Auto merge of #18592 - MortimerGoro:dom_texture, r=jdm

Implement DOM to texture

<!-- Please describe your changes on the following line: -->

This is a prototype of the WebGL DOMToTexture feature. The API should be fine as a privileged extension for now due to security concerns. The working group has been investigating the viability of unprivileged usage but the spec is not ready yet.

Demo video: https://www.youtube.com/watch?v=hpZqEM5hPao

---
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- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Cargo.lock

@@ -311,6 +311,7 @@ dependencies = [
  "log 0.3.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "num-traits 0.1.37 (registry+https://github.com/rust-lang/crates.io-index)",
  "offscreen_gl_context 0.11.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "servo_config 0.0.1",
  "webrender 0.52.1 (git+https://github.com/servo/webrender)",
  "webrender_api 0.52.1 (git+https://github.com/servo/webrender)",
 ]

components/canvas/Cargo.toml

@@ -21,5 +21,6 @@ ipc-channel = "0.9"
 log = "0.3.5"
 num-traits = "0.1.32"
 offscreen_gl_context = { version = "0.11", features = ["serde", "osmesa"] }
+servo_config = {path = "../config"}
 webrender = {git = "https://github.com/servo/webrender"}
 webrender_api = {git = "https://github.com/servo/webrender", features = ["ipc"]}

components/canvas/lib.rs

@@ -15,6 +15,7 @@ extern crate ipc_channel;
 #[macro_use] extern crate log;
 extern crate num_traits;
 extern crate offscreen_gl_context;
+extern crate servo_config;
 extern crate webrender;
 extern crate webrender_api;
 

components/canvas/webgl_mode/inprocess.rs

@@ -6,9 +6,12 @@ use ::gl_context::GLContextFactory;
 use ::webgl_thread::{WebGLExternalImageApi, WebGLExternalImageHandler, WebGLThreadObserver, WebGLThread};
 use canvas_traits::webgl::{WebGLChan, WebGLContextId, WebGLMsg, WebGLPipeline, WebGLReceiver};
 use canvas_traits::webgl::{WebGLSender, WebVRCommand, WebVRRenderHandler};
+use canvas_traits::webgl::DOMToTextureCommand;
 use canvas_traits::webgl::webgl_channel;
 use euclid::Size2D;
+use fnv::FnvHashMap;
 use gleam::gl;
+use servo_config::prefs::PREFS;
 use std::marker::PhantomData;
 use std::rc::Rc;
 use webrender;
@@ -23,14 +26,19 @@ impl WebGLThreads {
                webrender_gl: Rc<gl::Gl>,
                webrender_api_sender: webrender_api::RenderApiSender,
                webvr_compositor: Option<Box<WebVRRenderHandler>>)
-               -> (WebGLThreads, Box<webrender::ExternalImageHandler>) {
+               -> (WebGLThreads, Box<webrender::ExternalImageHandler>, Option<Box<webrender::OutputImageHandler>>) {
         // This implementation creates a single `WebGLThread` for all the pipelines.
         let channel = WebGLThread::start(gl_factory,
                                          webrender_api_sender,
                                          webvr_compositor.map(|c| WebVRRenderWrapper(c)),
                                          PhantomData);
+        let output_handler = if PREFS.is_dom_to_texture_enabled() {
+            Some(Box::new(OutputHandler::new(webrender_gl.clone(), channel.clone())))
+        } else {
+            None
+        };
         let external = WebGLExternalImageHandler::new(WebGLExternalImages::new(webrender_gl, channel.clone()));
-        (WebGLThreads(channel), Box::new(external))
+        (WebGLThreads(channel), Box::new(external), output_handler.map(|b| b as Box<_>))
     }
 
