text: /*
   * Copyright 2015 Google Inc.
   *
   * Use of this source code is governed by a BSD-style license that can be
   * found in the LICENSE file.
   */
  #include "GrGLSLBlend.h"
  #include "glsl/GrGLSLFragmentShaderBuilder.h"
  
  //////////////////////////////////////////////////////////////////////////////
  //  Advanced (non-coeff) blend helpers
  //////////////////////////////////////////////////////////////////////////////
  
  static void hard_light(GrGLSLFragmentBuilder* fsBuilder,
                         const char* final,
                         const char* src,
                         const char* dst) {
      static const char kComponents[] = { 'r', 'g', 'b' };
      for (size_t i = 0; i < SK_ARRAY_COUNT(kComponents); ++i) {
          char component = kComponents[i];
          fsBuilder->codeAppendf("if (2.0 * %s.%c <= %s.a) {", src, component, src);
          fsBuilder->codeAppendf("%s.%c = 2.0 * %s.%c * %s.%c;",
                                 final, component, src, component, dst, component);
          fsBuilder->codeAppend("} else {");
          fsBuilder->codeAppendf("%s.%c = %s.a * %s.a - 2.0 * (%s.a - %s.%c) * (%s.a - %s.%c);",
                                 final, component, src, dst, dst, dst, component, src, src,
                                 component);
          fsBuilder->codeAppend("}");
      }
      fsBuilder->codeAppendf("%s.rgb += %s.rgb * (1.0 - %s.a) + %s.rgb * (1.0 - %s.a);",
                             final, src, dst, dst, src);
  }
  
  // Does one component of color-dodge
  static void color_dodge_component(GrGLSLFragmentBuilder* fsBuilder,
                                    const char* final,
                                    const char* src,
                                    const char* dst,
                                    const char component) {
      fsBuilder->codeAppendf("if (0.0 == %s.%c) {", dst, component);
      fsBuilder->codeAppendf("%s.%c = %s.%c * (1.0 - %s.a);",
                             final, component, src, component, dst);
      fsBuilder->codeAppend("} else {");
      fsBuilder->codeAppendf("float d = %s.a - %s.%c;", src, src, component);
      fsBuilder->codeAppend("if (0.0 == d) {");
      fsBuilder->codeAppendf("%s.%c = %s.a * %s.a + %s.%c * (1.0 - %s.a) + %s.%c * (1.0 - %s.a);",
                             final, component, src, dst, src, component, dst, dst, component,
                             src);
      fsBuilder->codeAppend("} else {");
      fsBuilder->codeAppendf("d = min(%s.a, %s.%c * %s.a / d);",
                             dst, dst, component, src);
      fsBuilder->codeAppendf("%s.%c = d * %s.a + %s.%c * (1.0 - %s.a) + %s.%c * (1.0 - %s.a);",
                             final, component, src, src, component, dst, dst, component, src);
      fsBuilder->codeAppend("}");
      fsBuilder->codeAppend("}");
  }
  
  // Does one component of color-burn
  static void color_burn_component(GrGLSLFragmentBuilder* fsBuilder,
                                   const char* final,
                                   const char* src,
                                   const char* dst,
                                   const char component) {
      fsBuilder->codeAppendf("if (%s.a == %s.%c) {", dst, dst, component);
      fsBuilder->codeAppendf("%s.%c = %s.a * %s.a + %s.%c * (1.0 - %s.a) + %s.%c * (1.0 - %s.a);",
                             final, component, src, dst, src, component, dst, dst, component,
                             src);
      fsBuilder->codeAppendf("} else if (0.0 == %s.%c) {", src, component);
      fsBuilder->codeAppendf("%s.%c = %s.%c * (1.0 - %s.a);",
                             final, component, dst, component, src);
      fsBuilder->codeAppend("} else {");
      fsBuilder->codeAppendf("float d = max(0.0, %s.a - (%s.a - %s.%c) * %s.a / %s.%c);",
                             dst, dst, dst, component, src, src, component);
      fsBuilder->codeAppendf("%s.%c = %s.a * d + %s.%c * (1.0 - %s.a) + %s.%c * (1.0 - %s.a);",
                             final, component, src, src, component, dst, dst, component, src);
      fsBuilder->codeAppend("}");
  }
  
