };

        // ClearColor needs to be restored because after a resize the GLContext is recreated
        // and the framebuffer is cleared using the default black transparent color.
        let color = self.current_clear_color.get();
        self.send_command(WebGLCommand::ClearColor(color.0, color.1, color.2, color.3));

        // WebGL Spec: Scissor rect must not change if the canvas is resized.
        // See: webgl/conformance-1.0.3/conformance/rendering/gl-scissor-canvas-dimensions.html
        // NativeContext handling library changes the scissor after a resize, so we need to reset the
        // default scissor when the canvas was created or the last scissor that the user set.
        let rect = self.current_scissor.get();
        self.send_command(WebGLCommand::Scissor(rect.0, rect.1, rect.2, rect.3));

        // Bound texture must not change when the canvas is resized.
        // Right now offscreen_gl_context generates a new FBO and the bound texture is changed
        // in order to create a new render to texture attachment.
        // Send a command to re-bind the TEXTURE_2D, if any.
        if let Some(texture) = self
            .textures
            .active_texture_slot(constants::TEXTURE_2D)
            .unwrap()
            .get()
        {
            self.send_command(WebGLCommand::BindTexture(
                constants::TEXTURE_2D,
                Some(texture.id()),
            ));
        }

        // Bound framebuffer must not change when the canvas is resized.
        // Right now offscreen_gl_context generates a new FBO on resize.
        // Send a command to re-bind the framebuffer, if any.
        if let Some(fbo) = self.bound_framebuffer.get() {
            let id = WebGLFramebufferBindingRequest::Explicit(fbo.id());
            self.send_command(WebGLCommand::BindFramebuffer(constants::FRAMEBUFFER, id));
        }
    }

    pub fn webgl_sender(&self) -> WebGLMsgSender {
        self.webgl_sender.clone()
    }

    #[inline]
    pub fn send_command(&self, command: WebGLCommand) {
        self.webgl_sender.send(command, capture_webgl_backtrace(self)).unwrap();
    }

    #[inline]
    pub fn send_vr_command(&self, command: WebVRCommand) {
        self.webgl_sender.send_vr(command).unwrap();
    }

    pub fn webgl_error(&self, err: WebGLError) {
        // TODO(emilio): Add useful debug messages to this
        warn!(
            "WebGL error: {:?}, previous error was {:?}",
            err,
            self.last_error.get()
        );

        // If an error has been detected no further errors must be
        // recorded until `getError` has been called
        if self.last_error.get().is_none() {
            self.last_error.set(Some(err));
        }
    }

    // Helper function for validating framebuffer completeness in
    // calls touching the framebuffer.  From the GLES 2.0.25 spec,
    // page 119:
    //
    //    "Effects of Framebuffer Completeness on Framebuffer
    //     Operations
    //
    //     If the currently bound framebuffer is not framebuffer
    //     complete, then it is an error to attempt to use the
    //     framebuffer for writing or reading. This means that
    //     rendering commands such as DrawArrays and DrawElements, as
    //     well as commands that read the framebuffer such as
    //     ReadPixels and CopyTexSubImage, will generate the error
    //     INVALID_FRAMEBUFFER_OPERATION if called while the
    //     framebuffer is not framebuffer complete."
    //
    // The WebGL spec mentions a couple more operations that trigger
    // this: clear() and getParameter(IMPLEMENTATION_COLOR_READ_*).
    fn validate_framebuffer(&self) -> WebGLResult<()> {
        match self.bound_framebuffer.get() {
            Some(fb) => match fb.check_status_for_rendering() {
                CompleteForRendering::Complete => Ok(()),
                CompleteForRendering::Incomplete => Err(InvalidFramebufferOperation),
                CompleteForRendering::MissingColorAttachment => Err(InvalidOperation),
            },
            None => Ok(()),
        }
    }

    fn validate_ownership<T>(&self, object: &T) -> WebGLResult<()>
    where
        T: DerivedFrom<WebGLObject>,
    {
        if self != object.upcast().context() {
            return Err(InvalidOperation);
        }
        Ok(())
    }

    fn with_location<F>(&self, location: Option<&WebGLUniformLocation>, f: F)
    where
        F: FnOnce(&WebGLUniformLocation) -> WebGLResult<()>,
    {
        let location = match location {
            Some(loc) => loc,
            None => return,
        };
        match self.current_program.get() {
            Some(ref program)
                if program.id() == location.program_id() &&
                    program.link_generation() == location.link_generation() => {},
            _ => return self.webgl_error(InvalidOperation),
        }
        handle_potential_webgl_error!(self, f(location));
    }

    pub fn textures(&self) -> &Textures {
        &self.textures
    }

    fn tex_parameter(&self, target: u32, param: u32, value: TexParameterValue) {
        let texture_slot =
            handle_potential_webgl_error!(self, self.textures.active_texture_slot(target), return);
        let texture =
            handle_potential_webgl_error!(self, texture_slot.get().ok_or(InvalidOperation), return);

        if !self
            .extension_manager
            .is_get_tex_parameter_name_enabled(param)
        {
            return self.webgl_error(InvalidEnum);
        }

        handle_potential_webgl_error!(self, texture.tex_parameter(param, value), return);

        // Validate non filterable TEXTURE_2D data_types
        if target != constants::TEXTURE_2D {
            return;
        }

        let target = TexImageTarget::Texture2D;
        let info = texture.image_info_for_target(&target, 0);
        if info.is_initialized() {
            self.validate_filterable_texture(
                &texture,
                target,
                0,
                info.internal_format().unwrap_or(TexFormat::RGBA),
                info.width(),
                info.height(),
                info.data_type().unwrap_or(TexDataType::UnsignedByte),
            );
        }
    }

    fn mark_as_dirty(&self) {
        self.canvas
            .upcast::<Node>()
            .dirty(NodeDamage::OtherNodeDamage);
    }

    fn vertex_attrib(&self, indx: u32, x: f32, y: f32, z: f32, w: f32) {
        if indx >= self.limits.max_vertex_attribs {
            return self.webgl_error(InvalidValue);
        }

        if indx == 0 {
            self.current_vertex_attrib_0.set((x, y, z, w))
        }

        self.send_command(WebGLCommand::VertexAttrib(indx, x, y, z, w));
    }

    fn get_current_framebuffer_size(&self) -> Option<(i32, i32)> {
        match self.bound_framebuffer.get() {
            Some(fb) => return fb.size(),

            // The window system framebuffer is bound
            None => return Some((self.DrawingBufferWidth(), self.DrawingBufferHeight())),
        }
    }

    // LINEAR filtering may be forbidden when using WebGL extensions.
    // https://www.khronos.org/registry/webgl/extensions/OES_texture_float_linear/
    fn validate_filterable_texture(
        &self,
        texture: &WebGLTexture,
        target: TexImageTarget,
        level: u32,
        format: TexFormat,
        width: u32,
        height: u32,
        data_type: TexDataType,
    ) -> bool {
        if self
            .extension_manager
            .is_filterable(data_type.as_gl_constant()) ||
            !texture.is_using_linear_filtering()
        {
            return true;
        }

        // Handle validation failed: LINEAR filtering not valid for this texture
        // WebGL Conformance tests expect to fallback to [0, 0, 0, 255] RGBA UNSIGNED_BYTE
        let data_type = TexDataType::UnsignedByte;
        let expected_byte_length = width * height * 4;
        let mut pixels = vec![0u8; expected_byte_length as usize];
        for rgba8 in pixels.chunks_mut(4) {
            rgba8[3] = 255u8;
        }

        let pixels = self.prepare_pixels(format, data_type, width, height, 1, true, true, pixels);
        self.tex_image_2d(
            texture, target, data_type, format, level, width, height, 0, 1, pixels,
        );

        false
    }

    fn validate_stencil_actions(&self, action: u32) -> bool {
        match action {
            0 |
            constants::KEEP |
            constants::REPLACE |
            constants::INCR |
            constants::DECR |
            constants::INVERT |
            constants::INCR_WRAP |
            constants::DECR_WRAP => true,
            _ => false,
        }
    }

    fn get_image_pixels(
        &self,
        source: TexImageSource,
    ) -> Fallible<Option<(Vec<u8>, Size2D<u32>, bool)>> {
        Ok(Some(match source {
            TexImageSource::ImageData(image_data) => {
                (image_data.get_data_array(), image_data.get_size(), false)
            },
            TexImageSource::HTMLImageElement(image) => {
                let document = document_from_node(&*self.canvas);
                if !image.same_origin(document.origin()) {
                    return Err(Error::Security);
                }

                let img_url = match image.get_url() {
                    Some(url) => url,
                    None => return Ok(None),
                };

                let window = window_from_node(&*self.canvas);

                let img = match canvas_utils::request_image_from_cache(&window, img_url) {
                    ImageResponse::Loaded(img, _) => img,
                    ImageResponse::PlaceholderLoaded(_, _) |
                    ImageResponse::None |
                    ImageResponse::MetadataLoaded(_) => return Ok(None),
                };

                let size = Size2D::new(img.width, img.height);

                // For now Servo's images are all stored as BGRA8 internally.
                let mut data = match img.format {
                    PixelFormat::BGRA8 => img.bytes.to_vec(),
                    _ => unimplemented!(),
                };

                byte_swap(&mut data);

                (data, size, false)
            },
            // TODO(emilio): Getting canvas data is implemented in CanvasRenderingContext2D,
            // but we need to refactor it moving it to `HTMLCanvasElement` and support
            // WebGLContext (probably via GetPixels()).
            TexImageSource::HTMLCanvasElement(canvas) => {
                if !canvas.origin_is_clean() {
                    return Err(Error::Security);
                }
                if let Some((mut data, size)) = canvas.fetch_all_data() {
                    // Pixels got from Canvas have already alpha premultiplied
                    byte_swap(&mut data);
                    (data, size, true)
                } else {
                    return Ok(None);
                }
            },
            TexImageSource::HTMLVideoElement(_) => {
                // TODO: https://github.com/servo/servo/issues/6711
                return Ok(None);
            }
        }))
    }

    // TODO(emilio): Move this logic to a validator.
    fn validate_tex_image_2d_data(
        &self,
        width: u32,
        height: u32,
        format: TexFormat,
        data_type: TexDataType,
        unpacking_alignment: u32,
        data: &Option<ArrayBufferView>,
    ) -> Result<u32, ()> {
        let element_size = data_type.element_size();
        let components_per_element = data_type.components_per_element();
        let components = format.components();

        // If data is non-null, the type of pixels must match the type of the
        // data to be read.
        // If it is UNSIGNED_BYTE, a Uint8Array must be supplied;
        // if it is UNSIGNED_SHORT_5_6_5, UNSIGNED_SHORT_4_4_4_4,
        // or UNSIGNED_SHORT_5_5_5_1, a Uint16Array must be supplied.
        // or FLOAT, a Float32Array must be supplied.
        // If the types do not match, an INVALID_OPERATION error is generated.
        let received_size = match *data {
            None => element_size,
            Some(ref buffer) => match buffer.get_array_type() {
                Type::Uint8 => 1,
                Type::Uint16 => 2,
                Type::Float32 => 4,
                _ => {
                    self.webgl_error(InvalidOperation);
                    return Err(());
                },
            },
        };

        if received_size != element_size {
            self.webgl_error(InvalidOperation);
            return Err(());
        }

        // NOTE: width and height are positive or zero due to validate()
        if height == 0 {
            return Ok(0);
        } else {
            // We need to be careful here to not count unpack
            // alignment at the end of the image, otherwise (for
            // example) passing a single byte for uploading a 1x1
            // GL_ALPHA/GL_UNSIGNED_BYTE texture would throw an error.
            let cpp = element_size * components / components_per_element;
            let stride = (width * cpp + unpacking_alignment - 1) & !(unpacking_alignment - 1);
            return Ok(stride * (height - 1) + width * cpp);
        }
    }

    /// Flips the pixels in the Vec on the Y axis if
    /// UNPACK_FLIP_Y_WEBGL is currently enabled.
    fn flip_teximage_y(
        &self,
        pixels: Vec<u8>,
        internal_format: TexFormat,
        data_type: TexDataType,
        width: usize,
        height: usize,
        unpacking_alignment: usize,
    ) -> Vec<u8> {
        if !self
            .texture_unpacking_settings
            .get()
            .contains(TextureUnpacking::FLIP_Y_AXIS)
        {
            return pixels;
        }

        let cpp = (data_type.element_size() * internal_format.components() /
            data_type.components_per_element()) as usize;

        let stride = (width * cpp + unpacking_alignment - 1) & !(unpacking_alignment - 1);

        let mut flipped = Vec::<u8>::with_capacity(pixels.len());

        for y in 0..height {
            let flipped_y = height - 1 - y;
            let start = flipped_y * stride;

            flipped.extend_from_slice(&pixels[start..(start + width * cpp)]);
            flipped.extend(vec![0u8; stride - width * cpp]);
        }

        flipped
    }

    /// Performs premultiplication of the pixels if
    /// UNPACK_PREMULTIPLY_ALPHA_WEBGL is currently enabled.
    fn premultiply_pixels(&self, format: TexFormat, data_type: TexDataType, pixels: &mut [u8]) {
        if !self
            .texture_unpacking_settings
            .get()
            .contains(TextureUnpacking::PREMULTIPLY_ALPHA)
        {
            return;
        }

        match (format, data_type) {
            (TexFormat::RGBA, TexDataType::UnsignedByte) => {
                for rgba in pixels.chunks_mut(4) {
                    rgba[0] = multiply_u8_pixel(rgba[0], rgba[3]);
                    rgba[1] = multiply_u8_pixel(rgba[1], rgba[3]);
                    rgba[2] = multiply_u8_pixel(rgba[2], rgba[3]);
                }
            },
            (TexFormat::LuminanceAlpha, TexDataType::UnsignedByte) => {
                for la in pixels.chunks_mut(2) {
                    la[0] = multiply_u8_pixel(la[0], la[1]);
                }
            },
            (TexFormat::RGBA, TexDataType::UnsignedShort5551) => {
                for rgba in pixels.chunks_mut(2) {
                    if NativeEndian::read_u16(rgba) & 1 == 0 {
                        NativeEndian::write_u16(rgba, 0);
                    }
                }
            },
            (TexFormat::RGBA, TexDataType::UnsignedShort4444) => {
                for rgba in pixels.chunks_mut(2) {
                    let pix = NativeEndian::read_u16(rgba);
                    let extend_to_8_bits = |val| (val | val << 4) as u8;
                    let r = extend_to_8_bits(pix >> 12 & 0x0f);
                    let g = extend_to_8_bits(pix >> 8 & 0x0f);
                    let b = extend_to_8_bits(pix >> 4 & 0x0f);
                    let a = extend_to_8_bits(pix & 0x0f);
                    NativeEndian::write_u16(
                        rgba,
                        ((multiply_u8_pixel(r, a) & 0xf0) as u16) << 8 |
                            ((multiply_u8_pixel(g, a) & 0xf0) as u16) << 4 |
                            ((multiply_u8_pixel(b, a) & 0xf0) as u16) |
                            ((a & 0x0f) as u16),
                    );
                }
            },
            // Other formats don't have alpha, so return their data untouched.
            _ => {},
        }
    }

    fn prepare_pixels(
        &self,
        internal_format: TexFormat,
        data_type: TexDataType,
        width: u32,
        height: u32,
        unpacking_alignment: u32,
        source_premultiplied: bool,
        source_from_image_or_canvas: bool,
        mut pixels: Vec<u8>,
    ) -> Vec<u8> {
        let dest_premultiply = self
            .texture_unpacking_settings
            .get()
            .contains(TextureUnpacking::PREMULTIPLY_ALPHA);
        if !source_premultiplied && dest_premultiply {
            if source_from_image_or_canvas {
                // When the pixels come from image or canvas or imagedata, use RGBA8 format
                self.premultiply_pixels(TexFormat::RGBA, TexDataType::UnsignedByte, &mut pixels);
            } else {
                self.premultiply_pixels(internal_format, data_type, &mut pixels);
            }
        } else if source_premultiplied && !dest_premultiply {
            remove_premultiplied_alpha(&mut pixels);
        }

        if source_from_image_or_canvas {
            pixels = rgba8_image_to_tex_image_data(internal_format, data_type, pixels);
        }

        // FINISHME: Consider doing premultiply and flip in a single mutable Vec.
        self.flip_teximage_y(
            pixels,
            internal_format,
            data_type,
            width as usize,
            height as usize,
            unpacking_alignment as usize,
        )
    }

    fn tex_image_2d(
        &self,
        texture: &WebGLTexture,
        target: TexImageTarget,
        data_type: TexDataType,
        internal_format: TexFormat,
        level: u32,
        width: u32,
        height: u32,
        _border: u32,
        unpacking_alignment: u32,
        pixels: Vec<u8>,
    ) {
        // NB: pixels should NOT be premultipied

