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..b39e84b852890a359305e4d9bda78982e1d79a6f 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)
+{
+    return dstRect.width() < srcRect.width() && dstRect.height() < srcRect.height();
+}
+
 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, state().transform().mapRect(dstRect)))

[Justin Novosad, 2016/07/20 17:36:36]

Using mapRect is not correct.
For example if you scale by 0.9 and rotate by 45deg.  The image is being
scaled-down, but map rect s going to give you the bounding box of the tranformed
rect, which will be bigger than the original rect by a factor of 0.9*sqrt(2) =
1.27.

[xidachen, 2016/07/20 19:13:49]

Done.

+        imagePaint.setFilterQuality(kLow_SkFilterQuality);
+
     if (!imageSource->isVideoElement()) {
         imagePaint.setAntiAlias(shouldDrawImageAntialiased(dstRect));
         image->draw(c, imagePaint, dstRect, srcRect, DoNotRespectImageOrientation, Image::DoNotClampImageToSourceRect);