Bug 1850871 - Resize canvas in ImageBitmapRenderingContext::TransferF…

…romImageBitmap.

This patch makes it so that when a new ImageBitmap is provided to an
ImageBitmapRenderingContext, we resize its owning canvas rather than
crop or scale the given ImageBitmap to fit inside the canvas dimensions.

See discussion in whatwg/html#7833.

Differential Revision: https://phabricator.services.mozilla.com/D188126

dom/canvas/ImageBitmapRenderingContext.cpp

@@ -80,6 +80,18 @@ void ImageBitmapRenderingContext::TransferFromImageBitmap(
       return;
     }
 
+    // Note that this is reentrant and will call back into SetDimensions.
+    if (mCanvasElement) {
+      mCanvasElement->SetSize(mImage->GetSize(), aRv);
+    } else if (mOffscreenCanvas) {
+      mOffscreenCanvas->SetSize(mImage->GetSize(), aRv);
+    }
+
+    if (NS_WARN_IF(aRv.Failed())) {
+      mImage = nullptr;
+      return;
+    }
+
     if (aImageBitmap->IsWriteOnly()) {
       if (mCanvasElement) {
         mCanvasElement->SetWriteOnly();
@@ -96,6 +108,16 @@ NS_IMETHODIMP
 ImageBitmapRenderingContext::SetDimensions(int32_t aWidth, int32_t aHeight) {
   mWidth = aWidth;
   mHeight = aHeight;
+
+  if (mOffscreenCanvas) {
+    OffscreenCanvasDisplayData data;
+    data.mSize = {mWidth, mHeight};
+    data.mIsOpaque = GetIsOpaque();
+    data.mIsAlphaPremult = true;
+    data.mDoPaintCallbacks = false;
+    mOffscreenCanvas->UpdateDisplayData(data);
+  }
+
   return NS_OK;
 }
 

dom/canvas/OffscreenCanvas.cpp

@@ -147,6 +147,23 @@ void OffscreenCanvas::SetHeight(uint32_t aHeight, ErrorResult& aRv) {
   CanvasAttrChanged();
 }
 
+void OffscreenCanvas::SetSize(const nsIntSize& aSize, ErrorResult& aRv) {
+  if (mNeutered) {
+    aRv.ThrowInvalidStateError(
+        "Cannot set dimensions of placeholder canvas transferred to worker.");
+    return;
+  }
+
+  if (NS_WARN_IF(aSize.IsEmpty())) {
+    aRv.ThrowRangeError("OffscreenCanvas size is empty, must be non-empty.");
+    return;
+  }
+
+  mWidth = aSize.width;
+  mHeight = aSize.height;
+  CanvasAttrChanged();
+}
+
 void OffscreenCanvas::GetContext(
     JSContext* aCx, const OffscreenRenderingContextId& aContextId,
     JS::Handle<JS::Value> aContextOptions,

dom/canvas/OffscreenCanvas.h

@@ -150,6 +150,8 @@ class OffscreenCanvas final : public DOMEventTargetHelper,
 
   bool MayNeuter() const { return !mNeutered && !mCurrentContext; }
 
+  void SetSize(const nsIntSize& aSize, ErrorResult& aRv);
+
   nsIPrincipal* GetExpandedReader() const { return mExpandedReader; }
 
   void SetWriteOnly(RefPtr<nsIPrincipal>&& aExpandedReader);

dom/canvas/test/test_bitmaprenderer.html

@@ -43,11 +43,11 @@
   ctx.fillStyle = "#00FF00";
   ctx.fillRect(0, 0, canvasWidth, canvasHeight);
 
-  var canvasRef = createCanvas(90, 90);
+  var canvasRef = createCanvas(canvasWidth, canvasHeight);
   var ctx = canvasRef.getContext("2d");
   // Clear with black transparent first
   ctx.fillStyle = "rgba(0, 0, 0, 0)";
-  ctx.fillRect(0, 0, 90, 90);
+  ctx.fillRect(0, 0, canvasWidth, canvasHeight);
 
   ctx.fillStyle = "#00FF00";
   ctx.fillRect(0, 0, canvasWidth, canvasHeight);
@@ -89,52 +89,53 @@
   });
 }
 
-function scaleTest() {
-  var canvas1 = createCanvas(64, 64);
-  var ctx = canvas1.getContext("2d");
+function createSolidGreenBitmap(width, height) {
+  const canvas = createCanvas(width, height);
+  const ctx = canvas.getContext("2d");
   ctx.fillStyle = "#00FF00";
-  ctx.fillRect(0, 0, 64, 64);
-
-  var canvas2 = createCanvas(64, 64);
-  var ctx2 = canvas2.getContext("2d");
-  ctx2.fillStyle = "#00FF00";
-  ctx2.fillRect(0, 0, 64, 64);
+  ctx.fillRect(0, 0, width, height);
+  const bitmap = createImageBitmap(canvas);
+  document.body.removeChild(canvas);
+  return bitmap;
+}
 
