Add rendering fast path for when playing a display list canvas at 1:1 scale

Source/platform/graphics/GraphicsContext.cpp

Index: Source/platform/graphics/GraphicsContext.cpp
diff --git a/Source/platform/graphics/GraphicsContext.cpp b/Source/platform/graphics/GraphicsContext.cpp
index 6a237098cdcb61b57229a281df6ce90c46aad1ad..fb1d974483acbfb6b5bc01007b337a6883c78e7c 100644
--- a/Source/platform/graphics/GraphicsContext.cpp
+++ b/Source/platform/graphics/GraphicsContext.cpp
@@ -1125,6 +1125,15 @@ void GraphicsContext::drawImageBuffer(ImageBuffer* image, const FloatRect& dest,
     image->draw(this, dest, src, op, blendMode);
 }
 
+static inline bool pictureScaleIsApproximatelyOne(float x)
+{
+    // Function used to validate canvas scale factor.
+    // Numerical error should not reach 6th decimal except for highly degenerate cases,
+    // and effect of 6th decimal on scale is negligible over max span of a skia canvas
+    // which is 32k pixels.
+    return x > 0.999999 && x < 1.000001;

[Stephen White, 2014/10/16 17:38:00]

Could you put this in a constant at the start of the file (e.g.,
kPictureScaleEpsilon), and use fabsf(x - 1.0f) < kPictureScaleEpsilon? Would
that work?

+}
+
 void GraphicsContext::drawPicture(PassRefPtr<SkPicture> picture, const FloatRect& dest, const FloatRect& src, CompositeOperator op, WebBlendMode blendMode)
 {
     ASSERT(m_canvas);
@@ -1134,22 +1143,35 @@ void GraphicsContext::drawPicture(PassRefPtr<SkPicture> picture, const FloatRect
     SkMatrix ctm = m_canvas->getTotalMatrix();
     SkRect deviceDest;
     ctm.mapRect(&deviceDest, dest);
-    SkRect sourceBounds = WebCoreFloatRectToSKRect(src);
+    float scaleX = deviceDest.width() / src.width();
+    float scaleY = deviceDest.height() / src.height();
 
-    RefPtr<SkPictureImageFilter> pictureFilter = adoptRef(SkPictureImageFilter::Create(picture.get(), sourceBounds));
-    SkMatrix layerScale;
-    layerScale.setScale(deviceDest.width() / src.width(), deviceDest.height() / src.height());
-    RefPtr<SkMatrixImageFilter> matrixFilter = adoptRef(SkMatrixImageFilter::Create(layerScale, SkPaint::kLow_FilterLevel, pictureFilter.get()));
     SkPaint picturePaint;
     picturePaint.setXfermodeMode(WebCoreCompositeToSkiaComposite(op, blendMode));
-    picturePaint.setImageFilter(matrixFilter.get());
-    SkRect layerBounds = SkRect::MakeWH(std::max(deviceDest.width(), sourceBounds.width()), std::max(deviceDest.height(), sourceBounds.height()));
-    m_canvas->save();
-    m_canvas->resetMatrix();
-    m_canvas->translate(deviceDest.x(), deviceDest.y());
-    m_canvas->saveLayer(&layerBounds, &picturePaint);
-    m_canvas->restore();
-    m_canvas->restore();
+    SkRect sourceBounds = WebCoreFloatRectToSKRect(src);
+    if (pictureScaleIsApproximatelyOne(scaleX * m_deviceScaleFactor) && pictureScaleIsApproximatelyOne(scaleY * m_deviceScaleFactor)) {

[Stephen White, 2014/10/16 17:38:00]

I'm not certain, but I think that using the device scale factor at record time
may be problematic for devices which have both hiDPI and loDPI displays (e.g., a
retina MBP connected to a loDPI display), since we don't know which device will
be used at playback.

Anyway, not enough to block this change, but something to watch out for.

+        // Fast path for canvases that are rasterized at screen resolution
+        SkRect skBounds = WebCoreFloatRectToSKRect(dest);
+        m_canvas->saveLayer(&skBounds, &picturePaint);
+        SkMatrix pictureTransform;
+        pictureTransform.setRectToRect(sourceBounds, skBounds, SkMatrix::kFill_ScaleToFit);
+        m_canvas->concat(pictureTransform);
+        m_canvas->drawPicture(picture.get());
+        m_canvas->restore();
+    } else {
+        RefPtr<SkPictureImageFilter> pictureFilter = adoptRef(SkPictureImageFilter::Create(picture.get(), sourceBounds));
+        SkMatrix layerScale;
+        layerScale.setScale(scaleX, scaleY);
+        RefPtr<SkMatrixImageFilter> matrixFilter = adoptRef(SkMatrixImageFilter::Create(layerScale, SkPaint::kLow_FilterLevel, pictureFilter.get()));
+        picturePaint.setImageFilter(matrixFilter.get());
+        SkRect layerBounds = SkRect::MakeWH(std::max(deviceDest.width(), sourceBounds.width()), std::max(deviceDest.height(), sourceBounds.height()));
+        m_canvas->save();
+        m_canvas->resetMatrix();
+        m_canvas->translate(deviceDest.x(), deviceDest.y());
+        m_canvas->saveLayer(&layerBounds, &picturePaint);
+        m_canvas->restore();
+        m_canvas->restore();
+    }
 }
 
 void GraphicsContext::writePixels(const SkImageInfo& info, const void* pixels, size_t rowBytes, int x, int y)