Reland: [Impeller] adds a plus advanced blend for f16 pixel formats (flutter#51756)

Relands flutter#51589

The fix is in 74397bc. I couldn't
figure out how to get a test in the engine to cover it. The test is in
the devicelab.

Here's what I attempted:
```c++
TEST_P(AiksTest, BlendModePlusAlphaColorFilterAlphaWideGamut) {
  if (GetParam() != PlaygroundBackend::kMetal) {
    GTEST_SKIP_("This backend doesn't yet support wide gamut.");
  }
  EXPECT_EQ(GetContext()->GetCapabilities()->GetDefaultColorFormat(),
            PixelFormat::kR16G16B16A16Float);

  Canvas canvas;
  canvas.Scale(GetContentScale());
  canvas.DrawPaint({.color = Color(0.1, 0.2, 0.1, 0.5)});
  canvas.SaveLayer({
      .color_filter = ColorFilter::MakeBlend(BlendMode::kPlus,
                                             Color(Vector4{1, 0, 0, 0.5})),
  });
  Paint paint;
  paint.color = Color(1, 0, 0, 0.5);
  canvas.DrawRect(Rect::MakeXYWH(100, 100, 400, 400), paint);
  paint.color = Color::White();
  canvas.Restore();
  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
}
```

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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Author: gaaclarke

display_list/testing/dl_test_surface_metal.cc

@@ -115,7 +115,7 @@ sk_sp<DlImage> DlMetalSurfaceProvider::MakeImpellerImage(
 
 void DlMetalSurfaceProvider::InitScreenShotter() const {
   if (!snapshotter_) {
-    snapshotter_.reset(new MetalScreenshotter());
+    snapshotter_.reset(new MetalScreenshotter(/*enable_wide_gamut=*/false));
     auto typographer = impeller::TypographerContextSkia::Make();
     aiks_context_.reset(new impeller::AiksContext(
         snapshotter_->GetPlayground().GetContext(), typographer));

impeller/aiks/aiks_unittests.cc

@@ -1102,6 +1102,76 @@ TEST_P(AiksTest, PaintBlendModeIsRespected) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+// This makes sure the WideGamut named tests use 16bit float pixel format.
+TEST_P(AiksTest, F16WideGamut) {
+  if (GetParam() != PlaygroundBackend::kMetal) {
+    GTEST_SKIP_("This backend doesn't yet support wide gamut.");
+  }
+  EXPECT_EQ(GetContext()->GetCapabilities()->GetDefaultColorFormat(),
+            PixelFormat::kR16G16B16A16Float);
+  EXPECT_FALSE(IsAlphaClampedToOne(
+      GetContext()->GetCapabilities()->GetDefaultColorFormat()));
+}
+
+TEST_P(AiksTest, NotF16) {
+  EXPECT_TRUE(IsAlphaClampedToOne(
+      GetContext()->GetCapabilities()->GetDefaultColorFormat()));
+}
+
+// Bug: https://github.com/flutter/flutter/issues/142549
+TEST_P(AiksTest, BlendModePlusAlphaWideGamut) {
+  if (GetParam() != PlaygroundBackend::kMetal) {
+    GTEST_SKIP_("This backend doesn't yet support wide gamut.");
+  }
+  EXPECT_EQ(GetContext()->GetCapabilities()->GetDefaultColorFormat(),
+            PixelFormat::kR16G16B16A16Float);
+  auto texture = CreateTextureForFixture("airplane.jpg",
+                                         /*enable_mipmapping=*/true);
+
+  Canvas canvas;
+  canvas.Scale(GetContentScale());
+  canvas.DrawPaint({.color = Color(0.9, 1.0, 0.9, 1.0)});
+  canvas.SaveLayer({});
+  Paint paint;
+  paint.blend_mode = BlendMode::kPlus;
+  paint.color = Color::Red();
+  canvas.DrawRect(Rect::MakeXYWH(100, 100, 400, 400), paint);
+  paint.color = Color::White();
+  canvas.DrawImageRect(
+      std::make_shared<Image>(texture), Rect::MakeSize(texture->GetSize()),
+      Rect::MakeXYWH(100, 100, 400, 400).Expand(-100, -100), paint);
+  canvas.Restore();
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
+// Bug: https://github.com/flutter/flutter/issues/142549
+TEST_P(AiksTest, BlendModePlusAlphaColorFilterWideGamut) {
+  if (GetParam() != PlaygroundBackend::kMetal) {
+    GTEST_SKIP_("This backend doesn't yet support wide gamut.");
+  }
+  EXPECT_EQ(GetContext()->GetCapabilities()->GetDefaultColorFormat(),
+            PixelFormat::kR16G16B16A16Float);
+  auto texture = CreateTextureForFixture("airplane.jpg",
+                                         /*enable_mipmapping=*/true);
+
+  Canvas canvas;
+  canvas.Scale(GetContentScale());
+  canvas.DrawPaint({.color = Color(0.1, 0.2, 0.1, 1.0)});
+  canvas.SaveLayer({
+      .color_filter =
+          ColorFilter::MakeBlend(BlendMode::kPlus, Color(Vector4{1, 0, 0, 1})),
+  });
+  Paint paint;
+  paint.color = Color::Red();
+  canvas.DrawRect(Rect::MakeXYWH(100, 100, 400, 400), paint);
+  paint.color = Color::White();
+  canvas.DrawImageRect(
+      std::make_shared<Image>(texture), Rect::MakeSize(texture->GetSize()),
+      Rect::MakeXYWH(100, 100, 400, 400).Expand(-100, -100), paint);
+  canvas.Restore();
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 TEST_P(AiksTest, ColorWheel) {
   // Compare with https://fiddle.skia.org/c/@BlendModes
 