-  var p1 = createImageBitmap(canvas1);
-  var p2 = createImageBitmap(canvas2);
-  Promise.all([p1, p2]).then(async function(bitmaps) {
-    document.body.removeChild(canvas1);
-    document.body.removeChild(canvas2);
+async function scaleTestCase(name, refWidth, refHeight, testWidth, testHeight, canvasWidth, canvasHeight) {
+  const refBitmap = await createSolidGreenBitmap(refWidth, refHeight);
+  const refCanvas = createCanvas(refWidth, refHeight);
+  const refCtx = refCanvas.getContext("bitmaprenderer");
+  refCtx.transferFromImageBitmap(refBitmap);
+  is(refCanvas.width, refWidth, name + ": canvas width " + refCanvas.width + " is reference " + refWidth);
+  is(refCanvas.height, refHeight, name + ": canvas height " + refCanvas.height + " is reference " + refHeight);
+  const refSnapshot = await snapshotWindow(window);
+  document.body.removeChild(refCanvas);
+
+  const bitmap = await createSolidGreenBitmap(testWidth, testHeight);
+  const canvas = createCanvas(canvasWidth, canvasHeight);
+  const ctx = canvas.getContext("bitmaprenderer");
+  ctx.transferFromImageBitmap(bitmap);
+  is(canvas.width, testWidth, name + ": canvas width " + canvas.width + " is bitmap " + testWidth);
+  is(canvas.height, testHeight, name + ": canvas height " + canvas.height + " is bitmap " + testHeight);
+  if (refWidth !== testWidth) {
+    canvas.width = refWidth;
+  }
+  if (refHeight !== testHeight) {
+    canvas.height = refHeight;
+  }
+  const snapshot = await snapshotWindow(window);
+  document.body.removeChild(canvas);
+
+  const results = compareSnapshots(snapshot, refSnapshot, true);
+  ok(results[0], name + ": screenshots should be the same");
+  return Promise.resolve();
+}
 
-    // Create a large canvas then shrink.
-    var canvas3 = createCanvas(128, 128);
-    var ctx3 = canvas3.getContext("bitmaprenderer");
-    ctx3.transferFromImageBitmap(bitmaps[0]);
-    var snapshotLargeRef = await snapshotWindow(window);
-
-    canvas3.width = 32;
-    canvas3.height = 32;
-    var snapshotSmall = await snapshotWindow(window);
-    document.body.removeChild(canvas3);
-
-    // Create a small canvas then grow.
-    var canvas4 = createCanvas(32, 32);
-    var ctx4 = canvas4.getContext("bitmaprenderer");
-    ctx4.transferFromImageBitmap(bitmaps[1]);
-    var snapshotSmallRef = await snapshotWindow(window);
-
-    canvas4.width = 128;
-    canvas4.height = 128;
-    var snapshotLarge = await snapshotWindow(window);
-    document.body.removeChild(canvas4);
-
-    var resultsLarge = compareSnapshots(snapshotLarge, snapshotLargeRef, true);
-    ok(resultsLarge[0], "Screenshots should be the same");
-
-    var resultsSmall = compareSnapshots(snapshotSmall, snapshotSmallRef, true);
-    ok(resultsSmall[0], "Screenshots should be the same");
-    runTestOnWorker();
-  });
+async function scaleTest() {
+  await scaleTestCase("grow_unscaled", 64, 64, 64, 64, 32, 32); // Canvas grows, no scaling.
+  await scaleTestCase("grow_downscaled", 64, 64, 128, 128, 32, 32); // Canvas grows, scales down.
+  await scaleTestCase("same_downscaled", 64, 64, 128, 128, 128, 128); // Canvas unchanged, scales down.
+  await scaleTestCase("shrink_unscaled", 64, 64, 64, 64, 128, 128); // Canvas shrinks, no scaling.
+  await scaleTestCase("shrink_downscaled", 64, 64, 128, 128, 256, 256); // Canvas shrinks, scales down.
+  runTestOnWorker();
 }
 
 function runTestOnWorker() {

dom/html/HTMLCanvasElement.cpp

@@ -1095,6 +1095,26 @@ void HTMLCanvasElement::SetHeight(uint32_t aHeight, ErrorResult& aRv) {
   SetUnsignedIntAttr(nsGkAtoms::height, aHeight, DEFAULT_CANVAS_HEIGHT, aRv);
 }
 
+void HTMLCanvasElement::SetSize(const nsIntSize& aSize, ErrorResult& aRv) {
+  if (mOffscreenCanvas) {
+    aRv.ThrowInvalidStateError(
+        "Cannot set width of placeholder canvas transferred to "
+        "OffscreenCanvas.");
+    return;
+  }
+
+  if (NS_WARN_IF(aSize.IsEmpty())) {
+    aRv.ThrowRangeError("Canvas size is empty, must be non-empty.");
+    return;
+  }
+
+  SetUnsignedIntAttr(nsGkAtoms::width, aSize.width, DEFAULT_CANVAS_WIDTH, aRv);
+  MOZ_ASSERT(!aRv.Failed());
+  SetUnsignedIntAttr(nsGkAtoms::height, aSize.height, DEFAULT_CANVAS_HEIGHT,
+                     aRv);
+  MOZ_ASSERT(!aRv.Failed());
+}
+
 void HTMLCanvasElement::FlushOffscreenCanvas() {
   if (mOffscreenDisplay) {
     mOffscreenDisplay->FlushForDisplay();

dom/html/HTMLCanvasElement.h

@@ -175,6 +175,11 @@ class HTMLCanvasElement final : public nsGenericHTMLElement,
    */
   nsIntSize GetSize();
 
+  /**
+   * Set the size in pixels of this canvas element.
+   */
+  void SetSize(const nsIntSize& aSize, ErrorResult& aRv);
+
   /**
    * Determine whether the canvas is write-only.
    */