     /// Gets the WebGLThread handle for each script pipeline.
@@ -105,3 +113,47 @@ impl WebVRRenderHandler for WebVRRenderWrapper {
         self.0.handle(command, texture);
     }
 }
+
+/// struct used to implement DOMToTexture feature and webrender::OutputImageHandler trait.
+type OutputHandlerData = Option<(u32, Size2D<i32>)>;
+struct OutputHandler {
+    webrender_gl: Rc<gl::Gl>,
+    webgl_channel: WebGLSender<WebGLMsg>,
+    // Used to avoid creating a new channel on each received WebRender request.
+    lock_channel: (WebGLSender<OutputHandlerData>, WebGLReceiver<OutputHandlerData>),
+    sync_objects: FnvHashMap<webrender_api::PipelineId, gl::GLsync>,
+}
+
+impl OutputHandler {
+    fn new(webrender_gl: Rc<gl::Gl>, channel: WebGLSender<WebGLMsg>) -> Self {
+        Self {
+            webrender_gl,
+            webgl_channel: channel,
+            lock_channel: webgl_channel().unwrap(),
+            sync_objects: Default::default(),
+        }
+    }
+}
+
+/// Bridge between the WR frame outputs and WebGL to implement DOMToTexture synchronization.
+impl webrender::OutputImageHandler for OutputHandler {
+    fn lock(&mut self, id: webrender_api::PipelineId) -> Option<(u32, webrender_api::DeviceIntSize)> {
+        // Insert a fence in the WR command queue
+        let gl_sync = self.webrender_gl.fence_sync(gl::SYNC_GPU_COMMANDS_COMPLETE, 0);
+        // The lock command adds a WaitSync call on the WebGL command flow.
+        let command = DOMToTextureCommand::Lock(id, gl_sync as usize, self.lock_channel.0.clone());
+        self.webgl_channel.send(WebGLMsg::DOMToTextureCommand(command)).unwrap();
+        self.lock_channel.1.recv().unwrap().map(|(tex_id, size)| {
+            (tex_id, webrender_api::DeviceIntSize::new(size.width, size.height))
+        })
+    }
+
+    fn unlock(&mut self, id: webrender_api::PipelineId) {
+        if let Some(gl_sync) = self.sync_objects.remove(&id) {
+            // Flush the Sync object into the GPU's command queue to guarantee that it it's signaled.
+            self.webrender_gl.flush();
+            // Mark the sync object for deletion.
+            self.webrender_gl.delete_sync(gl_sync);
+        }
+    }
+}

components/canvas/webgl_thread.rs

@@ -36,6 +36,8 @@ pub struct WebGLThread<VR: WebVRRenderHandler + 'static, OB: WebGLThreadObserver
     webvr_compositor: Option<VR>,
     /// Generic observer that listens WebGLContext creation, resize or removal events.
     observer: OB,
+    /// Texture ids and sizes used in DOM to texture outputs.
+    dom_outputs: FnvHashMap<webrender_api::PipelineId, DOMToTextureData>,
 }
 
 impl<VR: WebVRRenderHandler + 'static, OB: WebGLThreadObserver> WebGLThread<VR, OB> {
@@ -52,6 +54,7 @@ impl<VR: WebVRRenderHandler + 'static, OB: WebGLThreadObserver> WebGLThread<VR,
             next_webgl_id: 0,
             webvr_compositor,
             observer: observer,
+            dom_outputs: Default::default(),
         }
     }
 
@@ -117,6 +120,9 @@ impl<VR: WebVRRenderHandler + 'static, OB: WebGLThreadObserver> WebGLThread<VR,
             WebGLMsg::UpdateWebRenderImage(ctx_id, sender) => {
                 self.handle_update_wr_image(ctx_id, sender);
             },
+            WebGLMsg::DOMToTextureCommand(command) => {
+                self.handle_dom_to_texture(command);
+            },
             WebGLMsg::Exit => {
                 return true;
             }
@@ -323,6 +329,54 @@ impl<VR: WebVRRenderHandler + 'static, OB: WebGLThreadObserver> WebGLThread<VR,
         sender.send(image_key).unwrap();
     }
 