impeller/compiler/shader_lib/impeller/color.glsl

@@ -46,4 +46,12 @@ f16vec4 IPHalfPremultiply(f16vec4 color) {
   return f16vec4(color.rgb * color.a, color.a);
 }
 
+/// Performs the plus blend on `src` and `dst` which are premultiplied colors.
+//`max` determines the values the results are clamped to.
+f16vec4 IPHalfPlusBlend(f16vec4 src, f16vec4 dst) {
+  float16_t min = 0.0hf;
+  float16_t max = 1.0hf;
+  return clamp(dst + src, min, max);
+}
+
 #endif

impeller/core/formats.h

@@ -138,6 +138,14 @@ constexpr bool IsStencilWritable(PixelFormat format) {
   }
 }
 
+/// Returns `true` if the pixel format has an implicit `clamp(x, 0, 1)` in the
+/// pixel format. This is important for example when performing the `Plus` blend
+/// where we don't want alpha values over 1.0.
+constexpr bool IsAlphaClampedToOne(PixelFormat pixel_format) {
+  return !(pixel_format == PixelFormat::kR32G32B32A32Float ||
+           pixel_format == PixelFormat::kR16G16B16A16Float);
+}
+
 constexpr const char* PixelFormatToString(PixelFormat format) {
   switch (format) {
     case PixelFormat::kUnknown:

