Bug 1360415 - Enable smoothing in canvas.drawImage when down-scaling, even with imageSmoothingEnabled=false. r=bzbarsky

dom/canvas/CanvasRenderingContext2D.cpp

@@ -5361,10 +5361,18 @@ CanvasRenderingContext2D::DrawImage(const CanvasImageSource& aImage,
     return;
   }
 
+  // Per spec, the smoothing setting applies only to scaling up a bitmap image.
+  // When down-scaling the user agent is free to choose whether or not to smooth
+  // the image. Nearest sampling when down-scaling is rarely desirable and
+  // smoothing when down-scaling matches chromium's behavior.
+  // If any dimension is up-scaled, we consider the image as being up-scaled.
+  auto scale = mTarget->GetTransform().ScaleFactors(true);
+  bool isDownScale = aDw * Abs(scale.width) < aSw && aDh * Abs(scale.height) < aSh;
+
   SamplingFilter samplingFilter;
   AntialiasMode antialiasMode;
 
-  if (CurrentState().imageSmoothingEnabled) {
+  if (CurrentState().imageSmoothingEnabled || isDownScale) {
     samplingFilter = gfx::SamplingFilter::LINEAR;
     antialiasMode = AntialiasMode::DEFAULT;
   } else {