Bug 1342571 - shrink dashed DrawTargetSkia::StrokeRect where possible. r=mstange

MozReview-Commit-ID: 7vu9j8jCwQn

gfx/2d/DrawTargetSkia.cpp

@@ -24,6 +24,7 @@
 #include "Logging.h"
 #include "Tools.h"
 #include "DataSurfaceHelpers.h"
+#include "PathHelpers.h"
 #include "Swizzle.h"
 #include <algorithm>
 
@@ -805,19 +806,99 @@ DrawTargetSkia::Stroke(const Path *aPath,
   mCanvas->drawPath(skiaPath->GetPath(), paint.mPaint);
 }
 
+static Float
+DashPeriodLength(const StrokeOptions& aStrokeOptions)
+{
+  Float length = 0;
+  for (size_t i = 0; i < aStrokeOptions.mDashLength; i++) {
+    length += aStrokeOptions.mDashPattern[i];
+  }
+  if (aStrokeOptions.mDashLength & 1) {
+    // "If an odd number of values is provided, then the list of values is
+    // repeated to yield an even number of values."
+    // Double the length.
+    length += length;
+  }
+  return length;
+}
+
+static inline Float
+RoundDownToMultiple(Float aValue, Float aFactor)
+{
+  return floorf(aValue / aFactor) * aFactor;
+}
+
+static Rect
+UserSpaceStrokeClip(const IntRect &aDeviceClip,
+                    const Matrix &aTransform,
+                    const StrokeOptions &aStrokeOptions)
+{
+  Matrix inverse = aTransform;
+  if (!inverse.Invert()) {
+    return Rect();
+  }
+  Rect deviceClip(aDeviceClip);
+  deviceClip.Inflate(MaxStrokeExtents(aStrokeOptions, aTransform));
+  return inverse.TransformBounds(deviceClip);
+}
+
+static Rect
+ShrinkClippedStrokedRect(const Rect &aStrokedRect, const IntRect &aDeviceClip,
+                         const Matrix &aTransform,
+                         const StrokeOptions &aStrokeOptions)
+{
+  Rect userSpaceStrokeClip =
+    UserSpaceStrokeClip(aDeviceClip, aTransform, aStrokeOptions);
+
+  Rect intersection = aStrokedRect.Intersect(userSpaceStrokeClip);
+  Float dashPeriodLength = DashPeriodLength(aStrokeOptions);
+  if (intersection.IsEmpty() || dashPeriodLength == 0.0f) {
+    return intersection;
+  }
+
+  // Reduce the rectangle side lengths in multiples of the dash period length
+  // so that the visible dashes stay in the same place.
+  Margin insetBy = aStrokedRect - intersection;
+  insetBy.top = RoundDownToMultiple(insetBy.top, dashPeriodLength);
+  insetBy.right = RoundDownToMultiple(insetBy.right, dashPeriodLength);
+  insetBy.bottom = RoundDownToMultiple(insetBy.bottom, dashPeriodLength);
+  insetBy.left = RoundDownToMultiple(insetBy.left, dashPeriodLength);
+
+  Rect shrunkRect = aStrokedRect;
+  shrunkRect.Deflate(insetBy);
+  return shrunkRect;
+}
+
 void
 DrawTargetSkia::StrokeRect(const Rect &aRect,
                            const Pattern &aPattern,
                            const StrokeOptions &aStrokeOptions,
                            const DrawOptions &aOptions)
 {
+  // Stroking large rectangles with dashes is expensive with Skia (fixed
+  // overhead based on the number of dashes, regardless of whether the dashes
+  // are visible), so we try to reduce the size of the stroked rectangle as
+  // much as possible before passing it on to Skia.
+  Rect rect = aRect;
+  if (aStrokeOptions.mDashLength > 0 && !rect.IsEmpty()) {
+    IntRect deviceClip(IntPoint(0, 0), mSize);
+    SkIRect clipBounds;
+    if (mCanvas->getClipDeviceBounds(&clipBounds)) {
+      deviceClip = SkIRectToIntRect(clipBounds);
+    }
+    rect = ShrinkClippedStrokedRect(rect, deviceClip, mTransform, aStrokeOptions);
+    if (rect.IsEmpty()) {
+      return;
+    }
+  }
+
   MarkChanged();
   AutoPaintSetup paint(mCanvas.get(), aOptions, aPattern);
   if (!StrokeOptionsToPaint(paint.mPaint, aStrokeOptions)) {
     return;
   }
 
-  mCanvas->drawRect(RectToSkRect(aRect), paint.mPaint);
+  mCanvas->drawRect(RectToSkRect(rect), paint.mPaint);
 }
 
 void

gfx/2d/HelpersSkia.h

@@ -297,6 +297,12 @@ IntRectToSkIRect(const IntRect& aRect)
   return SkIRect::MakeXYWH(aRect.x, aRect.y, aRect.width, aRect.height);
 }
 
+static inline IntRect
+SkIRectToIntRect(const SkIRect& aRect)
+{
+  return IntRect(aRect.x(), aRect.y(), aRect.width(), aRect.height());
+}
+
 static inline Point
 SkPointToPoint(const SkPoint &aPoint)
 {