Make GrGLConfigConversionEffect work on Intel GPUs

src/gpu/effects/GrConfigConversionEffect.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrConfigConversionEffect.h"
 #include "GrContext.h"
 #include "GrTBackendEffectFactory.h"
@@ skipping 32 matching lines @@
             builder->fsCodeAppendf("\t%s = %s.bgra;\n", outputColor, outputColor);
         } else {
             const char* swiz = fSwapRedAndBlue ? "bgr" : "rgb";
             switch (fPMConversion) {
                 case GrConfigConversionEffect::kMulByAlpha_RoundUp_PMConversion:
                     builder->fsCodeAppendf(
                         "\t\t%s = vec4(ceil(%s.%s * %s.a * 255.0) / 255.0, %s.a);\n",
                         outputColor, outputColor, swiz, outputColor, outputColor);
                     break;
                 case GrConfigConversionEffect::kMulByAlpha_RoundDown_PMConversion:
+                    // Add a compensation(0.001) here to avoid the side effect of the floor operation.
+                    // In Intel GPUs, the integer value converted from floor(%s.r * 255.0) / 255.0
+                    // is less than the integer value converted from  %s.r by 1 when the %s.r is
+                    // converted from the integer value 2^n, such as 1, 2, 4, 8, etc.
                     builder->fsCodeAppendf(
-                        "\t\t%s = vec4(floor(%s.%s * %s.a * 255.0) / 255.0, %s.a);\n",
+                        "\t\t%s = vec4(floor(%s.%s * %s.a * 255.0 + 0.001) / 255.0, %s.a);\n",
                         outputColor, outputColor, swiz, outputColor, outputColor);
                     break;
                 case GrConfigConversionEffect::kDivByAlpha_RoundUp_PMConversion:
                     builder->fsCodeAppendf("\t\t%s = %s.a <= 0.0 ? vec4(0,0,0,0) : vec4(ceil(%s.%s / %s.a * 255.0) / 255.0, %s.a);\n",
                         outputColor, outputColor, outputColor, swiz, outputColor, outputColor);
                     break;
                 case GrConfigConversionEffect::kDivByAlpha_RoundDown_PMConversion:
                     builder->fsCodeAppendf("\t\t%s = %s.a <= 0.0 ? vec4(0,0,0,0) : vec4(floor(%s.%s / %s.a * 255.0) / 255.0, %s.a);\n",
                         outputColor, outputColor, outputColor, swiz, outputColor, outputColor);
                     break;
@@ skipping 215 matching lines @@
             // The PM conversions assume colors are 0..255
             return NULL;
         }
         AutoEffectUnref effect(SkNEW_ARGS(GrConfigConversionEffect, (texture,
                                                                      swapRedAndBlue,
                                                                      pmConversion,
                                                                      matrix)));
         return CreateEffectRef(effect);
     }
 }