Introduce snapshot concept of canvas (#36119)

Each canvas context returns snapshot instead of just raw bytes. This
allows as to hold off conversions (BGRA <-> RGBA, (un)premultiply) to
when/if they are actually needed. For example when loading snapshot into
webgl we can load both RGBA and BGRA so no conversion is really needed.

Currently whole thing is designed to be able to be extend on
https://github.com/servo/ipc-channel/pull/356, to make less copies.
Hence some commented out code.


Fixes #35759
There are tests for these changes in WPT

---------

Signed-off-by: sagudev <16504129+sagudev@users.noreply.github.com>

components/canvas/Cargo.toml

@@ -38,6 +38,7 @@ pixels = { path = "../pixels" }
 range = { path = "../range" }
 raqote = "0.8.5"
 servo_arc = { workspace = true }
+snapshot = { workspace = true }
 stylo = { workspace = true }
 surfman = { workspace = true }
 unicode-script = { workspace = true }

components/canvas/canvas_data.rs

@@ -19,6 +19,7 @@ use log::warn;
 use num_traits::ToPrimitive;
 use range::Range;
 use servo_arc::Arc as ServoArc;
+use snapshot::Snapshot;
 use style::color::AbsoluteColor;
 use style::properties::style_structs::Font as FontStyleStruct;
 use unicode_script::Script;
@@ -521,8 +522,7 @@ pub trait GenericDrawTarget {
         stroke_options: &StrokeOptions,
         draw_options: &DrawOptions,
     );
-    fn snapshot_data(&self, f: &dyn Fn(&[u8]) -> Vec<u8>) -> Vec<u8>;
-    fn snapshot_data_owned(&self) -> Vec<u8>;
+    fn snapshot_data(&self) -> &[u8];
 }
 
 pub enum GradientStop {
@@ -605,9 +605,8 @@ impl<'a> CanvasData<'a> {
             offset: 0,
             flags: ImageDescriptorFlags::empty(),
         };
-        let data = SerializableImageData::Raw(IpcSharedMemory::from_bytes(
-            &draw_target.snapshot_data_owned(),
-        ));
+        let data =
+            SerializableImageData::Raw(IpcSharedMemory::from_bytes(draw_target.snapshot_data()));
         compositor_api.update_images(vec![ImageUpdate::AddImage(image_key, descriptor, data)]);
         CanvasData {
             backend,
@@ -628,7 +627,7 @@ impl<'a> CanvasData<'a> {
     pub fn draw_image(
         &mut self,
         image_data: &[u8],
-        image_size: Size2D<f64>,
+        image_size: Size2D<u64>,
         dest_rect: Rect<f64>,
         source_rect: Rect<f64>,
         smoothing_enabled: bool,
@@ -637,8 +636,8 @@ impl<'a> CanvasData<'a> {
         // We round up the floating pixel values to draw the pixels
         let source_rect = source_rect.ceil();
         // It discards the extra pixels (if any) that won't be painted
-        let image_data = if Rect::from_size(image_size).contains_rect(&source_rect) {
-            pixels::rgba8_get_rect(image_data, image_size.to_u64(), source_rect.to_u64()).into()
+        let image_data = if Rect::from_size(image_size.to_f64()).contains_rect(&source_rect) {
+            pixels::rgba8_get_rect(image_data, image_size, source_rect.to_u64()).into()
         } else {
             image_data.into()
         };
@@ -1412,12 +1411,8 @@ impl<'a> CanvasData<'a> {
         self.update_image_rendering();
     }
 
-    pub fn send_pixels(&mut self, chan: IpcSender<IpcSharedMemory>) {
-        self.drawtarget.snapshot_data(&|bytes| {
-            let data = IpcSharedMemory::from_bytes(bytes);
-            chan.send(data).unwrap();
-            vec![]
-        });
+    pub fn snapshot(&self) {
+        self.drawtarget.snapshot_data();
     }
 
