[Impeller] Added support to set polygon mode in Flutter GPU. (flutter/engine#55804)

Added support to set polygon mode in Flutter GPU. This fixes the issue [#142732](https://github.com/flutter/flutter/issues/142732).

engine/src/flutter/lib/gpu/formats.h

@@ -541,6 +541,25 @@ constexpr impeller::WindingOrder ToImpellerWindingOrder(int value) {
   return ToImpellerWindingOrder(static_cast<FlutterGPUWindingOrder>(value));
 }
 
+enum class FlutterGPUPolygonMode {
+  kFill,
+  kLine,
+};
+
+constexpr impeller::PolygonMode ToImpellerPolygonMode(
+    FlutterGPUPolygonMode value) {
+  switch (value) {
+    case FlutterGPUPolygonMode::kFill:
+      return impeller::PolygonMode::kFill;
+    case FlutterGPUPolygonMode::kLine:
+      return impeller::PolygonMode::kLine;
+  }
+}
+
+constexpr impeller::PolygonMode ToImpellerPolygonMode(int value) {
+  return ToImpellerPolygonMode(static_cast<FlutterGPUPolygonMode>(value));
+}
+
 }  // namespace gpu
 }  // namespace flutter
 

engine/src/flutter/lib/gpu/lib/src/formats.dart

@@ -146,6 +146,11 @@ enum WindingOrder {
   counterClockwise,
 }
 
+enum PolygonMode {
+  fill,
+  line
+}
+
 enum CompareFunction {
   /// Comparison test never passes.
   never,

engine/src/flutter/lib/gpu/lib/src/render_pass.dart

@@ -262,6 +262,9 @@ base class RenderPass extends NativeFieldWrapperClass1 {
     _setCullMode(cullMode.index);
   }
 
+  void setPolygonMode(PolygonMode polygonMode) {
+    _setPolygonMode(polygonMode.index);
+  }
 
   void setPrimitiveType(PrimitiveType primitiveType) {
     _setPrimitiveType(primitiveType.index);
@@ -433,6 +436,9 @@ base class RenderPass extends NativeFieldWrapperClass1 {
       symbol: 'InternalFlutterGpu_RenderPass_SetWindingOrder')
   external void _setWindingOrder(int windingOrder);
 
+  @Native<Void Function(Pointer<Void>, Int)>(
+      symbol: 'InternalFlutterGpu_RenderPass_SetPolygonMode')
+  external void _setPolygonMode(int polygonMode);
 
   @Native<Bool Function(Pointer<Void>)>(
       symbol: 'InternalFlutterGpu_RenderPass_Draw')

engine/src/flutter/lib/gpu/render_pass.cc

@@ -617,6 +617,15 @@ void InternalFlutterGpu_RenderPass_SetWindingOrder(
       flutter::gpu::ToImpellerWindingOrder(winding_order));
 }
 
+void InternalFlutterGpu_RenderPass_SetPolygonMode(
+    flutter::gpu::RenderPass* wrapper,
+    int polygon_mode) {
+  impeller::PipelineDescriptor& pipeline_descriptor =
+      wrapper->GetPipelineDescriptor();
+  pipeline_descriptor.SetPolygonMode(
+      flutter::gpu::ToImpellerPolygonMode(polygon_mode));
+}
+
 bool InternalFlutterGpu_RenderPass_Draw(flutter::gpu::RenderPass* wrapper) {
   return wrapper->Draw();
 }

engine/src/flutter/lib/gpu/render_pass.h

@@ -261,6 +261,11 @@ extern void InternalFlutterGpu_RenderPass_SetWindingOrder(
     flutter::gpu::RenderPass* wrapper,
     int winding_order);
 
+FLUTTER_GPU_EXPORT
+extern void InternalFlutterGpu_RenderPass_SetPolygonMode(
+    flutter::gpu::RenderPass* wrapper,
+    int polygon_mode);
+
 FLUTTER_GPU_EXPORT
 extern bool InternalFlutterGpu_RenderPass_Draw(
     flutter::gpu::RenderPass* wrapper);

engine/src/flutter/testing/dart/gpu_test.dart

@@ -371,6 +371,46 @@ void main() async {
     await comparer.addGoldenImage(image, 'flutter_gpu_test_triangle.png');
   }, skip: !impellerEnabled);
 
+  // Renders a green triangle pointing downwards using polygon mode line.
+  test('Can render triangle with polygon mode line.', () async {
+    final state = createSimpleRenderPass();
+
+    final gpu.RenderPipeline pipeline = createUnlitRenderPipeline();
+    state.renderPass.bindPipeline(pipeline);
+
+    // Configure blending with defaults (just to test the bindings).
+    state.renderPass.setColorBlendEnable(true);
+    state.renderPass.setColorBlendEquation(gpu.ColorBlendEquation());
+
+    // Set polygon mode.
+    state.renderPass.setPolygonMode(gpu.PolygonMode.line);
+
+    final gpu.HostBuffer transients = gpu.gpuContext.createHostBuffer();
+    final gpu.BufferView vertices = transients.emplace(float32(<double>[
+      -0.5, 0.5, //
+      0.0, -0.5, //
+      0.5, 0.5, //
+    ]));
+    final gpu.BufferView vertInfoData = transients.emplace(float32(<double>[
+      1, 0, 0, 0, // mvp
+      0, 1, 0, 0, // mvp
+      0, 0, 1, 0, // mvp
+      0, 0, 0, 1, // mvp
+      0, 1, 0, 1, // color
+    ]));
+    state.renderPass.bindVertexBuffer(vertices, 3);
+
+    final gpu.UniformSlot vertInfo =
+        pipeline.vertexShader.getUniformSlot('VertInfo');
+    state.renderPass.bindUniform(vertInfo, vertInfoData);
+    state.renderPass.draw();
+
+    state.commandBuffer.submit();
+
+    final ui.Image image = state.renderTexture.asImage();
+    await comparer.addGoldenImage(image, 'flutter_gpu_test_triangle_polygon_mode.png');
+  }, skip: !impellerEnabled);
+
   // Renders a hollow green triangle pointing downwards.
   test('Can render hollowed out triangle using stencil ops', () async {
     final state = createSimpleRenderPass();