+    fn handle_dom_to_texture(&mut self, command: DOMToTextureCommand) {
+        match command {
+            DOMToTextureCommand::Attach(context_id, texture_id, document_id, pipeline_id, size) => {
+                let ctx = Self::make_current_if_needed(context_id, &self.contexts, &mut self.bound_context_id)
+                                .expect("WebGLContext not found in a WebGL DOMToTextureCommand::Attach command");
+                // Initialize the texture that WR will use for frame outputs.
+                ctx.gl().tex_image_2d(gl::TEXTURE_2D,
+                                      0,
+                                      gl::RGBA as gl::GLint,
+                                      size.width,
+                                      size.height,
+                                      0,
+                                      gl::RGBA,
+                                      gl::UNSIGNED_BYTE,
+                                      None);
+                self.dom_outputs.insert(pipeline_id, DOMToTextureData {
+                    context_id, texture_id, document_id, size
+                });
+                self.webrender_api.enable_frame_output(document_id, pipeline_id, true);
+            },
+            DOMToTextureCommand::Lock(pipeline_id, gl_sync, sender) => {
+                let contexts = &self.contexts;
+                let bound_context_id = &mut self.bound_context_id;
+                let result = self.dom_outputs.get(&pipeline_id).and_then(|data| {
+                    let ctx = Self::make_current_if_needed(data.context_id, contexts, bound_context_id);
+                    ctx.and_then(|ctx| {
+                        // The next glWaitSync call is used to synchronize the two flows of
+                        // OpenGL commands (WR and WebGL) in order to avoid using semi-ready WR textures.
+                        // glWaitSync doesn't block WebGL CPU thread.
+                        ctx.gl().wait_sync(gl_sync as gl::GLsync, 0, gl::TIMEOUT_IGNORED);
+                        Some((data.texture_id.get(), data.size))
+                    })
+                });
+
+                // Send the texture id and size to WR.
+                sender.send(result).unwrap();
+            },
+            DOMToTextureCommand::Detach(texture_id) => {
+                if let Some((pipeline_id, document_id)) = self.dom_outputs.iter()
+                                                                          .find(|&(_, v)| v.texture_id == texture_id)
+                                                                          .map(|(k, v)| (*k, v.document_id)) {
+                    self.webrender_api.enable_frame_output(document_id, pipeline_id, false);
+                    self.dom_outputs.remove(&pipeline_id);
+                }
+            },
+        }
+    }
+
     /// Gets a reference to a GLContextWrapper for a given WebGLContextId and makes it current if required.
     fn make_current_if_needed<'a>(context_id: WebGLContextId,
                                   contexts: &'a FnvHashMap<WebGLContextId, GLContextWrapper>,
@@ -552,6 +606,14 @@ impl<T: WebGLExternalImageApi> webrender::ExternalImageHandler for WebGLExternal
     }
 }
 
+/// Data about the linked DOM<->WebGLTexture elements.
+struct DOMToTextureData {
+    context_id: WebGLContextId,
+    texture_id: WebGLTextureId,
+    document_id: webrender_api::DocumentId,
+    size: Size2D<i32>,
+}
+
 /// WebGL Commands Implementation
 pub struct WebGLImpl;
 

components/canvas_traits/webgl.rs

@@ -6,7 +6,7 @@ use euclid::Size2D;
 use nonzero::NonZeroU32;
 use offscreen_gl_context::{GLContextAttributes, GLLimits};
 use std::fmt;
-use webrender_api;
+use webrender_api::{DocumentId, ImageKey, PipelineId};
 
 /// Sender type used in WebGLCommands.
 pub use ::webgl_channel::WebGLSender;
@@ -46,7 +46,9 @@ pub enum WebGLMsg {
     /// Unlock messages are always sent after a Lock message.
     Unlock(WebGLContextId),
     /// Creates or updates the image keys required for WebRender.
-    UpdateWebRenderImage(WebGLContextId, WebGLSender<webrender_api::ImageKey>),
+    UpdateWebRenderImage(WebGLContextId, WebGLSender<ImageKey>),
+    /// Commands used for the DOMToTexture feature.
+    DOMToTextureCommand(DOMToTextureCommand),
     /// Frees all resources and closes the thread.
     Exit,
 }
@@ -86,6 +88,11 @@ impl WebGLMsgSender {
         }
     }
 
+    /// Returns the WebGLContextId associated to this sender
+    pub fn context_id(&self) -> WebGLContextId {
+        self.ctx_id
+    }
+
     /// Send a WebGLCommand message
     #[inline]
     pub fn send(&self, command: WebGLCommand) -> WebGLSendResult {
@@ -113,9 +120,13 @@ impl WebGLMsgSender {
     }
 
     #[inline]
-    pub fn send_update_wr_image(&self, sender: WebGLSender<webrender_api::ImageKey>) -> WebGLSendResult {
+    pub fn send_update_wr_image(&self, sender: WebGLSender<ImageKey>) -> WebGLSendResult {
         self.sender.send(WebGLMsg::UpdateWebRenderImage(self.ctx_id, sender))
     }
+
+    pub fn send_dom_to_texture(&self, command: DOMToTextureCommand) -> WebGLSendResult {
+        self.sender.send(WebGLMsg::DOMToTextureCommand(command))
+    }
 }
 