     /// Update image in WebRender
@@ -1430,7 +1425,7 @@ impl<'a> CanvasData<'a> {
             flags: ImageDescriptorFlags::empty(),
         };
         let data = SerializableImageData::Raw(IpcSharedMemory::from_bytes(
-            &self.drawtarget.snapshot_data_owned(),
+            self.drawtarget.snapshot_data(),
         ));
 
         self.compositor_api
@@ -1529,18 +1524,36 @@ impl<'a> CanvasData<'a> {
     /// canvas_size: The size of the canvas we're reading from
     /// read_rect: The area of the canvas we want to read from
     #[allow(unsafe_code)]
-    pub fn read_pixels(&self, read_rect: Rect<u64>, canvas_size: Size2D<u64>) -> Vec<u8> {
-        let canvas_rect = Rect::from_size(canvas_size);
-        if canvas_rect
-            .intersection(&read_rect)
-            .is_none_or(|rect| rect.is_empty())
-        {
-            return vec![];
-        }
+    pub fn read_pixels(
+        &self,
+        read_rect: Option<Rect<u64>>,
+        canvas_size: Option<Size2D<u64>>,
+    ) -> Snapshot {
+        let canvas_size = canvas_size.unwrap_or(self.drawtarget.get_size().cast());
 
-        self.drawtarget.snapshot_data(&|bytes| {
-            pixels::rgba8_get_rect(bytes, canvas_size, read_rect).into_owned()
-        })
+        let data = if let Some(read_rect) = read_rect {
+            let canvas_rect = Rect::from_size(canvas_size);
+            if canvas_rect
+                .intersection(&read_rect)
+                .is_none_or(|rect| rect.is_empty())
+            {
+                vec![]
+            } else {
+                let bytes = self.drawtarget.snapshot_data();
+                pixels::rgba8_get_rect(bytes, canvas_size, read_rect).to_vec()
+            }
+        } else {
+            self.drawtarget.snapshot_data().to_vec()
+        };
+
+        Snapshot::from_vec(
+            canvas_size,
+            snapshot::PixelFormat::BGRA,
+            snapshot::AlphaMode::Transparent {
+                premultiplied: true,
+            },
+            data,
+        )
     }
 }
 

components/canvas/canvas_paint_thread.rs

@@ -76,7 +76,11 @@ impl<'a> CanvasPaintThread<'a> {
                                 },
                                 Ok(CanvasMsg::FromScript(message, canvas_id)) => match message {
                                     FromScriptMsg::SendPixels(chan) => {
-                                        canvas_paint_thread.canvas(canvas_id).send_pixels(chan);
+                                        chan.send(canvas_paint_thread
+                                            .canvas(canvas_id)
+                                            .read_pixels(None, None)
+                                            .as_ipc()
+                                        ).unwrap();
                                     },
                                 },
                                 Err(e) => {
@@ -159,24 +163,21 @@ impl<'a> CanvasPaintThread<'a> {
             Canvas2dMsg::IsPointInPath(path, x, y, fill_rule, chan) => self
                 .canvas(canvas_id)
                 .is_point_in_path_(&path[..], x, y, fill_rule, chan),
-            Canvas2dMsg::DrawImage(
-                ref image_data,
-                image_size,
-                dest_rect,
-                source_rect,
-                smoothing_enabled,
-            ) => self.canvas(canvas_id).draw_image(
-                image_data,
-                image_size,
-                dest_rect,
-                source_rect,
-                smoothing_enabled,
-                true,
-            ),
+            Canvas2dMsg::DrawImage(snapshot, dest_rect, source_rect, smoothing_enabled) => {
+                let snapshot = snapshot.to_owned();
+                self.canvas(canvas_id).draw_image(
+                    snapshot.data(),
+                    snapshot.size(),
+                    dest_rect,
+                    source_rect,
+                    smoothing_enabled,
+                    !snapshot.alpha_mode().is_premultiplied(),
+                )
+            },
             Canvas2dMsg::DrawEmptyImage(image_size, dest_rect, source_rect) => {
                 self.canvas(canvas_id).draw_image(
                     &vec![0; image_size.area() as usize * 4],
-                    image_size,
+                    image_size.to_u64(),
                     dest_rect,
                     source_rect,
                     false,
@@ -192,10 +193,10 @@ impl<'a> CanvasPaintThread<'a> {
             ) => {
                 let image_data = self
                     .canvas(canvas_id)
-                    .read_pixels(source_rect.to_u64(), image_size.to_u64());
+                    .read_pixels(Some(source_rect.to_u64()), Some(image_size.to_u64()));
                 self.canvas(other_canvas_id).draw_image(
-                    &image_data,
-                    source_rect.size,
+                    image_data.data(),
+                    source_rect.size.to_u64(),
                     dest_rect,
                     source_rect,
                     smoothing,
@@ -244,8 +245,10 @@ impl<'a> CanvasPaintThread<'a> {
                 self.canvas(canvas_id).set_global_composition(op)
             },
             Canvas2dMsg::GetImageData(dest_rect, canvas_size, sender) => {
-                let pixels = self.canvas(canvas_id).read_pixels(dest_rect, canvas_size);
-                sender.send(&pixels).unwrap();
+                let snapshot = self
+                    .canvas(canvas_id)
+                    .read_pixels(Some(dest_rect), Some(canvas_size));
+                sender.send(snapshot.as_ipc()).unwrap();
             },
             Canvas2dMsg::PutImageData(rect, receiver) => {
                 self.canvas(canvas_id)