        // TexImage2D depth is always equal to 1
        handle_potential_webgl_error!(
            self,
            texture.initialize(
                target,
                width,
                height,
                1,
                internal_format,
                level,
                Some(data_type)
            )
        );

        // Set the unpack alignment.  For textures coming from arrays,
        // this will be the current value of the context's
        // GL_UNPACK_ALIGNMENT, while for textures from images or
        // canvas (produced by rgba8_image_to_tex_image_data()), it
        // will be 1.
        self.send_command(WebGLCommand::PixelStorei(
            constants::UNPACK_ALIGNMENT,
            unpacking_alignment as i32,
        ));

        let format = internal_format.as_gl_constant();
        let data_type = data_type.as_gl_constant();
        let internal_format = self
            .extension_manager
            .get_effective_tex_internal_format(format, data_type);

        // TODO(emilio): convert colorspace if requested
        let (sender, receiver) = ipc::bytes_channel().unwrap();
        self.send_command(WebGLCommand::TexImage2D(
            target.as_gl_constant(),
            level as i32,
            internal_format as i32,
            width as i32,
            height as i32,
            format,
            data_type,
            receiver,
        ));
        sender.send(&pixels).unwrap();

        if let Some(fb) = self.bound_framebuffer.get() {
            fb.invalidate_texture(&*texture);
        }
    }

    fn tex_sub_image_2d(
        &self,
        texture: DomRoot<WebGLTexture>,
        target: TexImageTarget,
        level: u32,
        xoffset: i32,
        yoffset: i32,
        width: u32,
        height: u32,
        format: TexFormat,
        data_type: TexDataType,
        unpacking_alignment: u32,
        pixels: Vec<u8>,
    ) {
        // We have already validated level
        let image_info = texture.image_info_for_target(&target, level);

        // GL_INVALID_VALUE is generated if:
        //   - xoffset or yoffset is less than 0
        //   - x offset plus the width is greater than the texture width
        //   - y offset plus the height is greater than the texture height
        if xoffset < 0 ||
            (xoffset as u32 + width) > image_info.width() ||
            yoffset < 0 ||
            (yoffset as u32 + height) > image_info.height()
        {
            return self.webgl_error(InvalidValue);
        }

        // NB: format and internal_format must match.
        if format != image_info.internal_format().unwrap() ||
            data_type != image_info.data_type().unwrap()
        {
            return self.webgl_error(InvalidOperation);
        }

        // Set the unpack alignment.  For textures coming from arrays,
        // this will be the current value of the context's
        // GL_UNPACK_ALIGNMENT, while for textures from images or
        // canvas (produced by rgba8_image_to_tex_image_data()), it
        // will be 1.
        self.send_command(WebGLCommand::PixelStorei(
            constants::UNPACK_ALIGNMENT,
            unpacking_alignment as i32,
        ));

        // TODO(emilio): convert colorspace if requested
        let (sender, receiver) = ipc::bytes_channel().unwrap();
        self.send_command(WebGLCommand::TexSubImage2D(
            target.as_gl_constant(),
            level as i32,
            xoffset,
            yoffset,
            width as i32,
            height as i32,
            format.as_gl_constant(),
            data_type.as_gl_constant(),
            receiver,
        ));
        sender.send(&pixels).unwrap();
    }

    fn get_gl_extensions(&self) -> String {
        let (sender, receiver) = webgl_channel().unwrap();
        self.send_command(WebGLCommand::GetExtensions(sender));
        receiver.recv().unwrap()
    }

    pub fn layout_handle(&self) -> webrender_api::ImageKey {
        match self.share_mode {
            WebGLContextShareMode::SharedTexture => {
                // WR using ExternalTexture requires a single update message.
                self.webrender_image.get().unwrap_or_else(|| {
                    let (sender, receiver) = webgl_channel().unwrap();
                    self.webgl_sender.send_update_wr_image(sender).unwrap();
                    let image_key = receiver.recv().unwrap();
                    self.webrender_image.set(Some(image_key));

                    image_key
                })
            },
            WebGLContextShareMode::Readback => {
                // WR using Readback requires to update WR image every frame
                // in order to send the new raw pixels.
                let (sender, receiver) = webgl_channel().unwrap();
                self.webgl_sender.send_update_wr_image(sender).unwrap();
                receiver.recv().unwrap()
            },
        }
    }

    // https://www.khronos.org/registry/webgl/extensions/ANGLE_instanced_arrays/
    pub fn draw_arrays_instanced(
        &self,
        mode: u32,
        first: i32,
        count: i32,
        primcount: i32,
    ) -> WebGLResult<()> {
        match mode {
            constants::POINTS |
            constants::LINE_STRIP |
            constants::LINE_LOOP |
            constants::LINES |
            constants::TRIANGLE_STRIP |
            constants::TRIANGLE_FAN |
            constants::TRIANGLES => {},
            _ => {
                return Err(InvalidEnum);
            },
        }
        if first < 0 || count < 0 || primcount < 0 {
            return Err(InvalidValue);
        }

        let current_program = self.current_program.get().ok_or(InvalidOperation)?;

        let required_len = if count > 0 {
            first
                .checked_add(count)
                .map(|len| len as u32)
                .ok_or(InvalidOperation)?
        } else {
            0
        };

        self.current_vao().validate_for_draw(
            required_len,
            primcount as u32,
            &current_program.active_attribs(),
        )?;

        self.validate_framebuffer()?;

        if count == 0 || primcount == 0 {
            return Ok(());
        }

        self.send_command(if primcount == 1 {
            WebGLCommand::DrawArrays { mode, first, count }
        } else {
            WebGLCommand::DrawArraysInstanced {
                mode,
                first,
                count,
                primcount,
            }
        });
        self.mark_as_dirty();
        Ok(())
    }

    // https://www.khronos.org/registry/webgl/extensions/ANGLE_instanced_arrays/
    pub fn draw_elements_instanced(
        &self,
        mode: u32,
        count: i32,
        type_: u32,
        offset: i64,
        primcount: i32,
    ) -> WebGLResult<()> {
        match mode {
            constants::POINTS |
            constants::LINE_STRIP |
            constants::LINE_LOOP |
            constants::LINES |
            constants::TRIANGLE_STRIP |
            constants::TRIANGLE_FAN |
            constants::TRIANGLES => {},
            _ => {
                return Err(InvalidEnum);
            },
        }
        if count < 0 || offset < 0 || primcount < 0 {
            return Err(InvalidValue);
        }
        let type_size = match type_ {
            constants::UNSIGNED_BYTE => 1,
            constants::UNSIGNED_SHORT => 2,
            constants::UNSIGNED_INT if self.extension_manager.is_element_index_uint_enabled() => 4,
            _ => return Err(InvalidEnum),
        };
        if offset % type_size != 0 {
            return Err(InvalidOperation);
        }

        let current_program = self.current_program.get().ok_or(InvalidOperation)?;
        let array_buffer = self
            .current_vao()
            .element_array_buffer()
            .get()
            .ok_or(InvalidOperation)?;

        if count > 0 && primcount > 0 {
            // This operation cannot overflow in u64 and we know all those values are nonnegative.
            let val = offset as u64 + (count as u64 * type_size as u64);
            if val > array_buffer.capacity() as u64 {
                return Err(InvalidOperation);
            }
        }

        // TODO(nox): Pass the correct number of vertices required.
        self.current_vao().validate_for_draw(
            0,
            primcount as u32,
            &current_program.active_attribs(),
        )?;

        self.validate_framebuffer()?;

        if count == 0 || primcount == 0 {
            return Ok(());
        }

        let offset = offset as u32;
        self.send_command(if primcount == 1 {
            WebGLCommand::DrawElements {
                mode,
                count,
                type_,
                offset,
            }
        } else {
            WebGLCommand::DrawElementsInstanced {
                mode,
                count,
                type_,
                offset,
                primcount,
            }
        });
        self.mark_as_dirty();
        Ok(())
    }

    pub fn vertex_attrib_divisor(&self, index: u32, divisor: u32) {
        if index >= self.limits.max_vertex_attribs {
            return self.webgl_error(InvalidValue);
        }

        self.current_vao().vertex_attrib_divisor(index, divisor);
        self.send_command(WebGLCommand::VertexAttribDivisor { index, divisor });
    }

    // Used by HTMLCanvasElement.toDataURL
    //
    // This emits errors quite liberally, but the spec says that this operation
    // can fail and that it is UB what happens in that case.
    //
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#2.2
    pub fn get_image_data(&self, width: u32, height: u32) -> Option<Vec<u8>> {
        handle_potential_webgl_error!(self, self.validate_framebuffer(), return None);

        let (fb_width, fb_height) = handle_potential_webgl_error!(
            self,
            self.get_current_framebuffer_size().ok_or(InvalidOperation),
            return None
        );
        let width = cmp::min(width, fb_width as u32);
        let height = cmp::min(height, fb_height as u32);

        let (sender, receiver) = ipc::bytes_channel().unwrap();
        self.send_command(WebGLCommand::ReadPixels(
            0,
            0,
            width as i32,
            height as i32,
            constants::RGBA,
            constants::UNSIGNED_BYTE,
            sender,
        ));
        Some(receiver.recv().unwrap())
    }

    pub fn array_buffer(&self) -> Option<DomRoot<WebGLBuffer>> {
        self.bound_buffer_array.get()
    }

    pub fn bound_buffer(&self, target: u32) -> WebGLResult<Option<DomRoot<WebGLBuffer>>> {
        match target {
            constants::ARRAY_BUFFER => Ok(self.bound_buffer_array.get()),
            constants::ELEMENT_ARRAY_BUFFER => Ok(self.current_vao().element_array_buffer().get()),
            _ => Err(WebGLError::InvalidEnum),
        }
    }

    pub fn create_vertex_array(&self) -> Option<DomRoot<WebGLVertexArrayObjectOES>> {
        let (sender, receiver) = webgl_channel().unwrap();
        self.send_command(WebGLCommand::CreateVertexArray(sender));
        receiver
            .recv()
            .unwrap()
            .map(|id| WebGLVertexArrayObjectOES::new(self, Some(id)))
    }

    pub fn delete_vertex_array(&self, vao: Option<&WebGLVertexArrayObjectOES>) {
        if let Some(vao) = vao {
            handle_potential_webgl_error!(self, self.validate_ownership(vao), return);
            // The default vertex array has no id and should never be passed around.
            assert!(vao.id().is_some());
            if vao.is_deleted() {
                return;
            }
            if vao == &*self.current_vao() {
                // Setting it to None will make self.current_vao() reset it to the default one
                // next time it is called.
                self.current_vao.set(None);
                self.send_command(WebGLCommand::BindVertexArray(None));
            }
            vao.delete();
        }
    }

    pub fn is_vertex_array(&self, vao: Option<&WebGLVertexArrayObjectOES>) -> bool {
        vao.map_or(false, |vao| {
            // The default vertex array has no id and should never be passed around.
            assert!(vao.id().is_some());
            self.validate_ownership(vao).is_ok() && vao.ever_bound() && !vao.is_deleted()
        })
    }

    pub fn bind_vertex_array(&self, vao: Option<&WebGLVertexArrayObjectOES>) {
        if let Some(vao) = vao {
            // The default vertex array has no id and should never be passed around.
            assert!(vao.id().is_some());
            handle_potential_webgl_error!(self, self.validate_ownership(vao), return);
            if vao.is_deleted() {
                return self.webgl_error(InvalidOperation);
            }
            vao.set_ever_bound();
        }
        self.send_command(WebGLCommand::BindVertexArray(vao.and_then(|vao| vao.id())));
        // Setting it to None will make self.current_vao() reset it to the default one
        // next time it is called.
        self.current_vao.set(vao);
    }

    fn validate_blend_mode(&self, mode: u32) -> WebGLResult<()> {
        match mode {
            constants::FUNC_ADD | constants::FUNC_SUBTRACT | constants::FUNC_REVERSE_SUBTRACT => {
                Ok(())
            },
            EXTBlendMinmaxConstants::MIN_EXT | EXTBlendMinmaxConstants::MAX_EXT
                if self.extension_manager.is_blend_minmax_enabled() =>
            {
                Ok(())
            },
            _ => Err(InvalidEnum),
        }
    }

    pub fn initialize_framebuffer(&self, clear_bits: u32) {
        if clear_bits == 0 {
            return;
        }
        self.send_command(WebGLCommand::InitializeFramebuffer {
            color: clear_bits & constants::COLOR_BUFFER_BIT != 0,
            depth: clear_bits & constants::DEPTH_BUFFER_BIT != 0,
            stencil: clear_bits & constants::STENCIL_BUFFER_BIT != 0,
        });
    }

    pub fn bound_framebuffer(&self) -> Option<DomRoot<WebGLFramebuffer>> {
        self.bound_framebuffer.get()
    }
}

#[cfg(not(feature = "webgl_backtrace"))]
#[inline]
pub fn capture_webgl_backtrace<T: DomObject>(_: &T) -> WebGLCommandBacktrace {
    WebGLCommandBacktrace {}
}

#[cfg(feature = "webgl_backtrace")]
#[cfg_attr(feature = "webgl_backtrace", allow(unsafe_code))]
pub fn capture_webgl_backtrace<T: DomObject>(obj: &T) -> WebGLCommandBacktrace {
    let bt = Backtrace::new();
    unsafe {
        capture_stack!(in(obj.global().get_cx()) let stack);
        WebGLCommandBacktrace {
            backtrace: format!("{:?}", bt),
            js_backtrace: stack.and_then(|s| s.as_string(None)),
        }
    }
}

impl Drop for WebGLRenderingContext {
    fn drop(&mut self) {
        let _ = self.webgl_sender.send_remove();
    }
}

impl WebGLRenderingContextMethods for WebGLRenderingContext {
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.1
    fn Canvas(&self) -> DomRoot<HTMLCanvasElement> {
        DomRoot::from_ref(&*self.canvas)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.11
    fn Flush(&self) {
        self.send_command(WebGLCommand::Flush);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.11
    fn Finish(&self) {
        let (sender, receiver) = webgl_channel().unwrap();
        self.send_command(WebGLCommand::Finish(sender));
        receiver.recv().unwrap()
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.1
    fn DrawingBufferWidth(&self) -> i32 {
        let (sender, receiver) = webgl_channel().unwrap();
        self.send_command(WebGLCommand::DrawingBufferWidth(sender));
        receiver.recv().unwrap()
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.1
    fn DrawingBufferHeight(&self) -> i32 {
        let (sender, receiver) = webgl_channel().unwrap();
        self.send_command(WebGLCommand::DrawingBufferHeight(sender));
        receiver.recv().unwrap()
    }

    #[allow(unsafe_code)]
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.5
    unsafe fn GetBufferParameter(&self, _cx: *mut JSContext, target: u32, parameter: u32) -> JSVal {
        let buffer = handle_potential_webgl_error!(
            self,
            self.bound_buffer(target)
                .and_then(|buf| buf.ok_or(InvalidOperation)),
            return NullValue()
        );

        match parameter {
            constants::BUFFER_SIZE => Int32Value(buffer.capacity() as i32),
            constants::BUFFER_USAGE => Int32Value(buffer.usage() as i32),
            _ => {
                self.webgl_error(InvalidEnum);
                NullValue()
            },
        }
    }

