Change filter quality when scaling-down in drawImage

third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp

Index: third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp
diff --git a/third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp b/third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp
index 2029de6d7d7f812aa189ca4529e5031003f1e253..edebc44d401d88710cace0a853989b5b072d6eaa 100644
--- a/third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp
+++ b/third_party/WebKit/Source/modules/canvas2d/BaseRenderingContext2D.cpp
@@ -941,6 +941,11 @@ bool BaseRenderingContext2D::shouldDrawImageAntialiased(const FloatRect& destRec
     return destRect.width() * fabs(widthExpansion) < 1 || destRect.height() * fabs(heightExpansion) < 1;
 }
 
+static bool isDrawScalingDown(const FloatRect& srcRect, const FloatRect& dstRect, double xScale, double yScale)
+{
+    return dstRect.width() * xScale < srcRect.width() && dstRect.height() * yScale < srcRect.height();

[Justin Novosad, 2016/07/20 19:43:56]

I had an idea: you can optimize out the expensive sqrt call inside xScale/yScale
by squaring both sides of the inequality.
The expression becomes: dstRect.width() * dstRect.width() * xScaleSquared <
srcRect.width() * srcRect.width()
Then you do the same for height.

The rects are stored in floats and the transform is in double.  The way it is
written now, the floats will be promoted to double in the expression. We wight
as well do all the arithmetic in float (like in the rest of the canvas
implementation), so you should make the args float.

[xidachen, 2016/07/20 20:31:33]

Done.

+}
+
 void BaseRenderingContext2D::drawImageInternal(SkCanvas* c, CanvasImageSource* imageSource, Image* image, const FloatRect& srcRect, const FloatRect& dstRect, const SkPaint* paint)
 {
     trackDrawCall(DrawImage);
@@ -972,6 +977,9 @@ void BaseRenderingContext2D::drawImageInternal(SkCanvas* c, CanvasImageSource* i
         imagePaint.setImageFilter(nullptr);
     }
 
+    if (!imageSmoothingEnabled() && isDrawScalingDown(srcRect, dstRect, state().transform().xScale(), state().transform().yScale()))
+        imagePaint.setFilterQuality(kLow_SkFilterQuality);
+
     if (!imageSource->isVideoElement()) {
         imagePaint.setAntiAlias(shouldDrawImageAntialiased(dstRect));
         image->draw(c, imagePaint, dstRect, srcRect, DoNotRespectImageOrientation, Image::DoNotClampImageToSourceRect);