 /// WebGL Commands for a specific WebGLContext
@@ -379,6 +390,17 @@ pub trait WebVRRenderHandler: Send {
     fn handle(&mut self, command: WebVRCommand, texture: Option<(u32, Size2D<i32>)>);
 }
 
+/// WebGL commands required to implement DOMToTexture feature.
+#[derive(Clone, Deserialize, Serialize)]
+pub enum DOMToTextureCommand {
+    /// Attaches a HTMLIFrameElement to a WebGLTexture.
+    Attach(WebGLContextId, WebGLTextureId, DocumentId, PipelineId, Size2D<i32>),
+    /// Releases the HTMLIFrameElement to WebGLTexture attachment.
+    Detach(WebGLTextureId),
+    /// Lock message used for a correct synchronization with WebRender GL flow.
+    Lock(PipelineId, usize, WebGLSender<Option<(u32, Size2D<i32>)>>),
+}
+
 impl fmt::Debug for WebGLCommand {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         use self::WebGLCommand::*;

components/config/prefs.rs

@@ -273,4 +273,8 @@ impl Preferences {
     pub fn is_webvr_enabled(&self) -> bool {
         self.get("dom.webvr.enabled").as_boolean().unwrap_or(false)
     }
+
+    pub fn is_dom_to_texture_enabled(&self) -> bool {
+        self.get("dom.webgl.dom_to_texture.enabled").as_boolean().unwrap_or(false)
+    }
 }

components/constellation/pipeline.rs

@@ -511,6 +511,7 @@ impl UnprivilegedPipelineContent {
             content_process_shutdown_chan: self.script_content_process_shutdown_chan,
             webgl_chan: self.webgl_chan,
             webvr_chan: self.webvr_chan,
+            webrender_document: self.webrender_document,
         }, self.load_data.clone());
 
         LTF::create(self.id,

components/script/dom/bindings/trace.rs

@@ -110,7 +110,7 @@ use style::stylesheets::keyframes_rule::Keyframe;
 use style::values::specified::Length;
 use time::Duration;
 use uuid::Uuid;
-use webrender_api::ImageKey;
+use webrender_api::{DocumentId, ImageKey};
 use webvr_traits::WebVRGamepadHand;
 
 /// A trait to allow tracing (only) DOM objects.
@@ -397,6 +397,7 @@ unsafe_no_jsmanaged_fields!(OpaqueStyleAndLayoutData);
 unsafe_no_jsmanaged_fields!(PathBuf);
 unsafe_no_jsmanaged_fields!(CSSErrorReporter);
 unsafe_no_jsmanaged_fields!(DrawAPaintImageResult);
+unsafe_no_jsmanaged_fields!(DocumentId);
 unsafe_no_jsmanaged_fields!(ImageKey);
 unsafe_no_jsmanaged_fields!(WebGLBufferId);
 unsafe_no_jsmanaged_fields!(WebGLChan);

components/script/dom/webglrenderingcontext.rs

@@ -6,6 +6,7 @@ use byteorder::{NativeEndian, ReadBytesExt, WriteBytesExt};
 use canvas_traits::canvas::{byte_swap, multiply_u8_pixel};
 use canvas_traits::webgl::{WebGLContextShareMode, WebGLCommand, WebGLError};
 use canvas_traits::webgl::{WebGLFramebufferBindingRequest, WebGLMsg, WebGLMsgSender, WebGLParameter, WebVRCommand};
+use canvas_traits::webgl::DOMToTextureCommand;
 use canvas_traits::webgl::WebGLError::*;
 use canvas_traits::webgl::webgl_channel;
 use core::cell::Ref;
@@ -25,6 +26,7 @@ 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, window_from_node};
 use dom::webgl_extensions::WebGLExtensions;
 use dom::webgl_validations::WebGLValidator;
@@ -3260,6 +3262,45 @@ impl WebGLRenderingContextMethods for WebGLRenderingContext {
         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) -> Fallible<()> {
+        // 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 = match self.bound_texture(constants::TEXTURE_2D) {
+            Some(texture) => texture,
+            None => {
+                return Ok(self.webgl_error(InvalidOperation));
+            }
+        };
+
+        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
     #[allow(unsafe_code)]
     unsafe fn TexSubImage2D(&self,