components/canvas/raqote_backend.rs

@@ -630,7 +630,7 @@ impl GenericDrawTarget for raqote::DrawTarget {
         self.set_transform(matrix);
     }
     fn snapshot(&self) -> SourceSurface {
-        SourceSurface::Raqote(self.snapshot_data_owned())
+        SourceSurface::Raqote(self.snapshot_data().to_vec())
     }
     fn stroke(
         &mut self,
@@ -694,20 +694,9 @@ impl GenericDrawTarget for raqote::DrawTarget {
         );
     }
     #[allow(unsafe_code)]
-    fn snapshot_data(&self, f: &dyn Fn(&[u8]) -> Vec<u8>) -> Vec<u8> {
+    fn snapshot_data(&self) -> &[u8] {
         let v = self.get_data();
-        f(
-            unsafe {
-                std::slice::from_raw_parts(v.as_ptr() as *const u8, std::mem::size_of_val(v))
-            },
-        )
-    }
-    #[allow(unsafe_code)]
-    fn snapshot_data_owned(&self) -> Vec<u8> {
-        let v = self.get_data();
-        unsafe {
-            std::slice::from_raw_parts(v.as_ptr() as *const u8, std::mem::size_of_val(v)).into()
-        }
+        unsafe { std::slice::from_raw_parts(v.as_ptr() as *const u8, std::mem::size_of_val(v)) }
     }
 }
 

components/canvas/webgl_thread.rs

@@ -31,6 +31,7 @@ use glow::{
     bytes_per_type, components_per_format,
 };
 use half::f16;
+use ipc_channel::ipc::IpcSharedMemory;
 use log::{debug, error, trace, warn};
 use pixels::{self, PixelFormat, unmultiply_inplace};
 use surfman::chains::{PreserveBuffer, SwapChains, SwapChainsAPI};
@@ -1212,7 +1213,13 @@ impl WebGLImpl {
                         glow::PixelPackData::Slice(Some(&mut pixels)),
                     )
                 };
-                sender.send(&pixels).unwrap();
+                let alpha_mode = match (attributes.alpha, attributes.premultiplied_alpha) {
+                    (true, premultiplied) => snapshot::AlphaMode::Transparent { premultiplied },
+                    (false, _) => snapshot::AlphaMode::Opaque,
+                };
+                sender
+                    .send((IpcSharedMemory::from_bytes(&pixels), alpha_mode))
+                    .unwrap();
             },
             WebGLCommand::ReadPixelsPP(rect, format, pixel_type, offset) => unsafe {
                 gl.read_pixels(