impeller/entity/contents/content_context.cc

@@ -124,6 +124,8 @@ void ContentContextOptions::ApplyToPipelineDescriptor(
       color0.src_color_blend_factor = BlendFactor::kOneMinusDestinationAlpha;
       break;
     case BlendMode::kPlus:
+      // The kPlusAdvanced should be used instead.
+      FML_DCHECK(IsAlphaClampedToOne(color_attachment_pixel_format));
       color0.dst_alpha_blend_factor = BlendFactor::kOne;
       color0.dst_color_blend_factor = BlendFactor::kOne;
       color0.src_alpha_blend_factor = BlendFactor::kOne;
@@ -324,6 +326,10 @@ ContentContext::ContentContext(
     framebuffer_blend_lighten_pipelines_.CreateDefault(
         *context_, options_trianglestrip,
         {static_cast<Scalar>(BlendSelectValues::kLighten), supports_decal});
+    framebuffer_blend_plus_advanced_pipelines_.CreateDefault(
+        *context_, options_trianglestrip,
+        {static_cast<Scalar>(BlendSelectValues::kPlusAdvanced),
+         supports_decal});
     framebuffer_blend_luminosity_pipelines_.CreateDefault(
         *context_, options_trianglestrip,
         {static_cast<Scalar>(BlendSelectValues::kLuminosity), supports_decal});
@@ -371,6 +377,9 @@ ContentContext::ContentContext(
   blend_lighten_pipelines_.CreateDefault(
       *context_, options_trianglestrip,
       {static_cast<Scalar>(BlendSelectValues::kLighten), supports_decal});
+  blend_plus_advanced_pipelines_.CreateDefault(
+      *context_, options_trianglestrip,
+      {static_cast<Scalar>(BlendSelectValues::kPlusAdvanced), supports_decal});
   blend_luminosity_pipelines_.CreateDefault(
       *context_, options_trianglestrip,
       {static_cast<Scalar>(BlendSelectValues::kLuminosity), supports_decal});

impeller/entity/contents/content_context.h

@@ -197,6 +197,8 @@ using BlendHuePipeline =
     RenderPipelineT<AdvancedBlendVertexShader, AdvancedBlendFragmentShader>;
 using BlendLightenPipeline =
     RenderPipelineT<AdvancedBlendVertexShader, AdvancedBlendFragmentShader>;
+using BlendPlusAdvancedPipeline =
+    RenderPipelineT<AdvancedBlendVertexShader, AdvancedBlendFragmentShader>;
 using BlendLuminosityPipeline =
     RenderPipelineT<AdvancedBlendVertexShader, AdvancedBlendFragmentShader>;
 using BlendMultiplyPipeline =
@@ -237,6 +239,9 @@ using FramebufferBlendHuePipeline =
 using FramebufferBlendLightenPipeline =
     RenderPipelineT<FramebufferBlendVertexShader,
                     FramebufferBlendFragmentShader>;
+using FramebufferBlendPlusAdvancedPipeline =
+    RenderPipelineT<FramebufferBlendVertexShader,
+                    FramebufferBlendFragmentShader>;
 using FramebufferBlendLuminosityPipeline =
     RenderPipelineT<FramebufferBlendVertexShader,
                     FramebufferBlendFragmentShader>;
@@ -640,6 +645,11 @@ class ContentContext {
     return GetPipeline(blend_lighten_pipelines_, opts);
   }
 
+  std::shared_ptr<Pipeline<PipelineDescriptor>> GetBlendPlusAdvancedPipeline(
+      ContentContextOptions opts) const {
+    return GetPipeline(blend_plus_advanced_pipelines_, opts);
+  }
+
   std::shared_ptr<Pipeline<PipelineDescriptor>> GetBlendLuminosityPipeline(
       ContentContextOptions opts) const {
     return GetPipeline(blend_luminosity_pipelines_, opts);
@@ -725,6 +735,12 @@ class ContentContext {
     return GetPipeline(framebuffer_blend_lighten_pipelines_, opts);
   }
 