components/script/dom/webgltexture.rs

@@ -5,6 +5,7 @@
 // https://www.khronos.org/registry/webgl/specs/latest/1.0/webgl.idl
 
 use canvas_traits::webgl::{webgl_channel, WebGLCommand, WebGLError, WebGLMsgSender, WebGLResult, WebGLTextureId};
+use canvas_traits::webgl::DOMToTextureCommand;
 use dom::bindings::cell::DomRefCell;
 use dom::bindings::codegen::Bindings::WebGLRenderingContextBinding::WebGLRenderingContextConstants as constants;
 use dom::bindings::codegen::Bindings::WebGLTextureBinding;
@@ -45,6 +46,8 @@ pub struct WebGLTexture {
     mag_filter: Cell<Option<u32>>,
     #[ignore_heap_size_of = "Defined in ipc-channel"]
     renderer: WebGLMsgSender,
+    /// True if this texture is used for the DOMToTexture feature.
+    attached_to_dom: Cell<bool>,
 }
 
 impl WebGLTexture {
@@ -62,6 +65,7 @@ impl WebGLTexture {
             mag_filter: Cell::new(None),
             image_info_array: DomRefCell::new([ImageInfo::new(); MAX_LEVEL_COUNT * MAX_FACE_COUNT]),
             renderer: renderer,
+            attached_to_dom: Cell::new(false),
         }
     }
 
@@ -179,6 +183,10 @@ impl WebGLTexture {
     pub fn delete(&self) {
         if !self.is_deleted.get() {
             self.is_deleted.set(true);
+            // Notify WR to release the frame output when using DOMToTexture feature
+            if self.attached_to_dom.get() {
+                let _ = self.renderer.send_dom_to_texture(DOMToTextureCommand::Detach(self.id));
+            }
             let _ = self.renderer.send(WebGLCommand::DeleteTexture(self.id));
         }
     }
@@ -374,6 +382,10 @@ impl WebGLTexture {
 
         Some(self.image_info_at_face(0, self.base_mipmap_level))
     }
+
+    pub fn set_attached_to_dom(&self) {
+        self.attached_to_dom.set(true);
+    }
 }
 
 impl Drop for WebGLTexture {

components/script/dom/webidls/WebGLRenderingContext.webidl

@@ -654,6 +654,9 @@ interface WebGLRenderingContextBase
     [Throws]
     void texImage2D(GLenum target, GLint level, GLenum internalformat,
                     GLenum format, GLenum type, TexImageSource? source); // May throw DOMException
+    [Throws, Pref="dom.webgl.dom_to_texture.enabled"]
+    void texImageDOM(GLenum target, GLint level, GLenum internalformat, GLsizei width, GLsizei height,
+                     GLenum format, GLenum type, HTMLIFrameElement source); // May throw DOMException
 
     void texParameterf(GLenum target, GLenum pname, GLfloat param);
     void texParameteri(GLenum target, GLenum pname, GLint param);

components/script/dom/window.rs

@@ -123,7 +123,7 @@ use timers::{IsInterval, TimerCallback};
 use tinyfiledialogs::{self, MessageBoxIcon};
 use url::Position;
 use webdriver_handlers::jsval_to_webdriver;
-use webrender_api::ClipId;
+use webrender_api::{ClipId, DocumentId};
 use webvr_traits::WebVRMsg;
 
 /// Current state of the window object
@@ -289,6 +289,9 @@ pub struct Window {
     test_worklet: MutNullableDom<Worklet>,
     /// https://drafts.css-houdini.org/css-paint-api-1/#paint-worklet
     paint_worklet: MutNullableDom<Worklet>,
+    /// The Webrender Document id associated with this window.
+    #[ignore_heap_size_of = "defined in webrender_api"]
+    webrender_document: DocumentId,
 }
 
 impl Window {
@@ -1760,6 +1763,10 @@ impl Window {
             .send(msg)
             .unwrap();
     }
+
+    pub fn webrender_document(&self) -> DocumentId {
+        self.webrender_document
+    }
 }
 
