revise row blits to keep intermediate precision so that color is preserved when blended against its…

src/core/SkBlitRow_D32.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBlitRow.h"
 #include "SkBlitMask.h"
 #include "SkColorPriv.h"
 #include "SkOpts.h"
 #include "SkUtils.h"
 
 #define UNROLL
 
 static void S32_Opaque_BlitRow32(SkPMColor* SK_RESTRICT dst,
                                  const SkPMColor* SK_RESTRICT src,
                                  int count, U8CPU alpha) {
     SkASSERT(255 == alpha);
     memcpy(dst, src, count * 4);
 }
 
 static void S32_Blend_BlitRow32(SkPMColor* SK_RESTRICT dst,
                                 const SkPMColor* SK_RESTRICT src,
                                 int count, U8CPU alpha) {
     SkASSERT(alpha <= 255);
     if (count > 0) {
         unsigned src_scale = SkAlpha255To256(alpha);
-        unsigned dst_scale = 256 - src_scale;
 
 #ifdef UNROLL
         if (count & 1) {
-            *dst = SkAlphaMulQ(*(src++), src_scale) + SkAlphaMulQ(*dst, dst_scale);
+            *dst = SkPMLerp(*src, *dst, src_scale);
+            src += 1;
             dst += 1;
             count -= 1;
         }
 
         const SkPMColor* SK_RESTRICT srcEnd = src + count;
         while (src != srcEnd) {
-            *dst = SkAlphaMulQ(*(src++), src_scale) + SkAlphaMulQ(*dst, dst_scale);
+            *dst = SkPMLerp(*src, *dst, src_scale);
+            src += 1;
             dst += 1;
-            *dst = SkAlphaMulQ(*(src++), src_scale) + SkAlphaMulQ(*dst, dst_scale);
+            *dst = SkPMLerp(*src, *dst, src_scale);
+            src += 1;
             dst += 1;
         }
 #else
         do {
-            *dst = SkAlphaMulQ(*src, src_scale) + SkAlphaMulQ(*dst, dst_scale);
+            *dst = SkPMLerp(*src, *dst, src_scale);
             src += 1;
             dst += 1;
         } while (--count > 0);
 #endif
     }
 }
 
 static void S32A_Blend_BlitRow32(SkPMColor* SK_RESTRICT dst,
                                  const SkPMColor* SK_RESTRICT src,
                                  int count, U8CPU alpha) {
@@ skipping 50 matching lines @@
     return proc;
 }
 
 void SkBlitRow::Color32(SkPMColor dst[], const SkPMColor src[], int count, SkPMColor color) {
     switch (SkGetPackedA32(color)) {
         case   0: memmove(dst, src, count * sizeof(SkPMColor)); return;
         case 255: sk_memset32(dst, color, count);               return;
     }
     return SkOpts::blit_row_color32(dst, src, count, color);
 }