Implement cc::FilterOperations::MapRect.

cc/output/filter_operation.cc

Index: cc/output/filter_operation.cc
diff --git a/cc/output/filter_operation.cc b/cc/output/filter_operation.cc
index a37d27d3b2b9728e896f9327c65d4a80dff5e7d2..c8afb679f595fda0dc0da44e8e4dd4b1d534645e 100644
--- a/cc/output/filter_operation.cc
+++ b/cc/output/filter_operation.cc
@@ -11,6 +11,9 @@
 #include "cc/base/math_util.h"
 #include "cc/output/filter_operation.h"
 #include "ui/gfx/animation/tween.h"
+#include "ui/gfx/geometry/rect.h"
+#include "ui/gfx/geometry/rect_conversions.h"
+#include "ui/gfx/skia_util.h"
 
 namespace cc {
 
@@ -317,4 +320,40 @@ void FilterOperation::AsValueInto(base::trace_event::TracedValue* value) const {
   }
 }
 
+static int SpreadForStdDeviation(float std_deviation) {
+  // https://dvcs.w3.org/hg/FXTF/raw-file/tip/filters/index.html#feGaussianBlurElement
+  // provides this approximation for evaluating a gaussian blur by a triple box
+  // filter.
+  float d = floorf(std_deviation * 3.f * sqrt(8.f * atan(1.f)) / 4.f + 0.5f);
+  return static_cast<int>(ceilf(d * 3.f / 2.f));
+}
+
+gfx::Rect FilterOperation::MapRect(const gfx::Rect& rect) const {
+  switch (type_) {
+    case FilterOperation::BLUR: {
+      int spread = SpreadForStdDeviation(amount());
+      gfx::Rect result = rect;
+      result.Inset(-spread, -spread, -spread, -spread);
+      return result;
+    }
+    case FilterOperation::DROP_SHADOW: {
+      int spread = SpreadForStdDeviation(amount());
+      gfx::Rect result = rect;
+      result.Inset(-spread, -spread, -spread, -spread);
+      result += drop_shadow_offset().OffsetFromOrigin();
+      result.Union(rect);
+      return result;
+    }
+    case FilterOperation::REFERENCE: {
+      if (!image_filter())
+        return rect;
+      SkIRect in_rect = gfx::RectToSkIRect(rect);
+      SkIRect out_rect = image_filter()->filterBounds(in_rect, SkMatrix::I());

[Stephen White, 2016/04/11 18:05:00]

SkImageFilter::filterBounds() defaults to the reverse mapping, but your comment
in the header states that this function is intended to do forward mapping. Which
is correct?

[jbroman, 2016/04/11 20:21:52]

Good catch. What's troubling to me is that GetOutsets seems to be used
principally for forward mapping, but GetOutsets called
SkImageFilter::filterBounds this way (i.e. with reverse mapping), which is why
my unit test passed. (I only checked that it matched what GetOutsets was doing).

[Stephen White, 2016/04/11 20:56:04]

Yeah, I think this was an existing bug. IIRC, this flavour of getOutsets() was
mostly used for background filter. I'm guessing there isn't a layout test that
would show the bug. We should probably add one.

+      return gfx::SkIRectToRect(out_rect);
+    }
+    default:
+      return rect;
+  }
+}
+
 }  // namespace cc