Draw thin slices of an image w/ anti-aliasing for 2D <canvas>

Source/platform/graphics/skia/NativeImageSkia.cpp

Index: Source/platform/graphics/skia/NativeImageSkia.cpp
diff --git a/Source/platform/graphics/skia/NativeImageSkia.cpp b/Source/platform/graphics/skia/NativeImageSkia.cpp
index f7e70af0ad4d05d15b9903954a24f59e89866f85..f7cc275f84297728558edc9c75acdbe341d01294 100644
--- a/Source/platform/graphics/skia/NativeImageSkia.cpp
+++ b/Source/platform/graphics/skia/NativeImageSkia.cpp
@@ -332,9 +332,22 @@ SkBitmap NativeImageSkia::resizedBitmap(const SkISize& scaledImageSize, const Sk
     return resizedSubset;
 }
 
-static bool hasNon90rotation(GraphicsContext* context)
+static bool shouldDrawAntiAliased(GraphicsContext* context, const SkRect& destRect)
 {
-    return !context->getTotalMatrix().rectStaysRect();
+    const SkMatrix totalMatrix = context->getTotalMatrix();
+    // Enable anti-aliasing if we're rotated or skewed.
+    if (!totalMatrix.rectStaysRect())
+        return true;
+    // Check if the dimensions of the destination are "small" (less than one
+    // destination pixel). To prevent sudden drop-outs. Since we know that
+    // kRectStaysRect_Mask is set, we can use the kAffine_Mask flag to select
+    // the relevant components of the matrix to compute the transformed vector.

[Justin Novosad, 2014/03/25 14:35:07]

Reformulate for clarity. Suggestion : Since we know that kRectStaysRect_Mask is
set, the matrix either has scale and no skew or vice versa. We can query the
kAffine_Mask flag to determine which case it is.

+    SkScalar widthExpansion, heightExpansion;
+    if (totalMatrix.getType() & SkMatrix::kAffine_Mask)
+        widthExpansion = totalMatrix[SkMatrix::kMSkewY], heightExpansion = totalMatrix[SkMatrix::kMSkewX];
+    else
+        widthExpansion = totalMatrix[SkMatrix::kMScaleX], heightExpansion = totalMatrix[SkMatrix::kMScaleY];
+    return destRect.width() * fabs(widthExpansion) < 1 || destRect.height() * fabs(heightExpansion) < 1;
 }
 
 void NativeImageSkia::draw(GraphicsContext* context, const SkRect& srcRect, const SkRect& destRect, PassRefPtr<SkXfermode> compOp) const
@@ -345,8 +358,7 @@ void NativeImageSkia::draw(GraphicsContext* context, const SkRect& srcRect, cons
     paint.setColorFilter(context->colorFilter());
     paint.setAlpha(context->getNormalizedAlpha());
     paint.setLooper(context->drawLooper());
-    // only antialias if we're rotated or skewed
-    paint.setAntiAlias(hasNon90rotation(context));
+    paint.setAntiAlias(shouldDrawAntiAliased(context, destRect));
 
     ResamplingMode resampling;
     if (context->isAccelerated()) {