    #[allow(unsafe_code)]
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    unsafe fn GetParameter(&self, cx: *mut JSContext, parameter: u32) -> JSVal {
        if !self
            .extension_manager
            .is_get_parameter_name_enabled(parameter)
        {
            self.webgl_error(WebGLError::InvalidEnum);
            return NullValue();
        }

        match parameter {
            constants::ARRAY_BUFFER_BINDING => {
                return optional_root_object_to_js_or_null!(cx, &self.bound_buffer_array.get());
            },
            constants::CURRENT_PROGRAM => {
                return optional_root_object_to_js_or_null!(cx, &self.current_program.get());
            },
            constants::ELEMENT_ARRAY_BUFFER_BINDING => {
                let buffer = self.current_vao().element_array_buffer().get();
                return optional_root_object_to_js_or_null!(cx, buffer);
            },
            constants::FRAMEBUFFER_BINDING => {
                return optional_root_object_to_js_or_null!(cx, &self.bound_framebuffer.get());
            },
            constants::RENDERBUFFER_BINDING => {
                return optional_root_object_to_js_or_null!(cx, &self.bound_renderbuffer.get());
            },
            constants::TEXTURE_BINDING_2D => {
                let texture = self
                    .textures
                    .active_texture_slot(constants::TEXTURE_2D)
                    .unwrap()
                    .get();
                return optional_root_object_to_js_or_null!(cx, texture);
            },
            constants::TEXTURE_BINDING_CUBE_MAP => {
                let texture = self
                    .textures
                    .active_texture_slot(constants::TEXTURE_CUBE_MAP)
                    .unwrap()
                    .get();
                return optional_root_object_to_js_or_null!(cx, texture);
            },
            OESVertexArrayObjectConstants::VERTEX_ARRAY_BINDING_OES => {
                let vao = self.current_vao.get().filter(|vao| vao.id().is_some());
                return optional_root_object_to_js_or_null!(cx, vao);
            },
            // In readPixels we currently support RGBA/UBYTE only.  If
            // we wanted to support other formats, we could ask the
            // driver, but we would need to check for
            // GL_OES_read_format support (assuming an underlying GLES
            // driver. Desktop is happy to format convert for us).
            constants::IMPLEMENTATION_COLOR_READ_FORMAT => {
                return Int32Value(constants::RGBA as i32);
            },
            constants::IMPLEMENTATION_COLOR_READ_TYPE => {
                return Int32Value(constants::UNSIGNED_BYTE as i32);
            },
            constants::COMPRESSED_TEXTURE_FORMATS => {
                // FIXME(nox): https://github.com/servo/servo/issues/20594
                rooted!(in(cx) let mut rval = ptr::null_mut::<JSObject>());
                let _ = Uint32Array::create(cx, CreateWith::Slice(&[]), rval.handle_mut()).unwrap();
                return ObjectValue(rval.get());
            },
            constants::VERSION => {
                rooted!(in(cx) let mut rval = UndefinedValue());
                "WebGL 1.0".to_jsval(cx, rval.handle_mut());
                return rval.get();
            },
            constants::RENDERER | constants::VENDOR => {
                rooted!(in(cx) let mut rval = UndefinedValue());
                "Mozilla/Servo".to_jsval(cx, rval.handle_mut());
                return rval.get();
            },
            constants::SHADING_LANGUAGE_VERSION => {
                rooted!(in(cx) let mut rval = UndefinedValue());
                "WebGL GLSL ES 1.0".to_jsval(cx, rval.handle_mut());
                return rval.get();
            },
            constants::UNPACK_FLIP_Y_WEBGL => {
                let unpack = self.texture_unpacking_settings.get();
                return BooleanValue(unpack.contains(TextureUnpacking::FLIP_Y_AXIS));
            },
            constants::UNPACK_PREMULTIPLY_ALPHA_WEBGL => {
                let unpack = self.texture_unpacking_settings.get();
                return BooleanValue(unpack.contains(TextureUnpacking::PREMULTIPLY_ALPHA));
            },
            constants::PACK_ALIGNMENT => {
                return UInt32Value(self.texture_packing_alignment.get() as u32);
            },
            constants::UNPACK_ALIGNMENT => {
                return UInt32Value(self.texture_unpacking_alignment.get());
            },
            constants::UNPACK_COLORSPACE_CONVERSION_WEBGL => {
                let unpack = self.texture_unpacking_settings.get();
                return UInt32Value(if unpack.contains(TextureUnpacking::CONVERT_COLORSPACE) {
                    constants::BROWSER_DEFAULT_WEBGL
                } else {
                    constants::NONE
                });
            },
            _ => {},
        }

        // Handle any MAX_ parameters by retrieving the limits that were stored
        // when this context was created.
        let limit = match parameter {
            constants::MAX_VERTEX_ATTRIBS => Some(self.limits.max_vertex_attribs),
            constants::MAX_TEXTURE_SIZE => Some(self.limits.max_tex_size),
            constants::MAX_CUBE_MAP_TEXTURE_SIZE => Some(self.limits.max_cube_map_tex_size),
            constants::MAX_COMBINED_TEXTURE_IMAGE_UNITS => {
                Some(self.limits.max_combined_texture_image_units)
            },
            constants::MAX_FRAGMENT_UNIFORM_VECTORS => {
                Some(self.limits.max_fragment_uniform_vectors)
            },
            constants::MAX_RENDERBUFFER_SIZE => Some(self.limits.max_renderbuffer_size),
            constants::MAX_TEXTURE_IMAGE_UNITS => Some(self.limits.max_texture_image_units),
            constants::MAX_VARYING_VECTORS => Some(self.limits.max_varying_vectors),
            constants::MAX_VERTEX_TEXTURE_IMAGE_UNITS => {
                Some(self.limits.max_vertex_texture_image_units)
            },
            constants::MAX_VERTEX_UNIFORM_VECTORS => Some(self.limits.max_vertex_uniform_vectors),
            _ => None,
        };
        if let Some(limit) = limit {
            return UInt32Value(limit);
        }

        if let Ok(value) = self.capabilities.is_enabled(parameter) {
            return BooleanValue(value);
        }

        match handle_potential_webgl_error!(
            self,
            Parameter::from_u32(parameter),
            return NullValue()
        ) {
            Parameter::Bool(param) => {
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetParameterBool(param, sender));
                BooleanValue(receiver.recv().unwrap())
            },
            Parameter::Bool4(param) => {
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetParameterBool4(param, sender));
                rooted!(in(cx) let mut rval = UndefinedValue());
                receiver.recv().unwrap().to_jsval(cx, rval.handle_mut());
                rval.get()
            },
            Parameter::Int(param) => {
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetParameterInt(param, sender));
                Int32Value(receiver.recv().unwrap())
            },
            Parameter::Int2(param) => {
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetParameterInt2(param, sender));
                rooted!(in(cx) let mut rval = ptr::null_mut::<JSObject>());
                let _ = Int32Array::create(
                    cx,
                    CreateWith::Slice(&receiver.recv().unwrap()),
                    rval.handle_mut(),
                ).unwrap();
                ObjectValue(rval.get())
            },
            Parameter::Int4(param) => {
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetParameterInt4(param, sender));
                rooted!(in(cx) let mut rval = ptr::null_mut::<JSObject>());
                let _ = Int32Array::create(
                    cx,
                    CreateWith::Slice(&receiver.recv().unwrap()),
                    rval.handle_mut(),
                ).unwrap();
                ObjectValue(rval.get())
            },
            Parameter::Float(param) => {
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetParameterFloat(param, sender));
                DoubleValue(receiver.recv().unwrap() as f64)
            },
            Parameter::Float2(param) => {
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetParameterFloat2(param, sender));
                rooted!(in(cx) let mut rval = ptr::null_mut::<JSObject>());
                let _ = Float32Array::create(
                    cx,
                    CreateWith::Slice(&receiver.recv().unwrap()),
                    rval.handle_mut(),
                ).unwrap();
                ObjectValue(rval.get())
            },
            Parameter::Float4(param) => {
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetParameterFloat4(param, sender));
                rooted!(in(cx) let mut rval = ptr::null_mut::<JSObject>());
                let _ = Float32Array::create(
                    cx,
                    CreateWith::Slice(&receiver.recv().unwrap()),
                    rval.handle_mut(),
                ).unwrap();
                ObjectValue(rval.get())
            },
        }
    }

    #[allow(unsafe_code)]
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    unsafe fn GetTexParameter(&self, _cx: *mut JSContext, target: u32, pname: u32) -> JSVal {
        let texture_slot = handle_potential_webgl_error!(
            self,
            self.textures.active_texture_slot(target),
            return NullValue()
        );
        let texture = handle_potential_webgl_error!(
            self,
            texture_slot.get().ok_or(InvalidOperation),
            return NullValue()
        );

        if !self
            .extension_manager
            .is_get_tex_parameter_name_enabled(pname)
        {
            self.webgl_error(InvalidEnum);
            return NullValue();
        }

        match pname {
            constants::TEXTURE_MAG_FILTER => return UInt32Value(texture.mag_filter()),
            constants::TEXTURE_MIN_FILTER => return UInt32Value(texture.min_filter()),
            _ => {},
        }

        match handle_potential_webgl_error!(self, TexParameter::from_u32(pname), return NullValue())
        {
            TexParameter::Float(param) => {
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetTexParameterFloat(target, param, sender));
                DoubleValue(receiver.recv().unwrap() as f64)
            },
            TexParameter::Int(param) => {
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetTexParameterInt(target, param, sender));
                Int32Value(receiver.recv().unwrap())
            },
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn GetError(&self) -> u32 {
        let error_code = if let Some(error) = self.last_error.get() {
            match error {
                WebGLError::InvalidEnum => constants::INVALID_ENUM,
                WebGLError::InvalidFramebufferOperation => constants::INVALID_FRAMEBUFFER_OPERATION,
                WebGLError::InvalidValue => constants::INVALID_VALUE,
                WebGLError::InvalidOperation => constants::INVALID_OPERATION,
                WebGLError::OutOfMemory => constants::OUT_OF_MEMORY,
                WebGLError::ContextLost => constants::CONTEXT_LOST_WEBGL,
            }
        } else {
            constants::NO_ERROR
        };
        self.last_error.set(None);
        error_code
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.2
    fn GetContextAttributes(&self) -> Option<WebGLContextAttributes> {
        let (sender, receiver) = webgl_channel().unwrap();

        // If the send does not succeed, assume context lost
        let backtrace = capture_webgl_backtrace(self);
        if self
            .webgl_sender
            .send(WebGLCommand::GetContextAttributes(sender), backtrace)
            .is_err()
        {
            return None;
        }

        let attrs = receiver.recv().unwrap();

        Some(WebGLContextAttributes {
            alpha: attrs.alpha,
            antialias: attrs.antialias,
            depth: attrs.depth,
            failIfMajorPerformanceCaveat: false,
            preferLowPowerToHighPerformance: false,
            premultipliedAlpha: attrs.premultiplied_alpha,
            preserveDrawingBuffer: attrs.preserve_drawing_buffer,
            stencil: attrs.stencil,
        })
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.14
    fn GetSupportedExtensions(&self) -> Option<Vec<DOMString>> {
        self.extension_manager
            .init_once(|| self.get_gl_extensions());
        let extensions = self.extension_manager.get_suported_extensions();
        Some(
            extensions
                .iter()
                .map(|name| DOMString::from(*name))
                .collect(),
        )
    }

    #[allow(unsafe_code)]
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.14
    unsafe fn GetExtension(
        &self,
        _cx: *mut JSContext,
        name: DOMString,
    ) -> Option<NonNull<JSObject>> {
        self.extension_manager
            .init_once(|| self.get_gl_extensions());
        self.extension_manager.get_or_init_extension(&name, self)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn ActiveTexture(&self, texture: u32) {
        handle_potential_webgl_error!(self, self.textures.set_active_unit_enum(texture), return);
        self.send_command(WebGLCommand::ActiveTexture(texture));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn BlendColor(&self, r: f32, g: f32, b: f32, a: f32) {
        self.send_command(WebGLCommand::BlendColor(r, g, b, a));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn BlendEquation(&self, mode: u32) {
        handle_potential_webgl_error!(self, self.validate_blend_mode(mode), return);
        self.send_command(WebGLCommand::BlendEquation(mode))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn BlendEquationSeparate(&self, mode_rgb: u32, mode_alpha: u32) {
        handle_potential_webgl_error!(self, self.validate_blend_mode(mode_rgb), return);
        handle_potential_webgl_error!(self, self.validate_blend_mode(mode_alpha), return);
        self.send_command(WebGLCommand::BlendEquationSeparate(mode_rgb, mode_alpha));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn BlendFunc(&self, src_factor: u32, dest_factor: u32) {
        // From the WebGL 1.0 spec, 6.13: Viewport Depth Range:
        //
        //     A call to blendFunc will generate an INVALID_OPERATION error if one of the two
        //     factors is set to CONSTANT_COLOR or ONE_MINUS_CONSTANT_COLOR and the other to
        //     CONSTANT_ALPHA or ONE_MINUS_CONSTANT_ALPHA.
        if has_invalid_blend_constants(src_factor, dest_factor) {
            return self.webgl_error(InvalidOperation);
        }
        if has_invalid_blend_constants(dest_factor, src_factor) {
            return self.webgl_error(InvalidOperation);
        }

        self.send_command(WebGLCommand::BlendFunc(src_factor, dest_factor));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn BlendFuncSeparate(&self, src_rgb: u32, dest_rgb: u32, src_alpha: u32, dest_alpha: u32) {
        // From the WebGL 1.0 spec, 6.13: Viewport Depth Range:
        //
        //     A call to blendFuncSeparate will generate an INVALID_OPERATION error if srcRGB is
        //     set to CONSTANT_COLOR or ONE_MINUS_CONSTANT_COLOR and dstRGB is set to
        //     CONSTANT_ALPHA or ONE_MINUS_CONSTANT_ALPHA or vice versa.
        if has_invalid_blend_constants(src_rgb, dest_rgb) {
            return self.webgl_error(InvalidOperation);
        }
        if has_invalid_blend_constants(dest_rgb, src_rgb) {
            return self.webgl_error(InvalidOperation);
        }

        self.send_command(WebGLCommand::BlendFuncSeparate(
            src_rgb, dest_rgb, src_alpha, dest_alpha,
        ));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn AttachShader(&self, program: &WebGLProgram, shader: &WebGLShader) {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return);
        handle_potential_webgl_error!(self, self.validate_ownership(shader), return);
        handle_potential_webgl_error!(self, program.attach_shader(shader));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn DetachShader(&self, program: &WebGLProgram, shader: &WebGLShader) {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return);
        handle_potential_webgl_error!(self, self.validate_ownership(shader), return);
        handle_potential_webgl_error!(self, program.detach_shader(shader));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn BindAttribLocation(&self, program: &WebGLProgram, index: u32, name: DOMString) {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return);
        handle_potential_webgl_error!(self, program.bind_attrib_location(index, name));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.5
    fn BindBuffer(&self, target: u32, buffer: Option<&WebGLBuffer>) {
        if let Some(buffer) = buffer {
            handle_potential_webgl_error!(self, self.validate_ownership(buffer), return);
        }

        let current_vao;
        let slot = match target {
            constants::ARRAY_BUFFER => &self.bound_buffer_array,
            constants::ELEMENT_ARRAY_BUFFER => {
                current_vao = self.current_vao();
                current_vao.element_array_buffer()
            },
            _ => return self.webgl_error(InvalidEnum),
        };

        if let Some(buffer) = buffer {
            if buffer.is_marked_for_deletion() {
                return self.webgl_error(InvalidOperation);
            }
            handle_potential_webgl_error!(self, buffer.set_target(target), return);
            buffer.increment_attached_counter();
        }
        self.send_command(WebGLCommand::BindBuffer(target, buffer.map(|b| b.id())));
        if let Some(old) = slot.get() {
            old.decrement_attached_counter();
        }
        slot.set(buffer);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.6
    fn BindFramebuffer(&self, target: u32, framebuffer: Option<&WebGLFramebuffer>) {
        if let Some(fb) = framebuffer {
            handle_potential_webgl_error!(self, self.validate_ownership(fb), return);
        }

        if target != constants::FRAMEBUFFER {
            return self.webgl_error(InvalidEnum);
        }

        if let Some(framebuffer) = framebuffer {
            if framebuffer.is_deleted() {
                // From the WebGL spec:
                //
                //     "An attempt to bind a deleted framebuffer will
                //      generate an INVALID_OPERATION error, and the
                //      current binding will remain untouched."
                return self.webgl_error(InvalidOperation);
            } else {
                framebuffer.bind(target);
                self.bound_framebuffer.set(Some(framebuffer));
            }
        } else {
            // Bind the default framebuffer
            let cmd =
                WebGLCommand::BindFramebuffer(target, WebGLFramebufferBindingRequest::Default);
            self.send_command(cmd);
            self.bound_framebuffer.set(framebuffer);
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.7
    fn BindRenderbuffer(&self, target: u32, renderbuffer: Option<&WebGLRenderbuffer>) {
        if let Some(rb) = renderbuffer {
            handle_potential_webgl_error!(self, self.validate_ownership(rb), return);
        }

        if target != constants::RENDERBUFFER {
            return self.webgl_error(InvalidEnum);
        }

        match renderbuffer {
            // Implementations differ on what to do in the deleted
            // case: Chromium currently unbinds, and Gecko silently
            // returns.  The conformance tests don't cover this case.
            Some(renderbuffer) if !renderbuffer.is_deleted() => {
                self.bound_renderbuffer.set(Some(renderbuffer));
                renderbuffer.bind(target);
            },
            _ => {
                if renderbuffer.is_some() {
                    self.webgl_error(InvalidOperation);
                }

                self.bound_renderbuffer.set(None);
                // Unbind the currently bound renderbuffer
                self.send_command(WebGLCommand::BindRenderbuffer(target, None));
            },
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn BindTexture(&self, target: u32, texture: Option<&WebGLTexture>) {
        if let Some(texture) = texture {
            handle_potential_webgl_error!(self, self.validate_ownership(texture), return);
        }

        let texture_slot =
            handle_potential_webgl_error!(self, self.textures.active_texture_slot(target), return);

