fix TriColorShader to respect the paint's alpha

src/core/SkDraw.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDraw.h"
 #include "SkBlitter.h"
 #include "SkBounder.h"
@@ skipping 2414 matching lines @@
 SkTriColorShader::TriColorShaderContext::TriColorShaderContext(const SkTriColorShader& shader,
                                                                const ContextRec& rec)
     : INHERITED(shader, rec) {}
 
 SkTriColorShader::TriColorShaderContext::~TriColorShaderContext() {}
 
 size_t SkTriColorShader::contextSize() const {
     return sizeof(TriColorShaderContext);
 }
 void SkTriColorShader::TriColorShaderContext::shadeSpan(int x, int y, SkPMColor dstC[], int count) {
+    const unsigned alpha = this->getPaintAlpha();
+    const int alphaScale = alpha + (alpha >> 7);

[scroggo, 2014/05/05 21:15:02]

From the comments in SkColorPriv, it looks like this is the more accurate
version of SkAlpha255To256; would it make sense to provide both as helpers?

[reed1, 2014/05/05 21:25:15]

Done.

+
     SkPoint src;
 
     for (int i = 0; i < count; i++) {
         fDstToUnit.mapXY(SkIntToScalar(x), SkIntToScalar(y), &src);
         x += 1;
 
         int scale1 = ScalarTo256(src.fX);
         int scale2 = ScalarTo256(src.fY);
         int scale0 = 256 - scale1 - scale2;
         if (scale0 < 0) {
             if (scale1 > scale2) {
                 scale2 = 256 - scale1;
             } else {
                 scale1 = 256 - scale2;
             }
             scale0 = 0;
         }
 
+        if (256 != alphaScale) {
+            scale0 = (scale0 * alphaScale) >> 8;

[scroggo, 2014/05/05 21:15:02]

Why not use SkAlphaMul?

[reed1, 2014/05/05 21:25:15]

Done.

+            scale1 = (scale1 * alphaScale) >> 8;
+            scale2 = (scale2 * alphaScale) >> 8;
+        }
+
         dstC[i] = SkAlphaMulQ(fColors[0], scale0) +
-        SkAlphaMulQ(fColors[1], scale1) +
-        SkAlphaMulQ(fColors[2], scale2);
+                  SkAlphaMulQ(fColors[1], scale1) +
+                  SkAlphaMulQ(fColors[2], scale2);
     }
 }
 
 #ifndef SK_IGNORE_TO_STRING
 void SkTriColorShader::toString(SkString* str) const {
     str->append("SkTriColorShader: (");
 
     this->INHERITED::toString(str);
 
     str->append(")");
@@ skipping 358 matching lines @@
         mask->fImage = SkMask::AllocImage(size);
         memset(mask->fImage, 0, mask->computeImageSize());
     }
 
     if (SkMask::kJustComputeBounds_CreateMode != mode) {
         draw_into_mask(*mask, devPath, style);
     }
 
     return true;
 }