 impl Window {
@@ -1794,6 +1801,7 @@ impl Window {
         webgl_chan: WebGLChan,
         webvr_chan: Option<IpcSender<WebVRMsg>>,
         microtask_queue: Rc<MicrotaskQueue>,
+        webrender_document: DocumentId,
     ) -> DomRoot<Self> {
         let layout_rpc: Box<LayoutRPC + Send> = {
             let (rpc_send, rpc_recv) = channel();
@@ -1868,6 +1876,7 @@ impl Window {
             unminified_js_dir: Default::default(),
             test_worklet: Default::default(),
             paint_worklet: Default::default(),
+            webrender_document,
         });
 
         unsafe {

components/script/script_thread.rs

@@ -124,6 +124,7 @@ use time::{get_time, precise_time_ns, Tm};
 use url::Position;
 use url::percent_encoding::percent_decode;
 use webdriver_handlers;
+use webrender_api::DocumentId;
 use webvr_traits::{WebVREvent, WebVRMsg};
 
 pub type ImageCacheMsg = (PipelineId, PendingImageResponse);
@@ -495,6 +496,9 @@ pub struct ScriptThread {
 
     /// https://html.spec.whatwg.org/multipage/#custom-element-reactions-stack
     custom_element_reaction_stack: CustomElementReactionStack,
+
+    /// The Webrender Document ID associated with this thread.
+    webrender_document: DocumentId,
 }
 
 /// In the event of thread panic, all data on the stack runs its destructor. However, there
@@ -871,6 +875,8 @@ impl ScriptThread {
             transitioning_nodes: Default::default(),
 
             custom_element_reaction_stack: CustomElementReactionStack::new(),
+
+            webrender_document: state.webrender_document,
         }
     }
 
@@ -2062,6 +2068,7 @@ impl ScriptThread {
             self.webgl_chan.channel(),
             self.webvr_chan.clone(),
             self.microtask_queue.clone(),
+            self.webrender_document,
         );
 
         // Initialize the browsing context for the window.

components/script_traits/lib.rs

@@ -69,7 +69,7 @@ use std::sync::mpsc::{Receiver, Sender, RecvTimeoutError};
 use style_traits::CSSPixel;
 use style_traits::SpeculativePainter;
 use webdriver_msg::{LoadStatus, WebDriverScriptCommand};
-use webrender_api::{ClipId, DevicePixel, ImageKey};
+use webrender_api::{ClipId, DevicePixel, DocumentId, ImageKey};
 use webvr_traits::{WebVREvent, WebVRMsg};
 
 pub use script_msg::{LayoutMsg, ScriptMsg, EventResult, LogEntry};
@@ -554,7 +554,9 @@ pub struct InitialScriptState {
     /// A channel to the webgl thread used in this pipeline.
     pub webgl_chan: WebGLPipeline,
     /// A channel to the webvr thread, if available.
-    pub webvr_chan: Option<IpcSender<WebVRMsg>>
+    pub webvr_chan: Option<IpcSender<WebVRMsg>>,
+    /// The Webrender document ID associated with this thread.
+    pub webrender_document: DocumentId,
 }
 
 /// This trait allows creating a `ScriptThread` without depending on the `script`

components/servo/lib.rs

@@ -553,13 +553,18 @@ fn create_constellation(user_agent: Cow<'static, str>,
     };
 
     // Initialize WebGL Thread entry point.
-    let (webgl_threads, image_handler) = WebGLThreads::new(gl_factory,
-                                                           window_gl,
-                                                           webrender_api_sender.clone(),
-                                                           webvr_compositor.map(|c| c as Box<_>));
+    let (webgl_threads, image_handler, output_handler) = WebGLThreads::new(gl_factory,
+                                                                           window_gl,
+                                                                           webrender_api_sender.clone(),
+                                                                           webvr_compositor.map(|c| c as Box<_>));
     // Set webrender external image handler for WebGL textures
     webrender.set_external_image_handler(image_handler);
 
+    // Set DOM to texture handler, if enabled.
+    if let Some(output_handler) = output_handler {
+        webrender.set_output_image_handler(output_handler);
+    }
+
     let initial_state = InitialConstellationState {
         compositor_proxy,
         embedder_proxy,

resources/prefs.json

@@ -15,6 +15,7 @@
   "dom.servoparser.async_html_tokenizer.enabled": false,
   "dom.testable_crash.enabled": false,
   "dom.testbinding.enabled": false,
+  "dom.webgl.dom_to_texture.enabled": false,
   "dom.webvr.enabled": false,
   "dom.webvr.event_polling_interval": 500,
   "js.asmjs.enabled": true,