  // Does one component of soft-light. Caller should have already checked that dst alpha > 0.
  static void soft_light_component_pos_dst_alpha(GrGLSLFragmentBuilder* fsBuilder,
                                                 const char* final,
                                                 const char* src,
                                                 const char* dst,
                                                 const char component) {
      // if (2S < Sa)
      fsBuilder->codeAppendf("if (2.0 * %s.%c <= %s.a) {", src, component, src);
      // (D^2 (Sa-2 S))/Da+(1-Da) S+D (-Sa+2 S+1)
      fsBuilder->codeAppendf("%s.%c = (%s.%c*%s.%c*(%s.a - 2.0*%s.%c)) / %s.a +"
                             "(1.0 - %s.a) * %s.%c + %s.%c*(-%s.a + 2.0*%s.%c + 1.0);",
                             final, component, dst, component, dst, component, src, src,
                             component, dst, dst, src, component, dst, component, src, src,
                             component);
      // else if (4D < Da)
      fsBuilder->codeAppendf("} else if (4.0 * %s.%c <= %s.a) {",
                             dst, component, dst);
      fsBuilder->codeAppendf("float DSqd = %s.%c * %s.%c;",
                             dst, component, dst, component);
      fsBuilder->codeAppendf("float DCub = DSqd * %s.%c;", dst, component);
      fsBuilder->codeAppendf("float DaSqd = %s.a * %s.a;", dst, dst);
      fsBuilder->codeAppendf("float DaCub = DaSqd * %s.a;", dst);
      // (Da^3 (-S)+Da^2 (S-D (3 Sa-6 S-1))+12 Da D^2 (Sa-2 S)-16 D^3 (Sa-2 S))/Da^2
      fsBuilder->codeAppendf("%s.%c ="
                             "(DaSqd*(%s.%c - %s.%c * (3.0*%s.a - 6.0*%s.%c - 1.0)) +"
                             " 12.0*%s.a*DSqd*(%s.a - 2.0*%s.%c) - 16.0*DCub * (%s.a - 2.0*%s.%c) -"
                             " DaCub*%s.%c) / DaSqd;",
                             final, component, src, component, dst, component,
                             src, src, component, dst, src, src, component, src, src,
                             component, src, component);
      fsBuilder->codeAppendf("} else {");
      // -sqrt(Da * D) (Sa-2 S)-Da S+D (Sa-2 S+1)+S
      fsBuilder->codeAppendf("%s.%c = %s.%c*(%s.a - 2.0*%s.%c + 1.0) + %s.%c -"
                             " sqrt(%s.a*%s.%c)*(%s.a - 2.0*%s.%c) - %s.a*%s.%c;",
                             final, component, dst, component, src, src, component, src, component,
                             dst, dst, component, src, src, component, dst, src, component);
      fsBuilder->codeAppendf("}");
  }
  
  // Adds a function that takes two colors and an alpha as input. It produces a color with the
  // hue and saturation of the first color, the luminosity of the second color, and the input
  // alpha. It has this signature:
  //      vec3 set_luminance(vec3 hueSatColor, float alpha, vec3 lumColor).
  static void add_lum_function(GrGLSLFragmentBuilder* fsBuilder, SkString* setLumFunction) {
      // Emit a helper that gets the luminance of a color.
      SkString getFunction;
      GrGLSLShaderVar getLumArgs[] = {
          GrGLSLShaderVar("color", kVec3f_GrSLType),
      };
      SkString getLumBody("return dot(vec3(0.3, 0.59, 0.11), color);");
      fsBuilder->emitFunction(kFloat_GrSLType,
                              "luminance",
                              SK_ARRAY_COUNT(getLumArgs), getLumArgs,
                              getLumBody.c_str(),
                              &getFunction);
  
      // Emit the set luminance function.
      GrGLSLShaderVar setLumArgs[] = {
          GrGLSLShaderVar("hueSat", kVec3f_GrSLType),
          GrGLSLShaderVar("alpha", kFloat_GrSLType),
          GrGLSLShaderVar("lumColor", kVec3f_GrSLType),
      };
      SkString setLumBody;
      setLumBody.printf("float diff = %s(lumColor - hueSat);", getFunction.c_str());
      setLumBody.append("vec3 outColor = hueSat + diff;");
      setLumBody.appendf("float outLum = %s(outColor);", getFunction.c_str());
      setLumBody.append("float minComp = min(min(outColor.r, outColor.g), outColor.b);"
                        "float maxComp = max(max(outColor.r, outColor.g), outColor.b);"
                        "if (minComp < 0.0 && outLum != minComp) {"
                        "outColor = outLum + ((outColor - vec3(outLum, outLum, outLum)) * outLum) /"
                        "(outLum - minComp);"
                        "}"
                        "if (maxComp > alpha && maxComp != outLum) {"
                        "outColor = outLum +"
                        "((outColor - vec3(outLum, outLum, outLum)) * (alpha - outLum)) /"
                        "(maxComp - outLum);"
                        "}"
                        "return outColor;");
      fsBuilder->emitFunction(kVec3f_GrSLType,
                              "set_luminance",
                              SK_ARRAY_COUNT(setLumArgs), setLumArgs,
                              setLumBody.c_str(),
                              setLumFunction);
  }
  
