Screenspace AA tessellated GPU path rendering.

This is an approach to antialiased concave path rendering
on the GPU without using MSAA. It uses GrTessellator to
extract boundary contours from the given path, then
inflates by half a pixel in screen space each direction,
then renders the result with zero alpha on the outer
contour and one alpha on in the inner contour. This
requires two passes through the tessellation code: one
to extract the boundaries, then one to tessellate the
result.

This gives approximately a 3X improvement on the IE
chalkboard demo in non-MSAA mode, a 30-40% improvement
on MotionMark's "Fill Paths", and a ~3X improvement on
MotionMark's "canvas arcTo segments".

It works best for large, simple paths, so there's currently
a limit of 10 verbs in the onCanDrawPath() check. This
dovetails nicely with the distance field path renderer's
support for small, detailed (and cached) paths.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1152733009

NOTRY=true

Review-Url: https://codereview.chromium.org/1152733009

1 files changed, 9 insertions, 3 deletions

diff --git a/src/gpu/GrTessellator.h b/src/gpu/GrTessellator.h
index 7cc4311aa5..dd92015cc2 100644
--- a/src/gpu/GrTessellator.h
+++ b/src/gpu/GrTessellator.h
@@ -8,6 +8,7 @@
 #ifndef GrTessellator_DEFINED
 #define GrTessellator_DEFINED
 
+#include "GrColor.h"
 #include "SkPoint.h"
 
 class SkPath;
@@ -23,9 +24,13 @@ namespace GrTessellator {
 
 class VertexAllocator {
 public:
+    VertexAllocator(size_t stride) : fStride(stride) {}
     virtual ~VertexAllocator() {}
-    virtual SkPoint* lock(int vertexCount) = 0;
+    virtual void* lock(int vertexCount) = 0;
     virtual void unlock(int actualCount) = 0;
+    size_t stride() const { return fStride; }
+private:
+    size_t fStride;
 };
 
 struct WindingVertex {
@@ -40,8 +45,9 @@ struct WindingVertex {
 int PathToVertices(const SkPath& path, SkScalar tolerance, const SkRect& clipBounds,
                    WindingVertex** verts);
 
-int PathToTriangles(const SkPath& path, SkScalar tolerance, const SkRect& clipBounds,
-                    VertexAllocator*, bool *isLinear);
+int PathToTriangles(const SkPath& path, SkScalar tolerance, const SkRect& clipBounds, 
+                    VertexAllocator*, bool antialias, const GrColor& color,
+                    bool canTweakAlphaForCoverage, bool *isLinear);
 }
 
 #endif