        if let Some(texture) = texture {
            handle_potential_webgl_error!(self, texture.bind(target), return);
        } else {
            self.send_command(WebGLCommand::BindTexture(target, None));
        }
        texture_slot.set(texture);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn GenerateMipmap(&self, target: u32) {
        let texture_slot =
            handle_potential_webgl_error!(self, self.textures.active_texture_slot(target), return);
        let texture =
            handle_potential_webgl_error!(self, texture_slot.get().ok_or(InvalidOperation), return);
        handle_potential_webgl_error!(self, texture.generate_mipmap());
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.5
    #[allow(unsafe_code)]
    fn BufferData(&self, target: u32, data: Option<ArrayBufferViewOrArrayBuffer>, usage: u32) {
        let data = handle_potential_webgl_error!(self, data.ok_or(InvalidValue), return);

        let bound_buffer = handle_potential_webgl_error!(self, self.bound_buffer(target), return);
        let bound_buffer =
            handle_potential_webgl_error!(self, bound_buffer.ok_or(InvalidOperation), return);

        let data = unsafe {
            // Safe because we don't do anything with JS until the end of the method.
            match data {
                ArrayBufferViewOrArrayBuffer::ArrayBuffer(ref data) => data.as_slice(),
                ArrayBufferViewOrArrayBuffer::ArrayBufferView(ref data) => data.as_slice(),
            }
        };
        handle_potential_webgl_error!(self, bound_buffer.buffer_data(data, usage));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.5
    fn BufferData_(&self, target: u32, size: i64, usage: u32) {
        let bound_buffer = handle_potential_webgl_error!(self, self.bound_buffer(target), return);
        let bound_buffer =
            handle_potential_webgl_error!(self, bound_buffer.ok_or(InvalidOperation), return);

        if size < 0 {
            return self.webgl_error(InvalidValue);
        }

        // FIXME: Allocating a buffer based on user-requested size is
        // not great, but we don't have a fallible allocation to try.
        let data = vec![0u8; size as usize];
        handle_potential_webgl_error!(self, bound_buffer.buffer_data(&data, usage));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.5
    #[allow(unsafe_code)]
    fn BufferSubData(&self, target: u32, offset: i64, data: ArrayBufferViewOrArrayBuffer) {
        let bound_buffer = handle_potential_webgl_error!(self, self.bound_buffer(target), return);
        let bound_buffer =
            handle_potential_webgl_error!(self, bound_buffer.ok_or(InvalidOperation), return);

        if offset < 0 {
            return self.webgl_error(InvalidValue);
        }

        let data = unsafe {
            // Safe because we don't do anything with JS until the end of the method.
            match data {
                ArrayBufferViewOrArrayBuffer::ArrayBuffer(ref data) => data.as_slice(),
                ArrayBufferViewOrArrayBuffer::ArrayBufferView(ref data) => data.as_slice(),
            }
        };
        if (offset as u64) + data.len() as u64 > bound_buffer.capacity() as u64 {
            return self.webgl_error(InvalidValue);
        }
        let (sender, receiver) = ipc::bytes_channel().unwrap();
        self.send_command(WebGLCommand::BufferSubData(
            target,
            offset as isize,
            receiver,
        ));
        sender.send(data).unwrap();
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn CompressedTexImage2D(
        &self,
        _target: u32,
        _level: i32,
        _internal_format: u32,
        _width: i32,
        _height: i32,
        _border: i32,
        _data: CustomAutoRooterGuard<ArrayBufferView>,
    ) {
        // FIXME: No compressed texture format is currently supported, so error out as per
        // https://www.khronos.org/registry/webgl/specs/latest/1.0/#COMPRESSED_TEXTURE_SUPPORT
        self.webgl_error(InvalidEnum);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn CompressedTexSubImage2D(
        &self,
        _target: u32,
        _level: i32,
        _xoffset: i32,
        _yoffset: i32,
        _width: i32,
        _height: i32,
        _format: u32,
        _data: CustomAutoRooterGuard<ArrayBufferView>,
    ) {
        // FIXME: No compressed texture format is currently supported, so error out as per
        // https://www.khronos.org/registry/webgl/specs/latest/1.0/#COMPRESSED_TEXTURE_SUPPORT
        self.webgl_error(InvalidEnum);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn CopyTexImage2D(
        &self,
        target: u32,
        level: i32,
        internal_format: u32,
        x: i32,
        y: i32,
        width: i32,
        height: i32,
        border: i32,
    ) {
        handle_potential_webgl_error!(self, self.validate_framebuffer(), return);

        let validator = CommonTexImage2DValidator::new(
            self,
            target,
            level,
            internal_format,
            width,
            height,
            border,
        );
        let CommonTexImage2DValidatorResult {
            texture,
            target,
            level,
            internal_format,
            width,
            height,
            border,
        } = match validator.validate() {
            Ok(result) => result,
            Err(_) => return,
        };

        // NB: TexImage2D depth is always equal to 1
        handle_potential_webgl_error!(
            self,
            texture.initialize(
                target,
                width as u32,
                height as u32,
                1,
                internal_format,
                level as u32,
                None
            )
        );

        let msg = WebGLCommand::CopyTexImage2D(
            target.as_gl_constant(),
            level as i32,
            internal_format.as_gl_constant(),
            x,
            y,
            width as i32,
            height as i32,
            border as i32,
        );

        self.send_command(msg);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn CopyTexSubImage2D(
        &self,
        target: u32,
        level: i32,
        xoffset: i32,
        yoffset: i32,
        x: i32,
        y: i32,
        width: i32,
        height: i32,
    ) {
        handle_potential_webgl_error!(self, self.validate_framebuffer(), return);

        // NB: We use a dummy (valid) format and border in order to reuse the
        // common validations, but this should have its own validator.
        let validator = CommonTexImage2DValidator::new(
            self,
            target,
            level,
            TexFormat::RGBA.as_gl_constant(),
            width,
            height,
            0,
        );
        let CommonTexImage2DValidatorResult {
            texture,
            target,
            level,
            width,
            height,
            ..
        } = match validator.validate() {
            Ok(result) => result,
            Err(_) => return,
        };

        let image_info = texture.image_info_for_target(&target, level);

        // GL_INVALID_VALUE is generated if:
        //   - xoffset or yoffset is less than 0
        //   - x offset plus the width is greater than the texture width
        //   - y offset plus the height is greater than the texture height
        if xoffset < 0 ||
            (xoffset as u32 + width) > image_info.width() ||
            yoffset < 0 ||
            (yoffset as u32 + height) > image_info.height()
        {
            self.webgl_error(InvalidValue);
            return;
        }

        let msg = WebGLCommand::CopyTexSubImage2D(
            target.as_gl_constant(),
            level as i32,
            xoffset,
            yoffset,
            x,
            y,
            width as i32,
            height as i32,
        );

        self.send_command(msg);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.11
    fn Clear(&self, mask: u32) {
        handle_potential_webgl_error!(self, self.validate_framebuffer(), return);
        if mask & !(constants::DEPTH_BUFFER_BIT |
            constants::STENCIL_BUFFER_BIT |
            constants::COLOR_BUFFER_BIT) !=
            0
        {
            return self.webgl_error(InvalidValue);
        }

        self.send_command(WebGLCommand::Clear(mask));
        self.mark_as_dirty();
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn ClearColor(&self, red: f32, green: f32, blue: f32, alpha: f32) {
        self.current_clear_color.set((red, green, blue, alpha));
        self.send_command(WebGLCommand::ClearColor(red, green, blue, alpha));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn ClearDepth(&self, depth: f32) {
        self.send_command(WebGLCommand::ClearDepth(depth))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn ClearStencil(&self, stencil: i32) {
        self.send_command(WebGLCommand::ClearStencil(stencil))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn ColorMask(&self, r: bool, g: bool, b: bool, a: bool) {
        self.send_command(WebGLCommand::ColorMask(r, g, b, a))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn CullFace(&self, mode: u32) {
        match mode {
            constants::FRONT | constants::BACK | constants::FRONT_AND_BACK => {
                self.send_command(WebGLCommand::CullFace(mode))
            },
            _ => self.webgl_error(InvalidEnum),
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn FrontFace(&self, mode: u32) {
        match mode {
            constants::CW | constants::CCW => self.send_command(WebGLCommand::FrontFace(mode)),
            _ => self.webgl_error(InvalidEnum),
        }
    }
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn DepthFunc(&self, func: u32) {
        match func {
            constants::NEVER |
            constants::LESS |
            constants::EQUAL |
            constants::LEQUAL |
            constants::GREATER |
            constants::NOTEQUAL |
            constants::GEQUAL |
            constants::ALWAYS => self.send_command(WebGLCommand::DepthFunc(func)),
            _ => self.webgl_error(InvalidEnum),
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn DepthMask(&self, flag: bool) {
        self.send_command(WebGLCommand::DepthMask(flag))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn DepthRange(&self, near: f32, far: f32) {
        // https://www.khronos.org/registry/webgl/specs/latest/1.0/#VIEWPORT_DEPTH_RANGE
        if near > far {
            return self.webgl_error(InvalidOperation);
        }
        self.send_command(WebGLCommand::DepthRange(near, far))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn Enable(&self, cap: u32) {
        if handle_potential_webgl_error!(self, self.capabilities.set(cap, true), return) {
            self.send_command(WebGLCommand::Enable(cap));
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn Disable(&self, cap: u32) {
        if handle_potential_webgl_error!(self, self.capabilities.set(cap, false), return) {
            self.send_command(WebGLCommand::Disable(cap));
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn CompileShader(&self, shader: &WebGLShader) {
        handle_potential_webgl_error!(self, self.validate_ownership(shader), return);
        handle_potential_webgl_error!(
            self,
            shader.compile(
                self.webgl_version,
                self.glsl_version,
                &self.limits,
                &self.extension_manager,
            )
        )
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.5
    fn CreateBuffer(&self) -> Option<DomRoot<WebGLBuffer>> {
        WebGLBuffer::maybe_new(self)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.6
    fn CreateFramebuffer(&self) -> Option<DomRoot<WebGLFramebuffer>> {
        WebGLFramebuffer::maybe_new(self)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.7
    fn CreateRenderbuffer(&self) -> Option<DomRoot<WebGLRenderbuffer>> {
        WebGLRenderbuffer::maybe_new(self)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn CreateTexture(&self) -> Option<DomRoot<WebGLTexture>> {
        WebGLTexture::maybe_new(self)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn CreateProgram(&self) -> Option<DomRoot<WebGLProgram>> {
        WebGLProgram::maybe_new(self)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn CreateShader(&self, shader_type: u32) -> Option<DomRoot<WebGLShader>> {
        match shader_type {
            constants::VERTEX_SHADER | constants::FRAGMENT_SHADER => {},
            _ => {
                self.webgl_error(InvalidEnum);
                return None;
            },
        }
        WebGLShader::maybe_new(self, shader_type)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.5
    fn DeleteBuffer(&self, buffer: Option<&WebGLBuffer>) {
        let buffer = match buffer {
            Some(buffer) => buffer,
            None => return,
        };
        handle_potential_webgl_error!(self, self.validate_ownership(buffer), return);
        if buffer.is_marked_for_deletion() {
            return;
        }
        self.current_vao().unbind_buffer(buffer);
        if self
            .bound_buffer_array
            .get()
            .map_or(false, |b| buffer == &*b)
        {
            self.bound_buffer_array.set(None);
            buffer.decrement_attached_counter();
        }
        buffer.mark_for_deletion();
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.6
    fn DeleteFramebuffer(&self, framebuffer: Option<&WebGLFramebuffer>) {
        if let Some(framebuffer) = framebuffer {
            handle_potential_webgl_error!(self, self.validate_ownership(framebuffer), return);
            handle_object_deletion!(
                self,
                self.bound_framebuffer,
                framebuffer,
                Some(WebGLCommand::BindFramebuffer(
                    constants::FRAMEBUFFER,
                    WebGLFramebufferBindingRequest::Default
                ))
            );
            framebuffer.delete()
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.7
    fn DeleteRenderbuffer(&self, renderbuffer: Option<&WebGLRenderbuffer>) {
        if let Some(renderbuffer) = renderbuffer {
            handle_potential_webgl_error!(self, self.validate_ownership(renderbuffer), return);
            handle_object_deletion!(
                self,
                self.bound_renderbuffer,
                renderbuffer,
                Some(WebGLCommand::BindRenderbuffer(
                    constants::RENDERBUFFER,
                    None
                ))
            );
            renderbuffer.delete()
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn DeleteTexture(&self, texture: Option<&WebGLTexture>) {
        if let Some(texture) = texture {
            handle_potential_webgl_error!(self, self.validate_ownership(texture), return);

            // From the GLES 2.0.25 spec, page 85:
            //
            //     "If a texture that is currently bound to one of the targets
            //      TEXTURE_2D, or TEXTURE_CUBE_MAP is deleted, it is as though
            //      BindTexture had been executed with the same target and texture
            //      zero."
            //
            // The same texture may be bound to multiple texture units.
            let mut active_unit_enum = self.textures.active_unit_enum();
            for (unit_enum, slot) in self.textures.iter() {
                if let Some(target) = slot.unbind(texture) {
                    if unit_enum != active_unit_enum {
                        self.send_command(WebGLCommand::ActiveTexture(unit_enum));
                        active_unit_enum = unit_enum;
                    }
                    self.send_command(WebGLCommand::BindTexture(target, None));
                }
            }

            // Restore bound texture unit if it has been changed.
            if active_unit_enum != self.textures.active_unit_enum() {
                self.send_command(WebGLCommand::ActiveTexture(
                    self.textures.active_unit_enum(),
                ));
            }

            texture.delete()
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn DeleteProgram(&self, program: Option<&WebGLProgram>) {
        if let Some(program) = program {
            handle_potential_webgl_error!(self, self.validate_ownership(program), return);
            program.mark_for_deletion()
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn DeleteShader(&self, shader: Option<&WebGLShader>) {
        if let Some(shader) = shader {
            handle_potential_webgl_error!(self, self.validate_ownership(shader), return);
            shader.mark_for_deletion()
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.11
    fn DrawArrays(&self, mode: u32, first: i32, count: i32) {
        handle_potential_webgl_error!(self, self.draw_arrays_instanced(mode, first, count, 1));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.11
    fn DrawElements(&self, mode: u32, count: i32, type_: u32, offset: i64) {
        handle_potential_webgl_error!(
            self,
            self.draw_elements_instanced(mode, count, type_, offset, 1)
        );
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn EnableVertexAttribArray(&self, attrib_id: u32) {
        if attrib_id >= self.limits.max_vertex_attribs {
            return self.webgl_error(InvalidValue);
        }

        self.current_vao()
            .enabled_vertex_attrib_array(attrib_id, true);
        self.send_command(WebGLCommand::EnableVertexAttribArray(attrib_id));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn DisableVertexAttribArray(&self, attrib_id: u32) {
        if attrib_id >= self.limits.max_vertex_attribs {
            return self.webgl_error(InvalidValue);
        }

        self.current_vao()
            .enabled_vertex_attrib_array(attrib_id, false);
        self.send_command(WebGLCommand::DisableVertexAttribArray(attrib_id));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn GetActiveUniform(
        &self,
        program: &WebGLProgram,
        index: u32,
    ) -> Option<DomRoot<WebGLActiveInfo>> {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return None);
        match program.get_active_uniform(index) {
            Ok(ret) => Some(ret),
            Err(e) => {
                self.webgl_error(e);
                return None;
            },
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn GetActiveAttrib(
        &self,
        program: &WebGLProgram,
        index: u32,
    ) -> Option<DomRoot<WebGLActiveInfo>> {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return None);
        handle_potential_webgl_error!(self, program.get_active_attrib(index).map(Some), None)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn GetAttribLocation(&self, program: &WebGLProgram, name: DOMString) -> i32 {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return -1);
        handle_potential_webgl_error!(self, program.get_attrib_location(name), -1)
    }

    #[allow(unsafe_code)]
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.6
    unsafe fn GetFramebufferAttachmentParameter(
        &self,
        cx: *mut JSContext,
        target: u32,
        attachment: u32,
        pname: u32,
    ) -> JSVal {
        // Check if currently bound framebuffer is non-zero as per spec.
        if self.bound_framebuffer.get().is_none() {
            self.webgl_error(InvalidOperation);
            return NullValue();
        }