  // Adds a function that creates a color with the hue and luminosity of one input color and
  // the saturation of another color. It will have this signature:
  //      float set_saturation(vec3 hueLumColor, vec3 satColor)
  static void add_sat_function(GrGLSLFragmentBuilder* fsBuilder, SkString* setSatFunction) {
      // Emit a helper that gets the saturation of a color
      SkString getFunction;
      GrGLSLShaderVar getSatArgs[] = { GrGLSLShaderVar("color", kVec3f_GrSLType) };
      SkString getSatBody;
      getSatBody.printf("return max(max(color.r, color.g), color.b) - "
                        "min(min(color.r, color.g), color.b);");
      fsBuilder->emitFunction(kFloat_GrSLType,
                              "saturation",
                              SK_ARRAY_COUNT(getSatArgs), getSatArgs,
                              getSatBody.c_str(),
                              &getFunction);
  
      // Emit a helper that sets the saturation given sorted input channels. This used
      // to use inout params for min, mid, and max components but that seems to cause
      // problems on PowerVR drivers. So instead it returns a vec3 where r, g ,b are the
      // adjusted min, mid, and max inputs, respectively.
      SkString helperFunction;
      GrGLSLShaderVar helperArgs[] = {
          GrGLSLShaderVar("minComp", kFloat_GrSLType),
          GrGLSLShaderVar("midComp", kFloat_GrSLType),
          GrGLSLShaderVar("maxComp", kFloat_GrSLType),
          GrGLSLShaderVar("sat", kFloat_GrSLType),
      };
      static const char kHelperBody[] = "if (minComp < maxComp) {"
          "vec3 result;"
          "result.r = 0.0;"
          "result.g = sat * (midComp - minComp) / (maxComp - minComp);"
          "result.b = sat;"
          "return result;"
          "} else {"
          "return vec3(0, 0, 0);"
          "}";
      fsBuilder->emitFunction(kVec3f_GrSLType,
                              "set_saturation_helper",
                              SK_ARRAY_COUNT(helperArgs), helperArgs,
                              kHelperBody,
                              &helperFunction);
  
      GrGLSLShaderVar setSatArgs[] = {
          GrGLSLShaderVar("hueLumColor", kVec3f_GrSLType),
          GrGLSLShaderVar("satColor", kVec3f_GrSLType),
      };
      const char* helpFunc = helperFunction.c_str();
      SkString setSatBody;
      setSatBody.appendf("float sat = %s(satColor);"
                         "if (hueLumColor.r <= hueLumColor.g) {"
                         "if (hueLumColor.g <= hueLumColor.b) {"
                         "hueLumColor.rgb = %s(hueLumColor.r, hueLumColor.g, hueLumColor.b, sat);"
                         "} else if (hueLumColor.r <= hueLumColor.b) {"
                         "hueLumColor.rbg = %s(hueLumColor.r, hueLumColor.b, hueLumColor.g, sat);"
                         "} else {"
                         "hueLumColor.brg = %s(hueLumColor.b, hueLumColor.r, hueLumColor.g, sat);"
                         "}"
                         "} else if (hueLumColor.r <= hueLumColor.b) {"
                         "hueLumColor.grb = %s(hueLumColor.g, hueLumColor.r, hueLumColor.b, sat);"
                         "} else if (hueLumColor.g <= hueLumColor.b) {"
                         "hueLumColor.gbr = %s(hueLumColor.g, hueLumColor.b, hueLumColor.r, sat);"
                         "} else {"
                         "hueLumColor.bgr = %s(hueLumColor.b, hueLumColor.g, hueLumColor.r, sat);"
                         "}"
                         "return hueLumColor;",
                         getFunction.c_str(), helpFunc, helpFunc, helpFunc, helpFunc,
                         helpFunc, helpFunc);
      fsBuilder->emitFunction(kVec3f_GrSLType,
                              "set_saturation",
                              SK_ARRAY_COUNT(setSatArgs), setSatArgs,
                              setSatBody.c_str(),
                              setSatFunction);
  }
  