+  std::shared_ptr<Pipeline<PipelineDescriptor>>
+  GetFramebufferBlendPlusAdvancedPipeline(ContentContextOptions opts) const {
+    FML_DCHECK(GetDeviceCapabilities().SupportsFramebufferFetch());
+    return GetPipeline(framebuffer_blend_plus_advanced_pipelines_, opts);
+  }
+
   std::shared_ptr<Pipeline<PipelineDescriptor>>
   GetFramebufferBlendLuminosityPipeline(ContentContextOptions opts) const {
     FML_DCHECK(GetDeviceCapabilities().SupportsFramebufferFetch());
@@ -989,6 +1005,7 @@ class ContentContext {
   mutable Variants<BlendHardLightPipeline> blend_hardlight_pipelines_;
   mutable Variants<BlendHuePipeline> blend_hue_pipelines_;
   mutable Variants<BlendLightenPipeline> blend_lighten_pipelines_;
+  mutable Variants<BlendPlusAdvancedPipeline> blend_plus_advanced_pipelines_;
   mutable Variants<BlendLuminosityPipeline> blend_luminosity_pipelines_;
   mutable Variants<BlendMultiplyPipeline> blend_multiply_pipelines_;
   mutable Variants<BlendOverlayPipeline> blend_overlay_pipelines_;
@@ -1014,6 +1031,8 @@ class ContentContext {
       framebuffer_blend_hue_pipelines_;
   mutable Variants<FramebufferBlendLightenPipeline>
       framebuffer_blend_lighten_pipelines_;
+  mutable Variants<FramebufferBlendPlusAdvancedPipeline>
+      framebuffer_blend_plus_advanced_pipelines_;
   mutable Variants<FramebufferBlendLuminosityPipeline>
       framebuffer_blend_luminosity_pipelines_;
   mutable Variants<FramebufferBlendMultiplyPipeline>

impeller/entity/contents/filters/blend_filter_contents.cc

@@ -338,6 +338,9 @@ std::optional<Entity> BlendFilterContents::CreateForegroundAdvancedBlend(
       case BlendMode::kColor:
         pass.SetPipeline(renderer.GetBlendColorPipeline(options));
         break;
+      case BlendMode::kPlusAdvanced:
+        pass.SetPipeline(renderer.GetBlendPlusAdvancedPipeline(options));
+        break;
       case BlendMode::kLuminosity:
         pass.SetPipeline(renderer.GetBlendLuminosityPipeline(options));
         break;
@@ -546,7 +549,7 @@ static std::optional<Entity> PipelineBlend(
 
 #define BLEND_CASE(mode)                                                      \
   case BlendMode::k##mode:                                                    \
-    advanced_blend_proc_ =                                                    \
+    return                                                                    \
         [](const FilterInput::Vector& inputs, const ContentContext& renderer, \
            const Entity& entity, const Rect& coverage, BlendMode blend_mode,  \
            std::optional<Color> fg_color,                                     \
@@ -559,35 +562,39 @@ static std::optional<Entity> PipelineBlend(
         };                                                                    \
     break;
 
+namespace {
+BlendFilterContents::AdvancedBlendProc GetAdvancedBlendProc(
+    BlendMode blend_mode) {
+  switch (blend_mode) {
+    BLEND_CASE(Screen)
+    BLEND_CASE(Overlay)
+    BLEND_CASE(Darken)
+    BLEND_CASE(Lighten)
+    BLEND_CASE(ColorDodge)
+    BLEND_CASE(ColorBurn)
+    BLEND_CASE(HardLight)
+    BLEND_CASE(SoftLight)
+    BLEND_CASE(Difference)
+    BLEND_CASE(Exclusion)
+    BLEND_CASE(Multiply)
+    BLEND_CASE(Hue)
+    BLEND_CASE(Saturation)
+    BLEND_CASE(Color)
+    BLEND_CASE(PlusAdvanced)
+    BLEND_CASE(Luminosity)
+    default:
+      FML_UNREACHABLE();
+  }
+}
+}  // namespace
+
 void BlendFilterContents::SetBlendMode(BlendMode blend_mode) {
   if (blend_mode > Entity::kLastAdvancedBlendMode) {
     VALIDATION_LOG << "Invalid blend mode " << static_cast<int>(blend_mode)
                    << " assigned to BlendFilterContents.";
   }
 