        // Note: commented out stuff is for the WebGL2 standard.
        let target_matches = match target {
            // constants::READ_FRAMEBUFFER |
            // constants::DRAW_FRAMEBUFFER => true,
            constants::FRAMEBUFFER => true,
            _ => false,
        };
        let attachment_matches = match attachment {
            // constants::MAX_COLOR_ATTACHMENTS ... gl::COLOR_ATTACHMENT0 |
            // constants::BACK |
            constants::COLOR_ATTACHMENT0 |
            constants::DEPTH_STENCIL_ATTACHMENT |
            constants::DEPTH_ATTACHMENT |
            constants::STENCIL_ATTACHMENT => true,
            _ => false,
        };
        let pname_matches = match pname {
            // constants::FRAMEBUFFER_ATTACHMENT_ALPHA_SIZE |
            // constants::FRAMEBUFFER_ATTACHMENT_BLUE_SIZE |
            // constants::FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING |
            // constants::FRAMEBUFFER_ATTACHMENT_COMPONENT_TYPE |
            // constants::FRAMEBUFFER_ATTACHMENT_DEPTH_SIZE |
            // constants::FRAMEBUFFER_ATTACHMENT_GREEN_SIZE |
            // constants::FRAMEBUFFER_ATTACHMENT_RED_SIZE |
            // constants::FRAMEBUFFER_ATTACHMENT_STENCIL_SIZE |
            // constants::FRAMEBUFFER_ATTACHMENT_TEXTURE_LAYER |
            constants::FRAMEBUFFER_ATTACHMENT_OBJECT_NAME |
            constants::FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE |
            constants::FRAMEBUFFER_ATTACHMENT_TEXTURE_CUBE_MAP_FACE |
            constants::FRAMEBUFFER_ATTACHMENT_TEXTURE_LEVEL => true,
            _ => false,
        };

        let bound_attachment_matches =
            match self.bound_framebuffer.get().unwrap().attachment(attachment) {
                Some(attachment_root) => match attachment_root {
                    WebGLFramebufferAttachmentRoot::Renderbuffer(_) => match pname {
                        constants::FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE |
                        constants::FRAMEBUFFER_ATTACHMENT_OBJECT_NAME => true,
                        _ => false,
                    },
                    WebGLFramebufferAttachmentRoot::Texture(_) => match pname {
                        constants::FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE |
                        constants::FRAMEBUFFER_ATTACHMENT_OBJECT_NAME |
                        constants::FRAMEBUFFER_ATTACHMENT_TEXTURE_LEVEL |
                        constants::FRAMEBUFFER_ATTACHMENT_TEXTURE_CUBE_MAP_FACE => true,
                        _ => false,
                    },
                },
                _ => match pname {
                    constants::FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE => true,
                    _ => false,
                },
            };

        if !target_matches || !attachment_matches || !pname_matches || !bound_attachment_matches {
            self.webgl_error(InvalidEnum);
            return NullValue();
        }

        // From the GLES2 spec:
        //
        //     If the value of FRAMEBUFFER_ATTACHMENT_OBJECT_TYPE is NONE,
        //     then querying any other pname will generate INVALID_ENUM.
        //
        // otherwise, return `WebGLRenderbuffer` or `WebGLTexture` dom object
        if pname == constants::FRAMEBUFFER_ATTACHMENT_OBJECT_NAME {
            // if fb is None, an INVALID_OPERATION is returned
            // at the beggining of the function, so `.unwrap()` will never panic
            let fb = self.bound_framebuffer.get().unwrap();
            if let Some(webgl_attachment) = fb.attachment(attachment) {
                match webgl_attachment {
                    WebGLFramebufferAttachmentRoot::Renderbuffer(rb) => {
                        rooted!(in(cx) let mut rval = NullValue());
                        rb.to_jsval(cx, rval.handle_mut());
                        return rval.get();
                    },
                    WebGLFramebufferAttachmentRoot::Texture(texture) => {
                        rooted!(in(cx) let mut rval = NullValue());
                        texture.to_jsval(cx, rval.handle_mut());
                        return rval.get();
                    },
                }
            }
            self.webgl_error(InvalidEnum);
            return NullValue();
        }

        let (sender, receiver) = webgl_channel().unwrap();
        self.send_command(WebGLCommand::GetFramebufferAttachmentParameter(
            target, attachment, pname, sender,
        ));

        Int32Value(receiver.recv().unwrap())
    }

    #[allow(unsafe_code)]
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.7
    unsafe fn GetRenderbufferParameter(
        &self,
        _cx: *mut JSContext,
        target: u32,
        pname: u32,
    ) -> JSVal {
        let target_matches = target == constants::RENDERBUFFER;

        let pname_matches = match pname {
            constants::RENDERBUFFER_WIDTH |
            constants::RENDERBUFFER_HEIGHT |
            constants::RENDERBUFFER_INTERNAL_FORMAT |
            constants::RENDERBUFFER_RED_SIZE |
            constants::RENDERBUFFER_GREEN_SIZE |
            constants::RENDERBUFFER_BLUE_SIZE |
            constants::RENDERBUFFER_ALPHA_SIZE |
            constants::RENDERBUFFER_DEPTH_SIZE |
            constants::RENDERBUFFER_STENCIL_SIZE => true,
            _ => false,
        };

        if !target_matches || !pname_matches {
            self.webgl_error(InvalidEnum);
            return NullValue();
        }

        if self.bound_renderbuffer.get().is_none() {
            self.webgl_error(InvalidOperation);
            return NullValue();
        }

        let result = if pname == constants::RENDERBUFFER_INTERNAL_FORMAT {
            let rb = self.bound_renderbuffer.get().unwrap();
            rb.internal_format() as i32
        } else {
            let (sender, receiver) = webgl_channel().unwrap();
            self.send_command(WebGLCommand::GetRenderbufferParameter(
                target, pname, sender,
            ));
            receiver.recv().unwrap()
        };

        Int32Value(result)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn GetProgramInfoLog(&self, program: &WebGLProgram) -> Option<DOMString> {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return None);
        match program.get_info_log() {
            Ok(value) => Some(DOMString::from(value)),
            Err(e) => {
                self.webgl_error(e);
                None
            },
        }
    }

    #[allow(unsafe_code)]
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    unsafe fn GetProgramParameter(
        &self,
        _: *mut JSContext,
        program: &WebGLProgram,
        param: u32,
    ) -> JSVal {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return NullValue());
        if program.is_deleted() {
            self.webgl_error(InvalidOperation);
            return NullValue();
        }
        match param {
            constants::DELETE_STATUS => BooleanValue(program.is_marked_for_deletion()),
            constants::LINK_STATUS => BooleanValue(program.is_linked()),
            constants::VALIDATE_STATUS => {
                // FIXME(nox): This could be cached on the DOM side when we call validateProgram
                // but I'm not sure when the value should be reset.
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetProgramValidateStatus(program.id(), sender));
                BooleanValue(receiver.recv().unwrap())
            },
            constants::ATTACHED_SHADERS => {
                // FIXME(nox): This allocates a vector and roots a couple of shaders for nothing.
                Int32Value(
                    program
                        .attached_shaders()
                        .map(|shaders| shaders.len() as i32)
                        .unwrap_or(0),
                )
            },
            constants::ACTIVE_ATTRIBUTES => Int32Value(program.active_attribs().len() as i32),
            constants::ACTIVE_UNIFORMS => Int32Value(program.active_uniforms().len() as i32),
            _ => {
                self.webgl_error(InvalidEnum);
                NullValue()
            },
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn GetShaderInfoLog(&self, shader: &WebGLShader) -> Option<DOMString> {
        handle_potential_webgl_error!(self, self.validate_ownership(shader), return None);
        Some(shader.info_log())
    }

    #[allow(unsafe_code)]
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    unsafe fn GetShaderParameter(
        &self,
        _: *mut JSContext,
        shader: &WebGLShader,
        param: u32,
    ) -> JSVal {
        handle_potential_webgl_error!(self, self.validate_ownership(shader), return NullValue());
        if shader.is_deleted() {
            self.webgl_error(InvalidValue);
            return NullValue();
        }
        match param {
            constants::DELETE_STATUS => BooleanValue(shader.is_marked_for_deletion()),
            constants::COMPILE_STATUS => BooleanValue(shader.successfully_compiled()),
            constants::SHADER_TYPE => UInt32Value(shader.gl_type()),
            _ => {
                self.webgl_error(InvalidEnum);
                NullValue()
            },
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn GetShaderPrecisionFormat(
        &self,
        shader_type: u32,
        precision_type: u32,
    ) -> Option<DomRoot<WebGLShaderPrecisionFormat>> {
        match precision_type {
            constants::LOW_FLOAT |
            constants::MEDIUM_FLOAT |
            constants::HIGH_FLOAT |
            constants::LOW_INT |
            constants::MEDIUM_INT |
            constants::HIGH_INT => (),
            _ => {
                self.webgl_error(InvalidEnum);
                return None;
            },
        }

        let (sender, receiver) = webgl_channel().unwrap();
        self.send_command(WebGLCommand::GetShaderPrecisionFormat(
            shader_type,
            precision_type,
            sender,
        ));

        let (range_min, range_max, precision) = receiver.recv().unwrap();
        Some(WebGLShaderPrecisionFormat::new(
            self.global().as_window(),
            range_min,
            range_max,
            precision,
        ))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn GetUniformLocation(
        &self,
        program: &WebGLProgram,
        name: DOMString,
    ) -> Option<DomRoot<WebGLUniformLocation>> {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return None);
        handle_potential_webgl_error!(self, program.get_uniform_location(name), None)
    }

    #[allow(unsafe_code)]
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    unsafe fn GetVertexAttrib(&self, cx: *mut JSContext, index: u32, param: u32) -> JSVal {
        let current_vao = self.current_vao();
        let data = handle_potential_webgl_error!(
            self,
            current_vao.get_vertex_attrib(index).ok_or(InvalidValue),
            return NullValue()
        );
        if param == constants::CURRENT_VERTEX_ATTRIB {
            let value = if index == 0 {
                let (x, y, z, w) = self.current_vertex_attrib_0.get();
                [x, y, z, w]
            } else {
                let (sender, receiver) = webgl_channel().unwrap();
                self.send_command(WebGLCommand::GetCurrentVertexAttrib(index, sender));
                receiver.recv().unwrap()
            };
            rooted!(in(cx) let mut result = ptr::null_mut::<JSObject>());
            let _ =
                Float32Array::create(cx, CreateWith::Slice(&value), result.handle_mut()).unwrap();
            return ObjectValue(result.get());
        }

        if !self
            .extension_manager
            .is_get_vertex_attrib_name_enabled(param)
        {
            self.webgl_error(WebGLError::InvalidEnum);
            return NullValue();
        }

        match param {
            constants::VERTEX_ATTRIB_ARRAY_ENABLED => BooleanValue(data.enabled_as_array),
            constants::VERTEX_ATTRIB_ARRAY_SIZE => Int32Value(data.size as i32),
            constants::VERTEX_ATTRIB_ARRAY_TYPE => Int32Value(data.type_ as i32),
            constants::VERTEX_ATTRIB_ARRAY_NORMALIZED => BooleanValue(data.normalized),
            constants::VERTEX_ATTRIB_ARRAY_STRIDE => Int32Value(data.stride as i32),
            constants::VERTEX_ATTRIB_ARRAY_BUFFER_BINDING => {
                rooted!(in(cx) let mut jsval = NullValue());
                if let Some(buffer) = data.buffer() {
                    buffer.to_jsval(cx, jsval.handle_mut());
                }
                jsval.get()
            },
            ANGLEInstancedArraysConstants::VERTEX_ATTRIB_ARRAY_DIVISOR_ANGLE => {
                UInt32Value(data.divisor)
            },
            _ => {
                self.webgl_error(InvalidEnum);
                NullValue()
            },
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn GetVertexAttribOffset(&self, index: u32, pname: u32) -> i64 {
        if pname != constants::VERTEX_ATTRIB_ARRAY_POINTER {
            self.webgl_error(InvalidEnum);
            return 0;
        }
        let vao = self.current_vao();
        let data = handle_potential_webgl_error!(
            self,
            vao.get_vertex_attrib(index).ok_or(InvalidValue),
            return 0
        );
        data.offset as i64
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn Hint(&self, target: u32, mode: u32) {
        if target != constants::GENERATE_MIPMAP_HINT &&
            !self.extension_manager.is_hint_target_enabled(target)
        {
            return self.webgl_error(InvalidEnum);
        }

        match mode {
            constants::FASTEST | constants::NICEST | constants::DONT_CARE => (),

            _ => return self.webgl_error(InvalidEnum),
        }

        self.send_command(WebGLCommand::Hint(target, mode));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.5
    fn IsBuffer(&self, buffer: Option<&WebGLBuffer>) -> bool {
        buffer.map_or(false, |buf| {
            self.validate_ownership(buf).is_ok() && buf.target().is_some() && !buf.is_deleted()
        })
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn IsEnabled(&self, cap: u32) -> bool {
        handle_potential_webgl_error!(self, self.capabilities.is_enabled(cap), false)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.6
    fn IsFramebuffer(&self, frame_buffer: Option<&WebGLFramebuffer>) -> bool {
        frame_buffer.map_or(false, |buf| {
            self.validate_ownership(buf).is_ok() && buf.target().is_some() && !buf.is_deleted()
        })
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn IsProgram(&self, program: Option<&WebGLProgram>) -> bool {
        program.map_or(false, |p| {
            self.validate_ownership(p).is_ok() && !p.is_deleted()
        })
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.7
    fn IsRenderbuffer(&self, render_buffer: Option<&WebGLRenderbuffer>) -> bool {
        render_buffer.map_or(false, |buf| {
            self.validate_ownership(buf).is_ok() && buf.ever_bound() && !buf.is_deleted()
        })
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn IsShader(&self, shader: Option<&WebGLShader>) -> bool {
        shader.map_or(false, |s| {
            self.validate_ownership(s).is_ok() && !s.is_deleted()
        })
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn IsTexture(&self, texture: Option<&WebGLTexture>) -> bool {
        texture.map_or(false, |tex| {
            self.validate_ownership(tex).is_ok() && tex.target().is_some() && !tex.is_deleted()
        })
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn LineWidth(&self, width: f32) {
        if width.is_nan() || width <= 0f32 {
            return self.webgl_error(InvalidValue);
        }

        self.send_command(WebGLCommand::LineWidth(width))
    }

    // NOTE: Usage of this function could affect rendering while we keep using
    //   readback to render to the page.
    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn PixelStorei(&self, param_name: u32, param_value: i32) {
        let mut texture_settings = self.texture_unpacking_settings.get();
        match param_name {
            constants::UNPACK_FLIP_Y_WEBGL => {
                texture_settings.set(TextureUnpacking::FLIP_Y_AXIS, param_value != 0);
            },
            constants::UNPACK_PREMULTIPLY_ALPHA_WEBGL => {
                texture_settings.set(TextureUnpacking::PREMULTIPLY_ALPHA, param_value != 0);
            },
            constants::UNPACK_COLORSPACE_CONVERSION_WEBGL => {
                let convert = match param_value as u32 {
                    constants::BROWSER_DEFAULT_WEBGL => true,
                    constants::NONE => false,
                    _ => return self.webgl_error(InvalidEnum),
                };
                texture_settings.set(TextureUnpacking::CONVERT_COLORSPACE, convert);
            },
            constants::UNPACK_ALIGNMENT => {
                match param_value {
                    1 | 2 | 4 | 8 => (),
                    _ => return self.webgl_error(InvalidValue),
                }
                self.texture_unpacking_alignment.set(param_value as u32);
                return;
            },
            constants::PACK_ALIGNMENT => {
                match param_value {
                    1 | 2 | 4 | 8 => (),
                    _ => return self.webgl_error(InvalidValue),
                }
                // We never actually change the actual value on the GL side
                // because it's better to receive the pixels without the padding
                // and then write the result at the right place in ReadPixels.
                self.texture_packing_alignment.set(param_value as u8);
                return;
            },
            _ => return self.webgl_error(InvalidEnum),
        }
        self.texture_unpacking_settings.set(texture_settings);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn PolygonOffset(&self, factor: f32, units: f32) {
        self.send_command(WebGLCommand::PolygonOffset(factor, units))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.12
    #[allow(unsafe_code)]
    fn ReadPixels(
        &self,
        x: i32,
        y: i32,
        width: i32,
        height: i32,
        format: u32,
        pixel_type: u32,
        mut pixels: CustomAutoRooterGuard<Option<ArrayBufferView>>,
    ) {
        let pixels =
            handle_potential_webgl_error!(self, pixels.as_mut().ok_or(InvalidValue), return);

        if width < 0 || height < 0 {
            return self.webgl_error(InvalidValue);
        }

        if format != constants::RGBA || pixel_type != constants::UNSIGNED_BYTE {
            return self.webgl_error(InvalidOperation);
        }

        if pixels.get_array_type() != Type::Uint8 {
            return self.webgl_error(InvalidOperation);
        }

        handle_potential_webgl_error!(self, self.validate_framebuffer(), return);
        let (fb_width, fb_height) = handle_potential_webgl_error!(
            self,
            self.get_current_framebuffer_size().ok_or(InvalidOperation),
            return
        );

        if width == 0 || height == 0 {
            return;
        }

        let bytes_per_pixel = 4;

        let row_len = handle_potential_webgl_error!(
            self,
            width.checked_mul(bytes_per_pixel).ok_or(InvalidOperation),
            return
        );