  static void emit_advanced_xfermode_code(GrGLSLFragmentBuilder* fsBuilder, const char* srcColor,
                                          const char* dstColor, const char* outputColor,
                                          SkXfermode::Mode mode) {
      SkASSERT(srcColor);
      SkASSERT(dstColor);
      SkASSERT(outputColor);
      // These all perform src-over on the alpha channel.
      fsBuilder->codeAppendf("%s.a = %s.a + (1.0 - %s.a) * %s.a;",
                             outputColor, srcColor, srcColor, dstColor);
  
      switch (mode) {
          case SkXfermode::kOverlay_Mode:
              // Overlay is Hard-Light with the src and dst reversed
              hard_light(fsBuilder, outputColor, dstColor, srcColor);
              break;
          case SkXfermode::kDarken_Mode:
              fsBuilder->codeAppendf("%s.rgb = min((1.0 - %s.a) * %s.rgb + %s.rgb, "
                                     "(1.0 - %s.a) * %s.rgb + %s.rgb);",
                                     outputColor,
                                     srcColor, dstColor, srcColor,
                                     dstColor, srcColor, dstColor);
              break;
          case SkXfermode::kLighten_Mode:
              fsBuilder->codeAppendf("%s.rgb = max((1.0 - %s.a) * %s.rgb + %s.rgb, "
                                     "(1.0 - %s.a) * %s.rgb + %s.rgb);",
                                     outputColor,
                                     srcColor, dstColor, srcColor,
                                     dstColor, srcColor, dstColor);
              break;
          case SkXfermode::kColorDodge_Mode:
              color_dodge_component(fsBuilder, outputColor, srcColor, dstColor, 'r');
              color_dodge_component(fsBuilder, outputColor, srcColor, dstColor, 'g');
              color_dodge_component(fsBuilder, outputColor, srcColor, dstColor, 'b');
              break;
          case SkXfermode::kColorBurn_Mode:
              color_burn_component(fsBuilder, outputColor, srcColor, dstColor, 'r');
              color_burn_component(fsBuilder, outputColor, srcColor, dstColor, 'g');
              color_burn_component(fsBuilder, outputColor, srcColor, dstColor, 'b');
              break;
          case SkXfermode::kHardLight_Mode:
              hard_light(fsBuilder, outputColor, srcColor, dstColor);
              break;
          case SkXfermode::kSoftLight_Mode:
              fsBuilder->codeAppendf("if (0.0 == %s.a) {", dstColor);
              fsBuilder->codeAppendf("%s.rgba = %s;", outputColor, srcColor);
              fsBuilder->codeAppendf("} else {");
              soft_light_component_pos_dst_alpha(fsBuilder, outputColor, srcColor, dstColor, 'r');
              soft_light_component_pos_dst_alpha(fsBuilder, outputColor, srcColor, dstColor, 'g');
              soft_light_component_pos_dst_alpha(fsBuilder, outputColor, srcColor, dstColor, 'b');
              fsBuilder->codeAppendf("}");
              break;
          case SkXfermode::kDifference_Mode:
              fsBuilder->codeAppendf("%s.rgb = %s.rgb + %s.rgb -"
                                     "2.0 * min(%s.rgb * %s.a, %s.rgb * %s.a);",
                                     outputColor, srcColor, dstColor, srcColor, dstColor,
                                     dstColor, srcColor);
              break;
          case SkXfermode::kExclusion_Mode:
              fsBuilder->codeAppendf("%s.rgb = %s.rgb + %s.rgb - "
                                     "2.0 * %s.rgb * %s.rgb;",
                                     outputColor, dstColor, srcColor, dstColor, srcColor);
              break;
          case SkXfermode::kMultiply_Mode:
              fsBuilder->codeAppendf("%s.rgb = (1.0 - %s.a) * %s.rgb + "
