color: avoid negative pow

Avoid calling pow() on negative numbers as that
causes undefined behavior in the shader.

Bug: 821967
Change-Id: Iab3dc9840c17d90aa5952adf17b8b24e2890d055
Reviewed-on: https://chromium-review.googlesource.com/962969Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Fredrik Hubinette <hubbe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543263}

ui/gfx/color_transform.cc

@@ -473,14 +473,9 @@ class ColorTransformSkTransferFn : public ColorTransformPerChannelTransferFn {
     if (std::abs(fn_.fE) > kEpsilon)
       nonlinear = nonlinear + " + " + Str(fn_.fE);
 
-    // Add both parts, skipping the if clause if possible.
-    if (fn_.fD > kEpsilon) {
-      *result << "  if (v < " << Str(fn_.fD) << ")" << endl;
-      *result << "    return " << linear << ";" << endl;
-      *result << "  return " << nonlinear << ";" << endl;
-    } else {
-      *result << "  return " << nonlinear << ";" << endl;
-    }
+    *result << "  if (v < " << Str(fn_.fD) << ")" << endl;
+    *result << "    return " << linear << ";" << endl;
+    *result << "  return " << nonlinear << ";" << endl;
   }
 
  private:

ui/gfx/color_transform_unittest.cc

@@ -380,55 +380,32 @@ TEST(SimpleColorSpace, DefaultToSRGB) {
   EXPECT_EQ(t2->NumberOfStepsForTesting(), 0u);
 }
 
-// This tests to make sure that we don't emit the "if" or "pow" parts of a
+// This tests to make sure that we don't emit "pow" parts of a
 // transfer function unless necessary.
 TEST(SimpleColorSpace, ShaderSourceTrFnOptimizations) {
   SkMatrix44 primaries;
   gfx::ColorSpace::CreateSRGB().GetPrimaryMatrix(&primaries);
 
-  SkColorSpaceTransferFn fn_no_pow_no_if = {
+  SkColorSpaceTransferFn fn_no_pow = {
       1.f, 2.f, 0.f, 1.f, 0.f, 0.f, 0.f,
   };
-  SkColorSpaceTransferFn fn_no_pow_yes_if = {
-      1.f, 2.f, 0.f, 1.f, 0.5f, 0.f, 0.f,
-  };
-  SkColorSpaceTransferFn fn_yes_pow_no_if = {
+  SkColorSpaceTransferFn fn_yes_pow = {
       2.f, 2.f, 0.f, 1.f, 0.f, 0.f, 0.f,
   };
-  SkColorSpaceTransferFn fn_yes_pow_yes_if = {
-      2.f, 2.f, 0.f, 1.f, 0.5f, 0.f, 0.f,
-  };
-
   gfx::ColorSpace src;
   gfx::ColorSpace dst = gfx::ColorSpace::CreateXYZD50();
   std::string shader_string;
 
-  src = gfx::ColorSpace::CreateCustom(primaries, fn_no_pow_no_if);
+  src = gfx::ColorSpace::CreateCustom(primaries, fn_no_pow);
   shader_string = ColorTransform::NewColorTransform(
                       src, dst, ColorTransform::Intent::INTENT_PERCEPTUAL)
                       ->GetShaderSource();
-  EXPECT_EQ(shader_string.find("if ("), std::string::npos);
   EXPECT_EQ(shader_string.find("pow("), std::string::npos);
 
-  src = gfx::ColorSpace::CreateCustom(primaries, fn_no_pow_yes_if);
-  shader_string = ColorTransform::NewColorTransform(
-                      src, dst, ColorTransform::Intent::INTENT_PERCEPTUAL)
-                      ->GetShaderSource();
-  EXPECT_NE(shader_string.find("if ("), std::string::npos);
-  EXPECT_EQ(shader_string.find("pow("), std::string::npos);
-
-  src = gfx::ColorSpace::CreateCustom(primaries, fn_yes_pow_no_if);
-  shader_string = ColorTransform::NewColorTransform(
-                      src, dst, ColorTransform::Intent::INTENT_PERCEPTUAL)
-                      ->GetShaderSource();
-  EXPECT_EQ(shader_string.find("if ("), std::string::npos);
-  EXPECT_NE(shader_string.find("pow("), std::string::npos);
-
-  src = gfx::ColorSpace::CreateCustom(primaries, fn_yes_pow_yes_if);
+  src = gfx::ColorSpace::CreateCustom(primaries, fn_yes_pow);
   shader_string = ColorTransform::NewColorTransform(
                       src, dst, ColorTransform::Intent::INTENT_PERCEPTUAL)
                       ->GetShaderSource();
-  EXPECT_NE(shader_string.find("if ("), std::string::npos);
   EXPECT_NE(shader_string.find("pow("), std::string::npos);
 }
 
@@ -456,6 +433,8 @@ TEST(SimpleColorSpace, MAYBE_SampleShaderSource) {
       "  return pow(9.47867334e-01 * v + 5.21326549e-02, 2.40000010e+00);\n"
       "}\n"
       "float TransferFn3(float v) {\n"
+      "  if (v < 0.00000000e+00)\n"
+      "    return v;\n"
       "  return pow(v, 3.57142866e-01);\n"
       "}\n"
       "vec3 DoColorConversion(vec3 color) {\n"