        let pack_alignment = self.texture_packing_alignment.get() as i32;
        let dest_padding = match row_len % pack_alignment {
            0 => 0,
            remainder => pack_alignment - remainder,
        };
        let dest_stride = row_len + dest_padding;

        let full_rows_len = handle_potential_webgl_error!(
            self,
            dest_stride.checked_mul(height - 1).ok_or(InvalidOperation),
            return
        );
        let required_dest_len = handle_potential_webgl_error!(
            self,
            full_rows_len.checked_add(row_len).ok_or(InvalidOperation),
            return
        );

        let dest = unsafe { pixels.as_mut_slice() };
        if dest.len() < required_dest_len as usize {
            return self.webgl_error(InvalidOperation);
        }

        let mut src_x = x;
        let mut src_y = y;
        let mut src_width = width;
        let mut src_height = height;
        let mut dest_offset = 0;

        if src_x < 0 {
            if src_width <= -src_x {
                return;
            }
            dest_offset += bytes_per_pixel * -src_x;
            src_width += src_x;
            src_x = 0;
        }
        if src_y < 0 {
            if src_height <= -src_y {
                return;
            }
            dest_offset += row_len * -src_y;
            src_height += src_y;
            src_y = 0;
        }

        if src_x + src_width > fb_width {
            src_width = fb_width - src_x;
        }
        if src_y + src_height > fb_height {
            src_height = fb_height - src_y;
        }

        let (sender, receiver) = ipc::bytes_channel().unwrap();
        self.send_command(WebGLCommand::ReadPixels(
            src_x, src_y, src_width, src_height, format, pixel_type, sender,
        ));

        let src = receiver.recv().unwrap();
        let src_row_len = (src_width * bytes_per_pixel) as usize;
        for i in 0..src_height {
            let dest_start = (dest_offset + i * dest_stride) as usize;
            let dest_end = dest_start + src_row_len;
            let src_start = i as usize * src_row_len;
            let src_end = src_start + src_row_len;
            dest[dest_start..dest_end].copy_from_slice(&src[src_start..src_end]);
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn SampleCoverage(&self, value: f32, invert: bool) {
        self.send_command(WebGLCommand::SampleCoverage(value, invert));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.4
    fn Scissor(&self, x: i32, y: i32, width: i32, height: i32) {
        if width < 0 || height < 0 {
            return self.webgl_error(InvalidValue);
        }

        let width = width as u32;
        let height = height as u32;

        self.current_scissor.set((x, y, width, height));
        self.send_command(WebGLCommand::Scissor(x, y, width, height));
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn StencilFunc(&self, func: u32, ref_: i32, mask: u32) {
        match func {
            constants::NEVER |
            constants::LESS |
            constants::EQUAL |
            constants::LEQUAL |
            constants::GREATER |
            constants::NOTEQUAL |
            constants::GEQUAL |
            constants::ALWAYS => self.send_command(WebGLCommand::StencilFunc(func, ref_, mask)),
            _ => self.webgl_error(InvalidEnum),
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn StencilFuncSeparate(&self, face: u32, func: u32, ref_: i32, mask: u32) {
        match face {
            constants::FRONT | constants::BACK | constants::FRONT_AND_BACK => (),
            _ => return self.webgl_error(InvalidEnum),
        }

        match func {
            constants::NEVER |
            constants::LESS |
            constants::EQUAL |
            constants::LEQUAL |
            constants::GREATER |
            constants::NOTEQUAL |
            constants::GEQUAL |
            constants::ALWAYS => {
                self.send_command(WebGLCommand::StencilFuncSeparate(face, func, ref_, mask))
            },
            _ => self.webgl_error(InvalidEnum),
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn StencilMask(&self, mask: u32) {
        self.send_command(WebGLCommand::StencilMask(mask))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn StencilMaskSeparate(&self, face: u32, mask: u32) {
        match face {
            constants::FRONT | constants::BACK | constants::FRONT_AND_BACK => {
                self.send_command(WebGLCommand::StencilMaskSeparate(face, mask))
            },
            _ => return self.webgl_error(InvalidEnum),
        };
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn StencilOp(&self, fail: u32, zfail: u32, zpass: u32) {
        if self.validate_stencil_actions(fail) &&
            self.validate_stencil_actions(zfail) &&
            self.validate_stencil_actions(zpass)
        {
            self.send_command(WebGLCommand::StencilOp(fail, zfail, zpass));
        } else {
            self.webgl_error(InvalidEnum)
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.3
    fn StencilOpSeparate(&self, face: u32, fail: u32, zfail: u32, zpass: u32) {
        match face {
            constants::FRONT | constants::BACK | constants::FRONT_AND_BACK => (),
            _ => return self.webgl_error(InvalidEnum),
        }

        if self.validate_stencil_actions(fail) &&
            self.validate_stencil_actions(zfail) &&
            self.validate_stencil_actions(zpass)
        {
            self.send_command(WebGLCommand::StencilOpSeparate(face, fail, zfail, zpass))
        } else {
            self.webgl_error(InvalidEnum)
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn LinkProgram(&self, program: &WebGLProgram) {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return);
        if program.is_deleted() {
            return self.webgl_error(InvalidValue);
        }
        handle_potential_webgl_error!(self, program.link());
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn ShaderSource(&self, shader: &WebGLShader, source: DOMString) {
        handle_potential_webgl_error!(self, self.validate_ownership(shader), return);
        shader.set_source(source)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn GetShaderSource(&self, shader: &WebGLShader) -> Option<DOMString> {
        handle_potential_webgl_error!(self, self.validate_ownership(shader), return None);
        Some(shader.source())
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform1f(&self, location: Option<&WebGLUniformLocation>, val: f32) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL | constants::FLOAT => {},
                _ => return Err(InvalidOperation),
            }
            self.send_command(WebGLCommand::Uniform1f(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform1i(&self, location: Option<&WebGLUniformLocation>, val: i32) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL | constants::INT => {},
                constants::SAMPLER_2D | constants::SAMPLER_CUBE => {
                    if val < 0 || val as u32 >= self.limits.max_combined_texture_image_units {
                        return Err(InvalidValue);
                    }
                },
                _ => return Err(InvalidOperation),
            }
            self.send_command(WebGLCommand::Uniform1i(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform1iv(&self, location: Option<&WebGLUniformLocation>, val: Int32ArrayOrLongSequence) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL |
                constants::INT |
                constants::SAMPLER_2D |
                constants::SAMPLER_CUBE => {},
                _ => return Err(InvalidOperation),
            }
            let val = match val {
                Int32ArrayOrLongSequence::Int32Array(v) => v.to_vec(),
                Int32ArrayOrLongSequence::LongSequence(v) => v,
            };
            if val.is_empty() {
                return Err(InvalidValue);
            }
            if location.size().is_none() && val.len() != 1 {
                return Err(InvalidOperation);
            }
            match location.type_() {
                constants::SAMPLER_2D | constants::SAMPLER_CUBE => {
                    for &v in val
                        .iter()
                        .take(cmp::min(location.size().unwrap_or(1) as usize, val.len()))
                    {
                        if v < 0 || v as u32 >= self.limits.max_combined_texture_image_units {
                            return Err(InvalidValue);
                        }
                    }
                },
                _ => {},
            }
            self.send_command(WebGLCommand::Uniform1iv(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform1fv(
        &self,
        location: Option<&WebGLUniformLocation>,
        val: Float32ArrayOrUnrestrictedFloatSequence,
    ) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL | constants::FLOAT => {},
                _ => return Err(InvalidOperation),
            }
            let val = match val {
                Float32ArrayOrUnrestrictedFloatSequence::Float32Array(v) => v.to_vec(),
                Float32ArrayOrUnrestrictedFloatSequence::UnrestrictedFloatSequence(v) => v,
            };
            if val.is_empty() {
                return Err(InvalidValue);
            }
            if location.size().is_none() && val.len() != 1 {
                return Err(InvalidOperation);
            }
            self.send_command(WebGLCommand::Uniform1fv(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform2f(&self, location: Option<&WebGLUniformLocation>, x: f32, y: f32) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC2 | constants::FLOAT_VEC2 => {},
                _ => return Err(InvalidOperation),
            }
            self.send_command(WebGLCommand::Uniform2f(location.id(), x, y));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform2fv(
        &self,
        location: Option<&WebGLUniformLocation>,
        val: Float32ArrayOrUnrestrictedFloatSequence,
    ) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC2 | constants::FLOAT_VEC2 => {},
                _ => return Err(InvalidOperation),
            }
            let val = match val {
                Float32ArrayOrUnrestrictedFloatSequence::Float32Array(v) => v.to_vec(),
                Float32ArrayOrUnrestrictedFloatSequence::UnrestrictedFloatSequence(v) => v,
            };
            if val.len() < 2 || val.len() % 2 != 0 {
                return Err(InvalidValue);
            }
            if location.size().is_none() && val.len() != 2 {
                return Err(InvalidOperation);
            }
            self.send_command(WebGLCommand::Uniform2fv(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform2i(&self, location: Option<&WebGLUniformLocation>, x: i32, y: i32) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC2 | constants::INT_VEC2 => {},
                _ => return Err(InvalidOperation),
            }
            self.send_command(WebGLCommand::Uniform2i(location.id(), x, y));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform2iv(&self, location: Option<&WebGLUniformLocation>, val: Int32ArrayOrLongSequence) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC2 | constants::INT_VEC2 => {},
                _ => return Err(InvalidOperation),
            }
            let val = match val {
                Int32ArrayOrLongSequence::Int32Array(v) => v.to_vec(),
                Int32ArrayOrLongSequence::LongSequence(v) => v,
            };
            if val.len() < 2 || val.len() % 2 != 0 {
                return Err(InvalidValue);
            }
            if location.size().is_none() && val.len() != 2 {
                return Err(InvalidOperation);
            }
            self.send_command(WebGLCommand::Uniform2iv(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform3f(&self, location: Option<&WebGLUniformLocation>, x: f32, y: f32, z: f32) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC3 | constants::FLOAT_VEC3 => {},
                _ => return Err(InvalidOperation),
            }
            self.send_command(WebGLCommand::Uniform3f(location.id(), x, y, z));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform3fv(
        &self,
        location: Option<&WebGLUniformLocation>,
        val: Float32ArrayOrUnrestrictedFloatSequence,
    ) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC3 | constants::FLOAT_VEC3 => {},
                _ => return Err(InvalidOperation),
            }
            let val = match val {
                Float32ArrayOrUnrestrictedFloatSequence::Float32Array(v) => v.to_vec(),
                Float32ArrayOrUnrestrictedFloatSequence::UnrestrictedFloatSequence(v) => v,
            };
            if val.len() < 3 || val.len() % 3 != 0 {
                return Err(InvalidValue);
            }
            if location.size().is_none() && val.len() != 3 {
                return Err(InvalidOperation);
            }
            self.send_command(WebGLCommand::Uniform3fv(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform3i(&self, location: Option<&WebGLUniformLocation>, x: i32, y: i32, z: i32) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC3 | constants::INT_VEC3 => {},
                _ => return Err(InvalidOperation),
            }
            self.send_command(WebGLCommand::Uniform3i(location.id(), x, y, z));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform3iv(&self, location: Option<&WebGLUniformLocation>, val: Int32ArrayOrLongSequence) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC3 | constants::INT_VEC3 => {},
                _ => return Err(InvalidOperation),
            }
            let val = match val {
                Int32ArrayOrLongSequence::Int32Array(v) => v.to_vec(),
                Int32ArrayOrLongSequence::LongSequence(v) => v,
            };
            if val.len() < 3 || val.len() % 3 != 0 {
                return Err(InvalidValue);
            }
            if location.size().is_none() && val.len() != 3 {
                return Err(InvalidOperation);
            }
            self.send_command(WebGLCommand::Uniform3iv(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform4i(&self, location: Option<&WebGLUniformLocation>, x: i32, y: i32, z: i32, w: i32) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC4 | constants::INT_VEC4 => {},
                _ => return Err(InvalidOperation),
            }
            self.send_command(WebGLCommand::Uniform4i(location.id(), x, y, z, w));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform4iv(&self, location: Option<&WebGLUniformLocation>, val: Int32ArrayOrLongSequence) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC4 | constants::INT_VEC4 => {},
                _ => return Err(InvalidOperation),
            }
            let val = match val {
                Int32ArrayOrLongSequence::Int32Array(v) => v.to_vec(),
                Int32ArrayOrLongSequence::LongSequence(v) => v,
            };
            if val.len() < 4 || val.len() % 4 != 0 {
                return Err(InvalidValue);
            }
            if location.size().is_none() && val.len() != 4 {
                return Err(InvalidOperation);
            }
            self.send_command(WebGLCommand::Uniform4iv(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform4f(&self, location: Option<&WebGLUniformLocation>, x: f32, y: f32, z: f32, w: f32) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC4 | constants::FLOAT_VEC4 => {},
                _ => return Err(InvalidOperation),
            }
            self.send_command(WebGLCommand::Uniform4f(location.id(), x, y, z, w));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn Uniform4fv(
        &self,
        location: Option<&WebGLUniformLocation>,
        val: Float32ArrayOrUnrestrictedFloatSequence,
    ) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::BOOL_VEC4 | constants::FLOAT_VEC4 => {},
                _ => return Err(InvalidOperation),
            }
            let val = match val {
                Float32ArrayOrUnrestrictedFloatSequence::Float32Array(v) => v.to_vec(),
                Float32ArrayOrUnrestrictedFloatSequence::UnrestrictedFloatSequence(v) => v,
            };
            if val.len() < 4 || val.len() % 4 != 0 {
                return Err(InvalidValue);
            }
            if location.size().is_none() && val.len() != 4 {
                return Err(InvalidOperation);
            }
            self.send_command(WebGLCommand::Uniform4fv(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn UniformMatrix2fv(
        &self,
        location: Option<&WebGLUniformLocation>,
        transpose: bool,
        val: Float32ArrayOrUnrestrictedFloatSequence,
    ) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::FLOAT_MAT2 => {},
                _ => return Err(InvalidOperation),
            }
            let val = match val {
                Float32ArrayOrUnrestrictedFloatSequence::Float32Array(v) => v.to_vec(),
                Float32ArrayOrUnrestrictedFloatSequence::UnrestrictedFloatSequence(v) => v,
            };
            if transpose {
                return Err(InvalidValue);
            }
            if val.len() < 4 || val.len() % 4 != 0 {
                return Err(InvalidValue);
            }
            if location.size().is_none() && val.len() != 4 {
                return Err(InvalidOperation);
            }
            self.send_command(WebGLCommand::UniformMatrix2fv(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn UniformMatrix3fv(
        &self,
        location: Option<&WebGLUniformLocation>,
        transpose: bool,
        val: Float32ArrayOrUnrestrictedFloatSequence,
    ) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::FLOAT_MAT3 => {},
                _ => return Err(InvalidOperation),
            }
            let val = match val {
                Float32ArrayOrUnrestrictedFloatSequence::Float32Array(v) => v.to_vec(),
                Float32ArrayOrUnrestrictedFloatSequence::UnrestrictedFloatSequence(v) => v,
            };
            if transpose {
                return Err(InvalidValue);
            }
            if val.len() < 9 || val.len() % 9 != 0 {
                return Err(InvalidValue);
            }
            if location.size().is_none() && val.len() != 9 {
                return Err(InvalidOperation);
            }
            self.send_command(WebGLCommand::UniformMatrix3fv(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn UniformMatrix4fv(
        &self,
        location: Option<&WebGLUniformLocation>,
        transpose: bool,
        val: Float32ArrayOrUnrestrictedFloatSequence,
    ) {
        self.with_location(location, |location| {
            match location.type_() {
                constants::FLOAT_MAT4 => {},
                _ => return Err(InvalidOperation),
            }
            let val = match val {
                Float32ArrayOrUnrestrictedFloatSequence::Float32Array(v) => v.to_vec(),
                Float32ArrayOrUnrestrictedFloatSequence::UnrestrictedFloatSequence(v) => v,
            };
            if transpose {
                return Err(InvalidValue);
            }
            if val.len() < 16 || val.len() % 16 != 0 {
                return Err(InvalidValue);
            }
            if location.size().is_none() && val.len() != 16 {
                return Err(InvalidOperation);
            }
            self.send_command(WebGLCommand::UniformMatrix4fv(location.id(), val));
            Ok(())
        });
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    #[allow(unsafe_code)]
    unsafe fn GetUniform(
        &self,
        cx: *mut JSContext,
        program: &WebGLProgram,
        location: &WebGLUniformLocation,
    ) -> JSVal {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return NullValue());

        if program.is_deleted() ||
            !program.is_linked() ||
            program.id() != location.program_id() ||
            program.link_generation() != location.link_generation()
        {
            self.webgl_error(InvalidOperation);
            return NullValue();
        }