   blend_mode_ = blend_mode;
-
-  if (blend_mode > Entity::kLastPipelineBlendMode) {
-    switch (blend_mode) {
-      BLEND_CASE(Screen)
-      BLEND_CASE(Overlay)
-      BLEND_CASE(Darken)
-      BLEND_CASE(Lighten)
-      BLEND_CASE(ColorDodge)
-      BLEND_CASE(ColorBurn)
-      BLEND_CASE(HardLight)
-      BLEND_CASE(SoftLight)
-      BLEND_CASE(Difference)
-      BLEND_CASE(Exclusion)
-      BLEND_CASE(Multiply)
-      BLEND_CASE(Hue)
-      BLEND_CASE(Saturation)
-      BLEND_CASE(Color)
-      BLEND_CASE(Luminosity)
-      default:
-        FML_UNREACHABLE();
-    }
-  }
 }
 
 void BlendFilterContents::SetForegroundColor(std::optional<Color> color) {
@@ -611,21 +618,29 @@ std::optional<Entity> BlendFilterContents::RenderFilter(
                          std::nullopt, GetAbsorbOpacity(), GetAlpha());
   }
 
-  if (blend_mode_ <= Entity::kLastPipelineBlendMode) {
-    return PipelineBlend(inputs, renderer, entity, coverage, blend_mode_,
+  BlendMode blend_mode = blend_mode_;
+  if (blend_mode == BlendMode::kPlus &&
+      !IsAlphaClampedToOne(
+          renderer.GetContext()->GetCapabilities()->GetDefaultColorFormat())) {
+    blend_mode = BlendMode::kPlusAdvanced;
+  }
+
+  if (blend_mode <= Entity::kLastPipelineBlendMode) {
+    return PipelineBlend(inputs, renderer, entity, coverage, blend_mode,
                          foreground_color_, GetAbsorbOpacity(), GetAlpha());
   }
 
-  if (blend_mode_ <= Entity::kLastAdvancedBlendMode) {
+  if (blend_mode <= Entity::kLastAdvancedBlendMode) {
     if (inputs.size() == 1 && foreground_color_.has_value() &&
         GetAbsorbOpacity() == ColorFilterContents::AbsorbOpacity::kYes) {
       return CreateForegroundAdvancedBlend(
           inputs[0], renderer, entity, coverage, foreground_color_.value(),
-          blend_mode_, GetAlpha(), GetAbsorbOpacity());
+          blend_mode, GetAlpha(), GetAbsorbOpacity());
     }
-    return advanced_blend_proc_(inputs, renderer, entity, coverage, blend_mode_,
-                                foreground_color_, GetAbsorbOpacity(),
-                                GetAlpha());
+    AdvancedBlendProc advanced_blend_proc = GetAdvancedBlendProc(blend_mode);
+    return advanced_blend_proc(inputs, renderer, entity, coverage, blend_mode,
+                               foreground_color_, GetAbsorbOpacity(),
+                               GetAlpha());
   }
 
   FML_UNREACHABLE();

impeller/entity/contents/filters/blend_filter_contents.h

@@ -79,7 +79,6 @@ class BlendFilterContents : public ColorFilterContents {
       ColorFilterContents::AbsorbOpacity absorb_opacity) const;
 
   BlendMode blend_mode_ = BlendMode::kSourceOver;
-  AdvancedBlendProc advanced_blend_proc_;
   std::optional<Color> foreground_color_;
 
   BlendFilterContents(const BlendFilterContents&) = delete;

impeller/entity/contents/framebuffer_blend_contents.cc

@@ -118,6 +118,10 @@ bool FramebufferBlendContents::Render(const ContentContext& renderer,
     case BlendMode::kColor:
       pass.SetPipeline(renderer.GetFramebufferBlendColorPipeline(options));
       break;
+    case BlendMode::kPlusAdvanced:
+      pass.SetPipeline(
+          renderer.GetFramebufferBlendPlusAdvancedPipeline(options));
+      break;
     case BlendMode::kLuminosity:
       pass.SetPipeline(renderer.GetFramebufferBlendLuminosityPipeline(options));
       break;

impeller/entity/contents/framebuffer_blend_contents.h

@@ -27,6 +27,7 @@ enum class BlendSelectValues {
   kHue,
   kSaturation,
   kColor,
+  kPlusAdvanced,
   kLuminosity,
 };