[Impeller] fix plus blend mode in porterduff shader. (#51792)

DrawAtlas already has the optimization in #51778 and so it is still rendering incorrectly with wide gamut.

Author: jonahwilliams

impeller/aiks/aiks_unittests.cc

@@ -3073,6 +3073,42 @@ TEST_P(AiksTest, MipmapGenerationWorksCorrectly) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, DrawAtlasPlusWideGamut) {
+  if (GetParam() != PlaygroundBackend::kMetal) {
+    GTEST_SKIP_("This backend doesn't yet support wide gamut.");
+  }
+
+  EXPECT_EQ(GetContext()->GetCapabilities()->GetDefaultColorFormat(),
+            PixelFormat::kR16G16B16A16Float);
+
+  // Draws the image as four squares stiched together.
+  auto atlas =
+      std::make_shared<Image>(CreateTextureForFixture("bay_bridge.jpg"));
+  auto size = atlas->GetSize();
+  // Divide image into four quadrants.
+  Scalar half_width = size.width / 2;
+  Scalar half_height = size.height / 2;
+  std::vector<Rect> texture_coordinates = {
+      Rect::MakeLTRB(0, 0, half_width, half_height),
+      Rect::MakeLTRB(half_width, 0, size.width, half_height),
+      Rect::MakeLTRB(0, half_height, half_width, size.height),
+      Rect::MakeLTRB(half_width, half_height, size.width, size.height)};
+  // Position quadrants adjacent to eachother.
+  std::vector<Matrix> transforms = {
+      Matrix::MakeTranslation({0, 0, 0}),
+      Matrix::MakeTranslation({half_width, 0, 0}),
+      Matrix::MakeTranslation({0, half_height, 0}),
+      Matrix::MakeTranslation({half_width, half_height, 0})};
+  std::vector<Color> colors = {Color::Red(), Color::Green(), Color::Blue(),
+                               Color::Yellow()};
+
+  Canvas canvas;
+  canvas.DrawAtlas(atlas, transforms, texture_coordinates, colors,
+                   BlendMode::kPlus, {}, std::nullopt, {});
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 }  // namespace testing
 }  // namespace impeller
 

impeller/entity/shaders/blending/porter_duff_blend.frag

@@ -36,6 +36,12 @@ f16vec4 Sample(f16sampler2D texture_sampler, vec2 texture_coords) {
   return IPHalfSampleDecal(texture_sampler, texture_coords);
 }
 
+float16_t ClampAlpha(float16_t alpha) {
+  float16_t min = 0.0hf;
+  float16_t max = 1.0hf;
+  return clamp(alpha, min, max);
+}
+
 void main() {
   f16vec4 dst =
       texture(texture_sampler_dst, v_texture_coords) * frag_info.input_alpha;
@@ -45,4 +51,9 @@ void main() {
       dst * (frag_info.dst_coeff + src.a * frag_info.dst_coeff_src_alpha +
              src * frag_info.dst_coeff_src_color);
   frag_color *= frag_info.output_alpha;
+  // This currently needs a clamp so that floating point textures blend
+  // correctly in wide gamut. Remove if we switch to a fixed point extended
+  // range format.
+  // See https://github.com/flutter/flutter/issues/145933 .
+  frag_color.a = ClampAlpha(frag_color.a);
 }

impeller/tools/malioc.json

@@ -8434,8 +8434,8 @@
               "varying"
             ],
             "longest_path_cycles": [
-              0.21875,
-              0.21875,
+              0.234375,
+              0.234375,
               0.0,
               0.0,
               0.0,
@@ -8455,8 +8455,8 @@
               "varying"
             ],
             "shortest_path_cycles": [
-              0.21875,
-              0.21875,
+              0.234375,
+              0.234375,
               0.0,
               0.0,
               0.0,
@@ -8467,8 +8467,8 @@
               "varying"
             ],
             "total_cycles": [
-              0.21875,
-              0.21875,
+              0.234375,
+              0.234375,
               0.0,
               0.0,
               0.0,

testing/impeller_golden_tests_output.txt

@@ -436,6 +436,7 @@ impeller_Play_AiksTest_DrawAdvancedBlendPartlyOffscreen_Vulkan.png
 impeller_Play_AiksTest_DrawAtlasAdvancedAndTransform_Metal.png
 impeller_Play_AiksTest_DrawAtlasAdvancedAndTransform_OpenGLES.png
 impeller_Play_AiksTest_DrawAtlasAdvancedAndTransform_Vulkan.png
+impeller_Play_AiksTest_DrawAtlasPlusWideGamut_Metal.png
 impeller_Play_AiksTest_DrawAtlasWithColorAdvancedAndTransform_Metal.png
 impeller_Play_AiksTest_DrawAtlasWithColorAdvancedAndTransform_OpenGLES.png
 impeller_Play_AiksTest_DrawAtlasWithColorAdvancedAndTransform_Vulkan.png