Improve computation of ctmScale{X,Y} in NativeImageSkia::drawPattern

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 394 matching lines @@
     info.fAlphaType = kPremul_SkAlphaType;
 
     SkBitmap result;
     result.allocPixels(info);
     result.eraseColor(SK_ColorTRANSPARENT);
     bitmap.copyPixelsTo(reinterpret_cast<uint8_t*>(result.getPixels()), result.rowBytes() * result.height(), result.rowBytes());
 
     return result;
 }
 
+static SkVector computeScaleVector(const SkMatrix& matrix)
+{
+    SkScalar scaleX = matrix.getScaleX();
+    SkScalar scaleY = matrix.getScaleY();
+    SkScalar skewX = matrix.getSkewX();
+    SkScalar skewY = matrix.getSkewY();
+    return SkVector::Make(sqrt(scaleX * scaleX + skewY * skewY), sqrt(scaleY * scaleY + skewX * skewX));

[Stephen White, 2014/04/03 12:25:04]

This looks a lot like AffineTransform::xScale() and AffineTransform::yScale().

[fs, 2014/04/03 14:01:31]

That was the intention...

+}
+
 void NativeImageSkia::drawPattern(
     GraphicsContext* context,
     const FloatRect& floatSrcRect,
     const FloatSize& scale,
     const FloatPoint& phase,
     CompositeOperator compositeOp,
     const FloatRect& destRect,
     blink::WebBlendMode blendMode,
     const IntSize& repeatSpacing) const
 {
     FloatRect normSrcRect = floatSrcRect;
     normSrcRect.intersect(FloatRect(0, 0, bitmap().width(), bitmap().height()));
     if (destRect.isEmpty() || normSrcRect.isEmpty())
         return; // nothing to draw
 
     SkMatrix totalMatrix = context->getTotalMatrix();
-    SkScalar ctmScaleX = totalMatrix.getScaleX();
-    SkScalar ctmScaleY = totalMatrix.getScaleY();
+    SkVector ctmScale = computeScaleVector(totalMatrix);

[Stephen White, 2014/04/03 12:25:04]

Could this be

AffineTransform ctm = context->getCTM();
float ctmScaleX = ctm.xScale();
float ctmScaleY = ctm.yScale();

?

[fs, 2014/04/03 14:01:31]

...because I failed to find this one. Changed to use getCTM.

+    SkScalar ctmScaleX = ctmScale.x();
+    SkScalar ctmScaleY = ctmScale.y();
     totalMatrix.preScale(scale.width(), scale.height());
 
     // Figure out what size the bitmap will be in the destination. The
     // destination rect is the bounds of the pattern, we need to use the
     // matrix to see how big it will be.
     SkRect destRectTarget;
     totalMatrix.mapRect(&destRectTarget, normSrcRect);
 
     float destBitmapWidth = SkScalarToFloat(destRectTarget.width());
     float destBitmapHeight = SkScalarToFloat(destRectTarget.height());
@@ skipping 144 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