Clarify when tweak alpha for coverage optimizaton can occur.

Also refer to it directly rather than using the term "modulate"

Change-Id: Ifa44a4d46e1be11b567943f58ead24e38f10d03b
Reviewed-on: https://skia-review.googlesource.com/8488
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>

2 files changed, 16 insertions, 7 deletions

diff --git a/include/gpu/GrFragmentProcessor.h b/include/gpu/GrFragmentProcessor.h
index 96ca1c830d..55dc08f7a0 100644
--- a/include/gpu/GrFragmentProcessor.h
+++ b/include/gpu/GrFragmentProcessor.h
@@ -99,11 +99,20 @@ public:
     }
 
     /**
-     * True if the processor's output is a modulation of its input color or alpha with a computed
-     * premultiplied color or alpha in the 0..1 range. If true and the blend mode allows it we may
-     * fold coverage into the first color fragment processor's input.
+     * A GrDrawOp may premultiply its antialiasing coverage into its GrGeometryProcessor's color
+     * output under the following scenario:
+     *   * all the color fragment processors report true to this query,
+     *   * all the coverage fragment processors report true to this query,
+     *   * the blend mode arithmetic allows for it it.
+     * To be compatible a fragment processor's output must be a modulation of its input color or
+     * alpha with a computed premultiplied color or alpha that is in 0..1 range. The computed color
+     * or alpha that is modulated against the input cannot depend on the input's alpha. The computed
+     * value cannot depend on the input's color channels unless it unpremultiplies the input color
+     * channels by the input alpha.
      */
-    bool modulatesInput() const { return SkToBool(fFlags & kModulatesInput_OptimizationFlag); }
+    bool compatibleWithCoverageAsAlpha() const {
+        return SkToBool(fFlags & kCompatibleWithCoverageAsAlpha_OptimizationFlag);
+    }
 
     /**
      * If this is true then all opaque input colors to the processor produce opaque output colors.
@@ -207,10 +216,10 @@ public:
 protected:
     enum OptimizationFlags : uint32_t {
         kNone_OptimizationFlags,
-        kModulatesInput_OptimizationFlag = 0x1,
+        kCompatibleWithCoverageAsAlpha_OptimizationFlag = 0x1,
         kPreservesOpaqueInput_OptimizationFlag = 0x2,
         kConstantOutputForConstantInput_OptimizationFlag = 0x4,
-        kAll_OptimizationFlags = kModulatesInput_OptimizationFlag |
+        kAll_OptimizationFlags = kCompatibleWithCoverageAsAlpha_OptimizationFlag |
                                  kPreservesOpaqueInput_OptimizationFlag |
                                  kConstantOutputForConstantInput_OptimizationFlag
     };
diff --git a/include/gpu/effects/GrConstColorProcessor.h b/include/gpu/effects/GrConstColorProcessor.h
index 44e8cb17fc..b543aa5069 100644
--- a/include/gpu/effects/GrConstColorProcessor.h
+++ b/include/gpu/effects/GrConstColorProcessor.h
@@ -46,7 +46,7 @@ private:
     static OptimizationFlags OptFlags(GrColor4f color, InputMode mode) {
         OptimizationFlags flags = kConstantOutputForConstantInput_OptimizationFlag;
         if (mode != kIgnore_InputMode) {
-            flags |= kModulatesInput_OptimizationFlag;
+            flags |= kCompatibleWithCoverageAsAlpha_OptimizationFlag;
         }
         if (color.isOpaque()) {
             flags |= kPreservesOpaqueInput_OptimizationFlag;