Preventing division by 0 in non-separable blend mode shaders.

src/core/SkXfermode.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 "SkXfermode.h"
@@ skipping 1077 matching lines @@
                                     getLumBody.c_str(),
                                     &getFunction);
 
             // Emit the set luminance function.
             GrGLShaderVar setLumArgs[] = {
                 GrGLShaderVar("hueSat", kVec3f_GrSLType),
                 GrGLShaderVar("alpha", kFloat_GrSLType),
                 GrGLShaderVar("lumColor", kVec3f_GrSLType),
             };
             SkString setLumBody;
             setLumBody.printf("\tfloat diff = %s(lumColor - hueSat);\n", getFunction.c_str());

[egdaniel, 2014/10/21 14:24:39]

Side note: This line looks like it does diff = Lum(lumColor - hueSat). The
software does this same calculation at line 490. However the software seems to
be doing diff = lumColor - Lum(hueSat). I haven't looked back at all the callers
to see what form all the inputs are in, but seems like this may produce
different things (but our gm's show no difference so maybe it's right). Do you
have any idea if these are doing what we expect?

[egdaniel, 2014/10/21 14:36:25]

Disregard this. It seems like the software/gpu just do all the various
multiplies at different stages in the call stack, but all end up equivalent in
the end.

             setLumBody.append("\tvec3 outColor = hueSat + diff;\n");
             setLumBody.appendf("\tfloat outLum = %s(outColor);\n", getFunction.c_str());
             setLumBody.append("\tfloat minComp = min(min(outColor.r, outColor.g), outColor.b);\n"
                               "\tfloat maxComp = max(max(outColor.r, outColor.g), outColor.b);\n"
-                              "\tif (minComp < 0.0) {\n"
+                              "\tif (minComp < 0.0 && outLum != minComp) {\n"
                               "\t\toutColor = outLum + ((outColor - vec3(outLum, outLum, outLum)) * outLum) / (outLum - minComp);\n"
                               "\t}\n"
-                              "\tif (maxComp > alpha) {\n"
+                              "\tif (maxComp > alpha && maxComp != outLum) {\n"
                               "\t\toutColor = outLum + ((outColor - vec3(outLum, outLum, outLum)) * (alpha - outLum)) / (maxComp - outLum);\n"
                               "\t}\n"
                               "\treturn outColor;\n");
             fsBuilder->emitFunction(kVec3f_GrSLType,
                                     "set_luminance",
                                     SK_ARRAY_COUNT(setLumArgs), setLumArgs,
                                     setLumBody.c_str(),
                                     setLumFunction);
         }
 
@@ skipping 836 matching lines @@
         } else {
             proc16 = rec.fProc16_General;
         }
     }
     return proc16;
 }
 
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_START(SkXfermode)
     SK_DEFINE_FLATTENABLE_REGISTRAR_ENTRY(SkProcCoeffXfermode)
 SK_DEFINE_FLATTENABLE_REGISTRAR_GROUP_END