                                     "(1.0 - %s.a) * %s.rgb + "
                                     "%s.rgb * %s.rgb;",
                                     outputColor, srcColor, dstColor, dstColor, srcColor,
                                     srcColor, dstColor);
              break;
          case SkXfermode::kHue_Mode: {
              //  SetLum(SetSat(S * Da, Sat(D * Sa)), Sa*Da, D*Sa) + (1 - Sa) * D + (1 - Da) * S
              SkString setSat, setLum;
              add_sat_function(fsBuilder, &setSat);
              add_lum_function(fsBuilder, &setLum);
              fsBuilder->codeAppendf("vec4 dstSrcAlpha = %s * %s.a;",
                                     dstColor, srcColor);
              fsBuilder->codeAppendf("%s.rgb = %s(%s(%s.rgb * %s.a, dstSrcAlpha.rgb),"
                                     "dstSrcAlpha.a, dstSrcAlpha.rgb);",
                                     outputColor, setLum.c_str(), setSat.c_str(), srcColor,
                                     dstColor);
              fsBuilder->codeAppendf("%s.rgb += (1.0 - %s.a) * %s.rgb + (1.0 - %s.a) * %s.rgb;",
                                     outputColor, srcColor, dstColor, dstColor, srcColor);
              break;
          }
          case SkXfermode::kSaturation_Mode: {
              // SetLum(SetSat(D * Sa, Sat(S * Da)), Sa*Da, D*Sa)) + (1 - Sa) * D + (1 - Da) * S
              SkString setSat, setLum;
              add_sat_function(fsBuilder, &setSat);
              add_lum_function(fsBuilder, &setLum);
              fsBuilder->codeAppendf("vec4 dstSrcAlpha = %s * %s.a;",
                                     dstColor, srcColor);
              fsBuilder->codeAppendf("%s.rgb = %s(%s(dstSrcAlpha.rgb, %s.rgb * %s.a),"
                                     "dstSrcAlpha.a, dstSrcAlpha.rgb);",
                                     outputColor, setLum.c_str(), setSat.c_str(), srcColor,
                                     dstColor);
              fsBuilder->codeAppendf("%s.rgb += (1.0 - %s.a) * %s.rgb + (1.0 - %s.a) * %s.rgb;",
                                     outputColor, srcColor, dstColor, dstColor, srcColor);
              break;
          }
          case SkXfermode::kColor_Mode: {
              //  SetLum(S * Da, Sa* Da, D * Sa) + (1 - Sa) * D + (1 - Da) * S
              SkString setLum;
              add_lum_function(fsBuilder, &setLum);
              fsBuilder->codeAppendf("vec4 srcDstAlpha = %s * %s.a;",
                                     srcColor, dstColor);
              fsBuilder->codeAppendf("%s.rgb = %s(srcDstAlpha.rgb, srcDstAlpha.a, %s.rgb * %s.a);",
                                     outputColor, setLum.c_str(), dstColor, srcColor);
              fsBuilder->codeAppendf("%s.rgb += (1.0 - %s.a) * %s.rgb + (1.0 - %s.a) * %s.rgb;",
                                     outputColor, srcColor, dstColor, dstColor, srcColor);
              break;
          }
          case SkXfermode::kLuminosity_Mode: {
              //  SetLum(D * Sa, Sa* Da, S * Da) + (1 - Sa) * D + (1 - Da) * S
              SkString setLum;
              add_lum_function(fsBuilder, &setLum);
              fsBuilder->codeAppendf("vec4 srcDstAlpha = %s * %s.a;",
                                     srcColor, dstColor);
              fsBuilder->codeAppendf("%s.rgb = %s(%s.rgb * %s.a, srcDstAlpha.a, srcDstAlpha.rgb);",
                                     outputColor, setLum.c_str(), dstColor, srcColor);
              fsBuilder->codeAppendf("%s.rgb += (1.0 - %s.a) * %s.rgb + (1.0 - %s.a) * %s.rgb;",
                                     outputColor, srcColor, dstColor, dstColor, srcColor);
              break;
          }
          default:
              SkFAIL("Unknown Custom Xfer mode.");
              break;
      }
  }
  