        fn get<T, F>(triple: (&WebGLRenderingContext, WebGLProgramId, i32), f: F) -> T
        where
            F: FnOnce(WebGLProgramId, i32, WebGLSender<T>) -> WebGLCommand,
            T: for<'de> Deserialize<'de> + Serialize,
        {
            let (sender, receiver) = webgl_channel().unwrap();
            triple.0.send_command(f(triple.1, triple.2, sender));
            receiver.recv().unwrap()
        }

        let triple = (self, program.id(), location.id());

        unsafe fn typed<T>(cx: *mut JSContext, value: &[T::Element]) -> JSVal
        where
            T: TypedArrayElementCreator,
        {
            rooted!(in(cx) let mut rval = ptr::null_mut::<JSObject>());
            <TypedArray<T, *mut JSObject>>::create(
                cx,
                CreateWith::Slice(&value),
                rval.handle_mut(),
            ).unwrap();
            ObjectValue(rval.get())
        }

        match location.type_() {
            constants::BOOL => BooleanValue(get(triple, WebGLCommand::GetUniformBool)),
            constants::BOOL_VEC2 => {
                rooted!(in(cx) let mut rval = NullValue());
                get(triple, WebGLCommand::GetUniformBool2).to_jsval(cx, rval.handle_mut());
                rval.get()
            },
            constants::BOOL_VEC3 => {
                rooted!(in(cx) let mut rval = NullValue());
                get(triple, WebGLCommand::GetUniformBool3).to_jsval(cx, rval.handle_mut());
                rval.get()
            },
            constants::BOOL_VEC4 => {
                rooted!(in(cx) let mut rval = NullValue());
                get(triple, WebGLCommand::GetUniformBool4).to_jsval(cx, rval.handle_mut());
                rval.get()
            },
            constants::INT | constants::SAMPLER_2D | constants::SAMPLER_CUBE => {
                Int32Value(get(triple, WebGLCommand::GetUniformInt))
            },
            constants::INT_VEC2 => typed::<Int32>(cx, &get(triple, WebGLCommand::GetUniformInt2)),
            constants::INT_VEC3 => typed::<Int32>(cx, &get(triple, WebGLCommand::GetUniformInt3)),
            constants::INT_VEC4 => typed::<Int32>(cx, &get(triple, WebGLCommand::GetUniformInt4)),
            constants::FLOAT => DoubleValue(get(triple, WebGLCommand::GetUniformFloat) as f64),
            constants::FLOAT_VEC2 => {
                typed::<Float32>(cx, &get(triple, WebGLCommand::GetUniformFloat2))
            },
            constants::FLOAT_VEC3 => {
                typed::<Float32>(cx, &get(triple, WebGLCommand::GetUniformFloat3))
            },
            constants::FLOAT_VEC4 | constants::FLOAT_MAT2 => {
                typed::<Float32>(cx, &get(triple, WebGLCommand::GetUniformFloat4))
            },
            constants::FLOAT_MAT3 => {
                typed::<Float32>(cx, &get(triple, WebGLCommand::GetUniformFloat9))
            },
            constants::FLOAT_MAT4 => {
                typed::<Float32>(cx, &get(triple, WebGLCommand::GetUniformFloat16))
            },
            _ => panic!("wrong uniform type"),
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn UseProgram(&self, program: Option<&WebGLProgram>) {
        if let Some(program) = program {
            handle_potential_webgl_error!(self, self.validate_ownership(program), return);
            if program.is_deleted() || !program.is_linked() {
                return self.webgl_error(InvalidOperation);
            }
            if program.is_in_use() {
                return;
            }
            program.in_use(true);
        }
        match self.current_program.get() {
            Some(ref current) if program != Some(&**current) => current.in_use(false),
            _ => {},
        }
        self.send_command(WebGLCommand::UseProgram(program.map(|p| p.id())));
        self.current_program.set(program);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn ValidateProgram(&self, program: &WebGLProgram) {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return);
        if let Err(e) = program.validate() {
            self.webgl_error(e);
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn VertexAttrib1f(&self, indx: u32, x: f32) {
        self.vertex_attrib(indx, x, 0f32, 0f32, 1f32)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn VertexAttrib1fv(&self, indx: u32, v: Float32ArrayOrUnrestrictedFloatSequence) {
        let values = match v {
            Float32ArrayOrUnrestrictedFloatSequence::Float32Array(v) => v.to_vec(),
            Float32ArrayOrUnrestrictedFloatSequence::UnrestrictedFloatSequence(v) => v,
        };
        if values.len() < 1 {
            // https://github.com/KhronosGroup/WebGL/issues/2700
            return self.webgl_error(InvalidValue);
        }
        self.vertex_attrib(indx, values[0], 0f32, 0f32, 1f32);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn VertexAttrib2f(&self, indx: u32, x: f32, y: f32) {
        self.vertex_attrib(indx, x, y, 0f32, 1f32)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn VertexAttrib2fv(&self, indx: u32, v: Float32ArrayOrUnrestrictedFloatSequence) {
        let values = match v {
            Float32ArrayOrUnrestrictedFloatSequence::Float32Array(v) => v.to_vec(),
            Float32ArrayOrUnrestrictedFloatSequence::UnrestrictedFloatSequence(v) => v,
        };
        if values.len() < 2 {
            // https://github.com/KhronosGroup/WebGL/issues/2700
            return self.webgl_error(InvalidValue);
        }
        self.vertex_attrib(indx, values[0], values[1], 0f32, 1f32);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn VertexAttrib3f(&self, indx: u32, x: f32, y: f32, z: f32) {
        self.vertex_attrib(indx, x, y, z, 1f32)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn VertexAttrib3fv(&self, indx: u32, v: Float32ArrayOrUnrestrictedFloatSequence) {
        let values = match v {
            Float32ArrayOrUnrestrictedFloatSequence::Float32Array(v) => v.to_vec(),
            Float32ArrayOrUnrestrictedFloatSequence::UnrestrictedFloatSequence(v) => v,
        };
        if values.len() < 3 {
            // https://github.com/KhronosGroup/WebGL/issues/2700
            return self.webgl_error(InvalidValue);
        }
        self.vertex_attrib(indx, values[0], values[1], values[2], 1f32);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn VertexAttrib4f(&self, indx: u32, x: f32, y: f32, z: f32, w: f32) {
        self.vertex_attrib(indx, x, y, z, w)
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn VertexAttrib4fv(&self, indx: u32, v: Float32ArrayOrUnrestrictedFloatSequence) {
        let values = match v {
            Float32ArrayOrUnrestrictedFloatSequence::Float32Array(v) => v.to_vec(),
            Float32ArrayOrUnrestrictedFloatSequence::UnrestrictedFloatSequence(v) => v,
        };
        if values.len() < 4 {
            // https://github.com/KhronosGroup/WebGL/issues/2700
            return self.webgl_error(InvalidValue);
        }
        self.vertex_attrib(indx, values[0], values[1], values[2], values[3]);
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.10
    fn VertexAttribPointer(
        &self,
        index: u32,
        size: i32,
        type_: u32,
        normalized: bool,
        stride: i32,
        offset: i64,
    ) {
        handle_potential_webgl_error!(
            self,
            self.current_vao()
                .vertex_attrib_pointer(index, size, type_, normalized, stride, offset, )
        );
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.4
    fn Viewport(&self, x: i32, y: i32, width: i32, height: i32) {
        if width < 0 || height < 0 {
            return self.webgl_error(InvalidValue);
        }

        self.send_command(WebGLCommand::SetViewport(x, y, width, height))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn TexImage2D(
        &self,
        target: u32,
        level: i32,
        internal_format: u32,
        width: i32,
        height: i32,
        border: i32,
        format: u32,
        data_type: u32,
        pixels: CustomAutoRooterGuard<Option<ArrayBufferView>>,
    ) -> ErrorResult {
        if !self.extension_manager.is_tex_type_enabled(data_type) {
            return Ok(self.webgl_error(InvalidEnum));
        }

        let validator = TexImage2DValidator::new(
            self,
            target,
            level,
            internal_format,
            width,
            height,
            border,
            format,
            data_type,
        );

        let TexImage2DValidatorResult {
            texture,
            target,
            width,
            height,
            level,
            border,
            format,
            data_type,
        } = match validator.validate() {
            Ok(result) => result,
            Err(_) => return Ok(()), // NB: The validator sets the correct error for us.
        };

        let unpacking_alignment = self.texture_unpacking_alignment.get();

        let expected_byte_length = match {
            self.validate_tex_image_2d_data(
                width,
                height,
                format,
                data_type,
                unpacking_alignment,
                &*pixels,
            )
        } {
            Ok(byte_length) => byte_length,
            Err(()) => return Ok(()),
        };

        // If data is null, a buffer of sufficient size
        // initialized to 0 is passed.
        let buff = match *pixels {
            None => vec![0u8; expected_byte_length as usize],
            Some(ref data) => data.to_vec(),
        };

        // From the WebGL spec:
        //
        //     "If pixels is non-null but its size is less than what
        //      is required by the specified width, height, format,
        //      type, and pixel storage parameters, generates an
        //      INVALID_OPERATION error."
        if buff.len() < expected_byte_length as usize {
            return Ok(self.webgl_error(InvalidOperation));
        }

        if !self
            .validate_filterable_texture(&texture, target, level, format, width, height, data_type)
        {
            return Ok(()); // The validator sets the correct error for use
        }

        let pixels = self.prepare_pixels(
            format,
            data_type,
            width,
            height,
            unpacking_alignment,
            false,
            false,
            buff,
        );

        self.tex_image_2d(
            &texture,
            target,
            data_type,
            format,
            level,
            width,
            height,
            border,
            unpacking_alignment,
            pixels,
        );

        Ok(())
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn TexImage2D_(
        &self,
        target: u32,
        level: i32,
        internal_format: u32,
        format: u32,
        data_type: u32,
        source: TexImageSource,
    ) -> ErrorResult {
        if !self.extension_manager.is_tex_type_enabled(data_type) {
            return Ok(self.webgl_error(InvalidEnum));
        }

        let (pixels, size, premultiplied) = match self.get_image_pixels(source)? {
            Some(triple) => triple,
            None => return Ok(()),
        };

        let validator = TexImage2DValidator::new(
            self,
            target,
            level,
            internal_format,
            size.width as i32,
            size.height as i32,
            0,
            format,
            data_type,
        );

        let TexImage2DValidatorResult {
            texture,
            target,
            width,
            height,
            level,
            border,
            format,
            data_type,
        } = match validator.validate() {
            Ok(result) => result,
            Err(_) => return Ok(()), // NB: The validator sets the correct error for us.
        };

        if !self
            .validate_filterable_texture(&texture, target, level, format, width, height, data_type)
        {
            return Ok(()); // The validator sets the correct error for use
        }

        let unpacking_alignment = 1;
        let pixels = self.prepare_pixels(
            format,
            data_type,
            width,
            height,
            unpacking_alignment,
            premultiplied,
            true,
            pixels,
        );

        self.tex_image_2d(
            &texture, target, data_type, format, level, width, height, border, 1, pixels,
        );
        Ok(())
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn TexImageDOM(
        &self,
        target: u32,
        level: i32,
        internal_format: u32,
        width: i32,
        height: i32,
        format: u32,
        data_type: u32,
        source: &HTMLIFrameElement,
    ) -> ErrorResult {
        // Currently DOMToTexture only supports TEXTURE_2D, RGBA, UNSIGNED_BYTE and no levels.
        if target != constants::TEXTURE_2D ||
            level != 0 ||
            internal_format != constants::RGBA ||
            format != constants::RGBA ||
            data_type != constants::UNSIGNED_BYTE
        {
            return Ok(self.webgl_error(InvalidValue));
        }

        // Get bound texture
        let texture = handle_potential_webgl_error!(
            self,
            self.textures
                .active_texture_slot(constants::TEXTURE_2D)
                .unwrap()
                .get()
                .ok_or(InvalidOperation),
            return Ok(())
        );

        let pipeline_id = source.pipeline_id().ok_or(Error::InvalidState)?;
        let document_id = self
            .global()
            .downcast::<Window>()
            .ok_or(Error::InvalidState)?
            .webrender_document();

        texture.set_attached_to_dom();

        let command = DOMToTextureCommand::Attach(
            self.webgl_sender.context_id(),
            texture.id(),
            document_id,
            pipeline_id.to_webrender(),
            Size2D::new(width, height),
        );
        self.webgl_sender.send_dom_to_texture(command).unwrap();

        Ok(())
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn TexSubImage2D(
        &self,
        target: u32,
        level: i32,
        xoffset: i32,
        yoffset: i32,
        width: i32,
        height: i32,
        format: u32,
        data_type: u32,
        pixels: CustomAutoRooterGuard<Option<ArrayBufferView>>,
    ) -> ErrorResult {
        let validator = TexImage2DValidator::new(
            self, target, level, format, width, height, 0, format, data_type,
        );
        let TexImage2DValidatorResult {
            texture,
            target,
            width,
            height,
            level,
            format,
            data_type,
            ..
        } = match validator.validate() {
            Ok(result) => result,
            Err(_) => return Ok(()), // NB: The validator sets the correct error for us.
        };

        let unpacking_alignment = self.texture_unpacking_alignment.get();

        let expected_byte_length = match {
            self.validate_tex_image_2d_data(
                width,
                height,
                format,
                data_type,
                unpacking_alignment,
                &*pixels,
            )
        } {
            Ok(byte_length) => byte_length,
            Err(()) => return Ok(()),
        };

        // If data is null, a buffer of sufficient size
        // initialized to 0 is passed.
        let buff = handle_potential_webgl_error!(
            self,
            pixels.as_ref().map(|p| p.to_vec()).ok_or(InvalidValue),
            return Ok(())
        );

        // From the WebGL spec:
        //
        //     "If pixels is non-null but its size is less than what
        //      is required by the specified width, height, format,
        //      type, and pixel storage parameters, generates an
        //      INVALID_OPERATION error."
        if buff.len() < expected_byte_length as usize {
            return Ok(self.webgl_error(InvalidOperation));
        }

        let unpacking_alignment = self.texture_unpacking_alignment.get();
        let pixels = self.prepare_pixels(
            format,
            data_type,
            width,
            height,
            unpacking_alignment,
            false,
            false,
            buff,
        );

        self.tex_sub_image_2d(
            texture,
            target,
            level,
            xoffset,
            yoffset,
            width,
            height,
            format,
            data_type,
            unpacking_alignment,
            pixels,
        );
        Ok(())
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn TexSubImage2D_(
        &self,
        target: u32,
        level: i32,
        xoffset: i32,
        yoffset: i32,
        format: u32,
        data_type: u32,
        source: TexImageSource,
    ) -> ErrorResult {
        let (pixels, size, premultiplied) = match self.get_image_pixels(source)? {
            Some(triple) => triple,
            None => return Ok(()),
        };

        let validator = TexImage2DValidator::new(
            self,
            target,
            level,
            format,
            size.width as i32,
            size.height as i32,
            0,
            format,
            data_type,
        );
        let TexImage2DValidatorResult {
            texture,
            target,
            width,
            height,
            level,
            format,
            data_type,
            ..
        } = match validator.validate() {
            Ok(result) => result,
            Err(_) => return Ok(()), // NB: The validator sets the correct error for us.
        };

        let unpacking_alignment = 1;
        let pixels = self.prepare_pixels(
            format,
            data_type,
            width,
            height,
            unpacking_alignment,
            premultiplied,
            true,
            pixels,
        );

        self.tex_sub_image_2d(
            texture, target, level, xoffset, yoffset, width, height, format, data_type, 1, pixels,
        );
        Ok(())
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn TexParameterf(&self, target: u32, name: u32, value: f32) {
        self.tex_parameter(target, name, TexParameterValue::Float(value))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.8
    fn TexParameteri(&self, target: u32, name: u32, value: i32) {
        self.tex_parameter(target, name, TexParameterValue::Int(value))
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.6
    fn CheckFramebufferStatus(&self, target: u32) -> u32 {
        // From the GLES 2.0.25 spec, 4.4 ("Framebuffer Objects"):
        //
        //    "If target is not FRAMEBUFFER, INVALID_ENUM is
        //     generated. If CheckFramebufferStatus generates an
        //     error, 0 is returned."
        if target != constants::FRAMEBUFFER {
            self.webgl_error(InvalidEnum);
            return 0;
        }

        match self.bound_framebuffer.get() {
            Some(fb) => return fb.check_status(),
            None => return constants::FRAMEBUFFER_COMPLETE,
        }
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.7
    fn RenderbufferStorage(&self, target: u32, internal_format: u32, width: i32, height: i32) {
        if target != constants::RENDERBUFFER {
            return self.webgl_error(InvalidEnum);
        }

        let max = self.limits.max_renderbuffer_size;

        if width < 0 || width as u32 > max || height < 0 || height as u32 > max {
            return self.webgl_error(InvalidValue);
        }

        match self.bound_renderbuffer.get() {
            Some(rb) => {
                handle_potential_webgl_error!(self, rb.storage(internal_format, width, height));
                if let Some(fb) = self.bound_framebuffer.get() {
                    fb.invalidate_renderbuffer(&*rb);
                }
            },
            None => self.webgl_error(InvalidOperation),
        };

