Color: handle pre-multiplied alpha correctly

Without this change, we're doing color transforms on
the multiplied-in alpha values, which is wrong.

Bug: 835322
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel
Change-Id: I30a9533837f8d6392394eab9d859968cec93e4da
Reviewed-on: https://chromium-review.googlesource.com/1036618Reviewed-by: Fredrik Hubinette <hubbe@chromium.org>
Reviewed-by: ccameron <ccameron@chromium.org>
Commit-Queue: Fredrik Hubinette <hubbe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555226}

components/viz/service/display/renderer_pixeltest.cc

@@ -3997,10 +3997,11 @@ TEST_P(ColorTransformPixelTest, Basic) {
     color.set_y(input_colors[4 * i + 1] / 255.f);
     color.set_z(input_colors[4 * i + 2] / 255.f);
     float alpha = input_colors[4 * i + 3] / 255.f;
-    if (!premultiplied_alpha_) {
-      color.Scale(alpha);
+    if (premultiplied_alpha_ && alpha > 0.0) {
+      color.Scale(1.0f / alpha);
     }
     transform->Transform(&color, 1);
+    color.Scale(alpha);
     color.set_x(std::min(std::max(0.f, color.x()), 1.f));
     color.set_y(std::min(std::max(0.f, color.y()), 1.f));
     color.set_z(std::min(std::max(0.f, color.z()), 1.f));

components/viz/service/display/shader.cc

@@ -938,12 +938,6 @@ std::string FragmentShader::GetShaderSource() const {
       break;
   }
 
-  // Premultiply by alpha.
-  if (premultiply_alpha_mode_ == NON_PREMULTIPLIED_ALPHA) {
-    SRC("// Premultiply alpha");
-    SRC("texColor.rgb *= texColor.a;");
-  }
-
   // Apply color conversion.
   switch (color_conversion_mode_) {
     case COLOR_CONVERSION_MODE_LUT:
@@ -961,13 +955,30 @@ std::string FragmentShader::GetShaderSource() const {
       HDR("             LutLookup(sampler, pos.xy + vec2(0, 1.0 / size)),");
       HDR("             pos.z - layer);");
       HDR("}");
+      // Un-premultiply by alpha.
+      if (premultiply_alpha_mode_ != NON_PREMULTIPLIED_ALPHA) {
+        SRC("// un-premultiply alpha");
+        SRC("if (texColor.a > 0.0) texColor.rgb /= texColor.a;");
+      }
       SRC("texColor.rgb = LUT(lut_texture, texColor.xyz, lut_size).xyz;");
+      SRC("texColor.rgb *= texColor.a;");
       break;
     case COLOR_CONVERSION_MODE_SHADER:
       header += color_transform_->GetShaderSource();
+      // Un-premultiply by alpha.
+      if (premultiply_alpha_mode_ != NON_PREMULTIPLIED_ALPHA) {
+        SRC("// un-premultiply alpha");
+        SRC("if (texColor.a > 0.0) texColor.rgb /= texColor.a;");
+      }
       SRC("texColor.rgb = DoColorConversion(texColor.xyz);");
+      SRC("texColor.rgb *= texColor.a;");
       break;
     case COLOR_CONVERSION_MODE_NONE:
+      // Premultiply by alpha.
+      if (premultiply_alpha_mode_ == NON_PREMULTIPLIED_ALPHA) {
+        SRC("// Premultiply alpha");
+        SRC("texColor.rgb *= texColor.a;");
+      }
       break;
   }