  //////////////////////////////////////////////////////////////////////////////
  //  Porter-Duff blend helper
  //////////////////////////////////////////////////////////////////////////////
  
  static bool append_porterduff_term(GrGLSLFragmentBuilder* fsBuilder, SkXfermode::Coeff coeff,
                                     const char* colorName, const char* srcColorName,
                                     const char* dstColorName, bool hasPrevious) {
      if (SkXfermode::kZero_Coeff == coeff) {
          return hasPrevious;
      } else {
          if (hasPrevious) {
              fsBuilder->codeAppend(" + ");
          }
          fsBuilder->codeAppendf("%s", colorName);
          switch (coeff) {
              case SkXfermode::kOne_Coeff:
                  break;
              case SkXfermode::kSC_Coeff:
                  fsBuilder->codeAppendf(" * %s", srcColorName);
                  break;
              case SkXfermode::kISC_Coeff:
                  fsBuilder->codeAppendf(" * (vec4(1.0) - %s)", srcColorName);
                  break;
              case SkXfermode::kDC_Coeff:
                  fsBuilder->codeAppendf(" * %s", dstColorName);
                  break;
              case SkXfermode::kIDC_Coeff:
                  fsBuilder->codeAppendf(" * (vec4(1.0) - %s)", dstColorName);
                  break;
              case SkXfermode::kSA_Coeff:
                  fsBuilder->codeAppendf(" * %s.a", srcColorName);
                  break;
              case SkXfermode::kISA_Coeff:
                  fsBuilder->codeAppendf(" * (1.0 - %s.a)", srcColorName);
                  break;
              case SkXfermode::kDA_Coeff:
                  fsBuilder->codeAppendf(" * %s.a", dstColorName);
                  break;
              case SkXfermode::kIDA_Coeff:
                  fsBuilder->codeAppendf(" * (1.0 - %s.a)", dstColorName);
                  break;
              default:
                  SkFAIL("Unsupported Blend Coeff");
          }
          return true;
      }
  }
  
  //////////////////////////////////////////////////////////////////////////////
  
  void GrGLSLBlend::AppendMode(GrGLSLFragmentBuilder* fsBuilder, const char* srcColor,
                               const char* dstColor, const char* outColor,
                               SkXfermode::Mode mode) {
  
      SkXfermode::Coeff srcCoeff, dstCoeff;
      if (SkXfermode::ModeAsCoeff(mode, &srcCoeff, &dstCoeff)) {
          fsBuilder->codeAppendf("%s = ", outColor);
          // append src blend
          bool didAppend = append_porterduff_term(fsBuilder, srcCoeff, srcColor, srcColor, dstColor,
                                                  false);
          // append dst blend
          if(!append_porterduff_term(fsBuilder, dstCoeff, dstColor, srcColor, dstColor, didAppend)) {
              fsBuilder->codeAppend("vec4(0, 0, 0, 0)");
          }
          fsBuilder->codeAppend(";");
      } else {
          emit_advanced_xfermode_code(fsBuilder, srcColor, dstColor, outColor, mode);
      }
  }
  
  void GrGLSLBlend::AppendRegionOp(GrGLSLFragmentBuilder* fsBuilder, const char* srcColor,
                                   const char* dstColor, const char* outColor,
                                   SkRegion::Op regionOp) {
      SkXfermode::Coeff srcCoeff, dstCoeff;
      switch (regionOp) {
          case SkRegion::kReplace_Op:
              srcCoeff = SkXfermode::kOne_Coeff;
              dstCoeff = SkXfermode::kZero_Coeff;
              break;
          case SkRegion::kIntersect_Op:
              srcCoeff = SkXfermode::kDC_Coeff;
              dstCoeff = SkXfermode::kZero_Coeff;
              break;
          case SkRegion::kUnion_Op:
              srcCoeff = SkXfermode::kOne_Coeff;
              dstCoeff = SkXfermode::kISC_Coeff;
              break;
          case SkRegion::kXOR_Op:
              srcCoeff = SkXfermode::kIDC_Coeff;
              dstCoeff = SkXfermode::kISC_Coeff;
              break;
          case SkRegion::kDifference_Op:
              srcCoeff = SkXfermode::kZero_Coeff;
              dstCoeff = SkXfermode::kISC_Coeff;
              break;
          case SkRegion::kReverseDifference_Op:
              srcCoeff = SkXfermode::kIDC_Coeff;
              dstCoeff = SkXfermode::kZero_Coeff;
              break;
          default:
              SkFAIL("Unsupported Op");
              // We should never get here but to make compiler happy
              srcCoeff = SkXfermode::kZero_Coeff;
              dstCoeff = SkXfermode::kZero_Coeff;
      }
      fsBuilder->codeAppendf("%s = ", outColor);
      // append src blend
      bool didAppend = append_porterduff_term(fsBuilder, srcCoeff, srcColor, srcColor, dstColor,
                                              false);
      // append dst blend
      if(!append_porterduff_term(fsBuilder, dstCoeff, dstColor, srcColor, dstColor, didAppend)) {
          fsBuilder->codeAppend("vec4(0, 0, 0, 0)");
      }
      fsBuilder->codeAppend(";");
  }
  