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

Source/platform/graphics/skia/NativeImageSkia.cpp

 /*
  * Copyright (c) 2008, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 314 matching lines @@
 
         m_resizedImage = resizedImage;
     }
 
     SkBitmap resizedSubset;
     SkIRect resizedSubsetRect = m_cachedImageInfo.rectInSubset(scaledImageSubset);
     m_resizedImage.extractSubset(&resizedSubset, resizedSubsetRect);
     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
 {
     TRACE_EVENT0("skia", "NativeImageSkia::draw");
     SkPaint paint;
     paint.setXfermode(compOp.get());
     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()) {
         resampling = LinearResampling;
     } else if (context->printing()) {
         resampling = NoResampling;
     } else {
         // Take into account scale applied to the canvas when computing sampling mode (e.g. CSS scale or page scale).
         SkRect destRectTarget = destRect;
         SkMatrix totalMatrix = context->getTotalMatrix();
@@ skipping 227 matching lines @@
 SkIRect NativeImageSkia::ImageResourceInfo::rectInSubset(const SkIRect& otherScaledImageSubset)
 {
     if (!scaledImageSubset.contains(otherScaledImageSubset))
         return SkIRect::MakeEmpty();
     SkIRect subsetRect = otherScaledImageSubset;
     subsetRect.offset(-scaledImageSubset.x(), -scaledImageSubset.y());
     return subsetRect;
 }
 
 } // namespace WebCore