        // FIXME: We need to clear the renderbuffer before it can be
        // accessed.  See https://github.com/servo/servo/issues/13710
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.6
    fn FramebufferRenderbuffer(
        &self,
        target: u32,
        attachment: u32,
        renderbuffertarget: u32,
        rb: Option<&WebGLRenderbuffer>,
    ) {
        if let Some(rb) = rb {
            handle_potential_webgl_error!(self, self.validate_ownership(rb), return);
        }

        if target != constants::FRAMEBUFFER || renderbuffertarget != constants::RENDERBUFFER {
            return self.webgl_error(InvalidEnum);
        }

        match self.bound_framebuffer.get() {
            Some(fb) => handle_potential_webgl_error!(self, fb.renderbuffer(attachment, rb)),
            None => self.webgl_error(InvalidOperation),
        };
    }

    // https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.6
    fn FramebufferTexture2D(
        &self,
        target: u32,
        attachment: u32,
        textarget: u32,
        texture: Option<&WebGLTexture>,
        level: i32,
    ) {
        if let Some(texture) = texture {
            handle_potential_webgl_error!(self, self.validate_ownership(texture), return);
        }

        if target != constants::FRAMEBUFFER {
            return self.webgl_error(InvalidEnum);
        }

        match self.bound_framebuffer.get() {
            Some(fb) => handle_potential_webgl_error!(
                self,
                fb.texture2d(attachment, textarget, texture, level)
            ),
            None => self.webgl_error(InvalidOperation),
        };
    }

    /// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.9
    fn GetAttachedShaders(&self, program: &WebGLProgram) -> Option<Vec<DomRoot<WebGLShader>>> {
        handle_potential_webgl_error!(self, self.validate_ownership(program), return None);
        handle_potential_webgl_error!(self, program.attached_shaders().map(Some), None)
    }
}

pub trait LayoutCanvasWebGLRenderingContextHelpers {
    #[allow(unsafe_code)]
    unsafe fn canvas_data_source(&self) -> HTMLCanvasDataSource;
}

impl LayoutCanvasWebGLRenderingContextHelpers for LayoutDom<WebGLRenderingContext> {
    #[allow(unsafe_code)]
    unsafe fn canvas_data_source(&self) -> HTMLCanvasDataSource {
        HTMLCanvasDataSource::WebGL((*self.unsafe_get()).layout_handle())
    }
}

#[derive(Default, JSTraceable, MallocSizeOf)]
struct Capabilities {
    value: Cell<CapFlags>,
}

impl Capabilities {
    fn set(&self, cap: u32, set: bool) -> WebGLResult<bool> {
        let cap = CapFlags::from_enum(cap)?;
        let mut value = self.value.get();
        if value.contains(cap) == set {
            return Ok(false);
        }
        value.set(cap, set);
        self.value.set(value);
        Ok(true)
    }

    fn is_enabled(&self, cap: u32) -> WebGLResult<bool> {
        Ok(self.value.get().contains(CapFlags::from_enum(cap)?))
    }
}

impl Default for CapFlags {
    fn default() -> Self {
        CapFlags::DITHER
    }
}

macro_rules! capabilities {
    ($name:ident, $next:ident, $($rest:ident,)*) => {
        capabilities!($name, $next, $($rest,)* [$name = 1;]);
    };
    ($prev:ident, $name:ident, $($rest:ident,)* [$($tt:tt)*]) => {
        capabilities!($name, $($rest,)* [$($tt)* $name = Self::$prev.bits << 1;]);
    };
    ($prev:ident, [$($name:ident = $value:expr;)*]) => {
        bitflags! {
            #[derive(JSTraceable, MallocSizeOf)]
            struct CapFlags: u16 {
                $(const $name = $value;)*
            }
        }

        impl CapFlags {
            fn from_enum(cap: u32) -> WebGLResult<Self> {
                match cap {
                    $(constants::$name => Ok(Self::$name),)*
                    _ => Err(InvalidEnum),
                }
            }
        }
    };
}

capabilities! {
    BLEND,
    CULL_FACE,
    DEPTH_TEST,
    DITHER,
    POLYGON_OFFSET_FILL,
    SAMPLE_ALPHA_TO_COVERAGE,
    SAMPLE_COVERAGE,
    SCISSOR_TEST,
    STENCIL_TEST,
}

#[must_root]
#[derive(JSTraceable, MallocSizeOf)]
pub struct Textures {
    active_unit: Cell<u32>,
    units: Box<[TextureUnit]>,
}

impl Textures {
    fn new(max_combined_textures: u32) -> Self {
        Self {
            active_unit: Default::default(),
            units: (0..max_combined_textures)
                .map(|_| Default::default())
                .collect::<Vec<_>>()
                .into(),
        }
    }

    fn active_unit_enum(&self) -> u32 {
        self.active_unit.get() + constants::TEXTURE0
    }

    fn set_active_unit_enum(&self, index: u32) -> WebGLResult<()> {
        if index < constants::TEXTURE0 || (index - constants::TEXTURE0) as usize > self.units.len()
        {
            return Err(InvalidEnum);
        }
        self.active_unit.set(index - constants::TEXTURE0);
        Ok(())
    }

    fn active_texture_slot(&self, target: u32) -> WebGLResult<&MutNullableDom<WebGLTexture>> {
        let active_unit = self.active_unit();
        match target {
            constants::TEXTURE_2D => Ok(&active_unit.tex_2d),
            constants::TEXTURE_CUBE_MAP => Ok(&active_unit.tex_cube_map),
            _ => Err(InvalidEnum),
        }
    }

    pub fn active_texture_for_image_target(
        &self,
        target: TexImageTarget,
    ) -> Option<DomRoot<WebGLTexture>> {
        let active_unit = self.active_unit();
        match target {
            TexImageTarget::Texture2D => active_unit.tex_2d.get(),
            TexImageTarget::CubeMapPositiveX |
            TexImageTarget::CubeMapNegativeX |
            TexImageTarget::CubeMapPositiveY |
            TexImageTarget::CubeMapNegativeY |
            TexImageTarget::CubeMapPositiveZ |
            TexImageTarget::CubeMapNegativeZ => active_unit.tex_cube_map.get(),
        }
    }

    fn active_unit(&self) -> &TextureUnit {
        &self.units[self.active_unit.get() as usize]
    }

    fn iter(&self) -> impl Iterator<Item = (u32, &TextureUnit)> {
        self.units
            .iter()
            .enumerate()
            .map(|(index, unit)| (index as u32 + constants::TEXTURE0, unit))
    }
}

#[must_root]
#[derive(Default, JSTraceable, MallocSizeOf)]
struct TextureUnit {
    tex_2d: MutNullableDom<WebGLTexture>,
    tex_cube_map: MutNullableDom<WebGLTexture>,
}

impl TextureUnit {
    fn unbind(&self, texture: &WebGLTexture) -> Option<u32> {
        let fields = [
            (&self.tex_2d, constants::TEXTURE_2D),
            (&self.tex_cube_map, constants::TEXTURE_CUBE_MAP),
        ];
        for &(slot, target) in &fields {
            if slot.get().map_or(false, |t| texture == &*t) {
                slot.set(None);
                return Some(target);
            }
        }
        None
    }
}

// Remove premultiplied alpha.
// This is only called when texImage2D is called using a canvas2d source and
// UNPACK_PREMULTIPLY_ALPHA_WEBGL is disabled. Pixels got from a canvas2D source
// are always RGBA8 with premultiplied alpha, so we don't have to worry about
// additional formats as happens in the premultiply_pixels method.
fn remove_premultiplied_alpha(pixels: &mut [u8]) {
    for rgba in pixels.chunks_mut(4) {
        let a = (rgba[3] as f32) / 255.0;
        rgba[0] = (rgba[0] as f32 / a) as u8;
        rgba[1] = (rgba[1] as f32 / a) as u8;
        rgba[2] = (rgba[2] as f32 / a) as u8;
    }
}

/// Translates an image in rgba8 (red in the first byte) format to
/// the format that was requested of TexImage.
///
/// From the WebGL 1.0 spec, 5.14.8:
///
///     "The source image data is conceptually first converted to
///      the data type and format specified by the format and type
///      arguments, and then transferred to the WebGL
///      implementation. If a packed pixel format is specified
///      which would imply loss of bits of precision from the image
///      data, this loss of precision must occur."
fn rgba8_image_to_tex_image_data(
    format: TexFormat,
    data_type: TexDataType,
    mut pixels: Vec<u8>,
) -> Vec<u8> {
    // hint for vector allocation sizing.
    let pixel_count = pixels.len() / 4;

    match (format, data_type) {
        (TexFormat::RGBA, TexDataType::UnsignedByte) => pixels,
        (TexFormat::RGB, TexDataType::UnsignedByte) => {
            for i in 0..pixel_count {
                let rgb = {
                    let rgb = &pixels[i * 4..i * 4 + 3];
                    [rgb[0], rgb[1], rgb[2]]
                };
                pixels[i * 3..i * 3 + 3].copy_from_slice(&rgb);
            }
            pixels.truncate(pixel_count * 3);
            pixels
        },
        (TexFormat::Alpha, TexDataType::UnsignedByte) => {
            for i in 0..pixel_count {
                let p = pixels[i * 4 + 3];
                pixels[i] = p;
            }
            pixels.truncate(pixel_count);
            pixels
        },
        (TexFormat::Luminance, TexDataType::UnsignedByte) => {
            for i in 0..pixel_count {
                let p = pixels[i * 4];
                pixels[i] = p;
            }
            pixels.truncate(pixel_count);
            pixels
        },
        (TexFormat::LuminanceAlpha, TexDataType::UnsignedByte) => {
            for i in 0..pixel_count {
                let (lum, a) = {
                    let rgba = &pixels[i * 4..i * 4 + 4];
                    (rgba[0], rgba[3])
                };
                pixels[i * 2] = lum;
                pixels[i * 2 + 1] = a;
            }
            pixels.truncate(pixel_count * 2);
            pixels
        },
        (TexFormat::RGBA, TexDataType::UnsignedShort4444) => {
            for i in 0..pixel_count {
                let p = {
                    let rgba = &pixels[i * 4..i * 4 + 4];
                    (rgba[0] as u16 & 0xf0) << 8 |
                        (rgba[1] as u16 & 0xf0) << 4 |
                        (rgba[2] as u16 & 0xf0) |
                        (rgba[3] as u16 & 0xf0) >> 4
                };
                NativeEndian::write_u16(&mut pixels[i * 2..i * 2 + 2], p);
            }
            pixels.truncate(pixel_count * 2);
            pixels
        },
        (TexFormat::RGBA, TexDataType::UnsignedShort5551) => {
            for i in 0..pixel_count {
                let p = {
                    let rgba = &pixels[i * 4..i * 4 + 4];
                    (rgba[0] as u16 & 0xf8) << 8 |
                        (rgba[1] as u16 & 0xf8) << 3 |
                        (rgba[2] as u16 & 0xf8) >> 2 |
                        (rgba[3] as u16) >> 7
                };
                NativeEndian::write_u16(&mut pixels[i * 2..i * 2 + 2], p);
            }
            pixels.truncate(pixel_count * 2);
            pixels
        },
        (TexFormat::RGB, TexDataType::UnsignedShort565) => {
            for i in 0..pixel_count {
                let p = {
                    let rgb = &pixels[i * 4..i * 4 + 3];
                    (rgb[0] as u16 & 0xf8) << 8 |
                        (rgb[1] as u16 & 0xfc) << 3 |
                        (rgb[2] as u16 & 0xf8) >> 3
                };
                NativeEndian::write_u16(&mut pixels[i * 2..i * 2 + 2], p);
            }
            pixels.truncate(pixel_count * 2);
            pixels
        },
        (TexFormat::RGBA, TexDataType::Float) => {
            let mut rgbaf32 = Vec::<u8>::with_capacity(pixel_count * 16);
            for rgba8 in pixels.chunks(4) {
                rgbaf32.write_f32::<NativeEndian>(rgba8[0] as f32).unwrap();
                rgbaf32.write_f32::<NativeEndian>(rgba8[1] as f32).unwrap();
                rgbaf32.write_f32::<NativeEndian>(rgba8[2] as f32).unwrap();
                rgbaf32.write_f32::<NativeEndian>(rgba8[3] as f32).unwrap();
            }
            rgbaf32
        },

        (TexFormat::RGB, TexDataType::Float) => {
            let mut rgbf32 = Vec::<u8>::with_capacity(pixel_count * 12);
            for rgba8 in pixels.chunks(4) {
                rgbf32.write_f32::<NativeEndian>(rgba8[0] as f32).unwrap();
                rgbf32.write_f32::<NativeEndian>(rgba8[1] as f32).unwrap();
                rgbf32.write_f32::<NativeEndian>(rgba8[2] as f32).unwrap();
            }
            rgbf32
        },

        (TexFormat::Alpha, TexDataType::Float) => {
            for rgba8 in pixels.chunks_mut(4) {
                let p = rgba8[3] as f32;
                NativeEndian::write_f32(rgba8, p);
            }
            pixels
        },

        (TexFormat::Luminance, TexDataType::Float) => {
            for rgba8 in pixels.chunks_mut(4) {
                let p = rgba8[0] as f32;
                NativeEndian::write_f32(rgba8, p);
            }
            pixels
        },

        (TexFormat::LuminanceAlpha, TexDataType::Float) => {
            let mut data = Vec::<u8>::with_capacity(pixel_count * 8);
            for rgba8 in pixels.chunks(4) {
                data.write_f32::<NativeEndian>(rgba8[0] as f32).unwrap();
                data.write_f32::<NativeEndian>(rgba8[3] as f32).unwrap();
            }
            data
        },

        (TexFormat::RGBA, TexDataType::HalfFloat) => {
            let mut rgbaf16 = Vec::<u8>::with_capacity(pixel_count * 8);
            for rgba8 in pixels.chunks(4) {
                rgbaf16
                    .write_u16::<NativeEndian>(f16::from_f32(rgba8[0] as f32).as_bits())
                    .unwrap();
                rgbaf16
                    .write_u16::<NativeEndian>(f16::from_f32(rgba8[1] as f32).as_bits())
                    .unwrap();
                rgbaf16
                    .write_u16::<NativeEndian>(f16::from_f32(rgba8[2] as f32).as_bits())
                    .unwrap();
                rgbaf16
                    .write_u16::<NativeEndian>(f16::from_f32(rgba8[3] as f32).as_bits())
                    .unwrap();
            }
            rgbaf16
        },

        (TexFormat::RGB, TexDataType::HalfFloat) => {
            let mut rgbf16 = Vec::<u8>::with_capacity(pixel_count * 6);
            for rgba8 in pixels.chunks(4) {
                rgbf16
                    .write_u16::<NativeEndian>(f16::from_f32(rgba8[0] as f32).as_bits())
                    .unwrap();
                rgbf16
                    .write_u16::<NativeEndian>(f16::from_f32(rgba8[1] as f32).as_bits())
                    .unwrap();
                rgbf16
                    .write_u16::<NativeEndian>(f16::from_f32(rgba8[2] as f32).as_bits())
                    .unwrap();
            }
            rgbf16
        },
        (TexFormat::Alpha, TexDataType::HalfFloat) => {
            for i in 0..pixel_count {
                let p = f16::from_f32(pixels[i * 4 + 3] as f32).as_bits();
                NativeEndian::write_u16(&mut pixels[i * 2..i * 2 + 2], p);
            }
            pixels.truncate(pixel_count * 2);
            pixels
        },
        (TexFormat::Luminance, TexDataType::HalfFloat) => {
            for i in 0..pixel_count {
                let p = f16::from_f32(pixels[i * 4] as f32).as_bits();
                NativeEndian::write_u16(&mut pixels[i * 2..i * 2 + 2], p);
            }
            pixels.truncate(pixel_count * 2);
            pixels
        },
        (TexFormat::LuminanceAlpha, TexDataType::HalfFloat) => {
            for rgba8 in pixels.chunks_mut(4) {
                let lum = f16::from_f32(rgba8[0] as f32).as_bits();
                let a = f16::from_f32(rgba8[3] as f32).as_bits();
                NativeEndian::write_u16(&mut rgba8[0..2], lum);
                NativeEndian::write_u16(&mut rgba8[2..4], a);
            }
            pixels
        },

        // Validation should have ensured that we only hit the
        // above cases, but we haven't turned the (format, type)
        // into an enum yet so there's a default case here.
        _ => unreachable!("Unsupported formats {:?} {:?}", format, data_type),
    }
}