[Flutter GPU] Add WindingOrder.

lib/gpu/formats.h

@@ -522,6 +522,25 @@ constexpr impeller::CullMode ToImpellerCullMode(int value) {
   return ToImpellerCullMode(static_cast<FlutterGPUCullMode>(value));
 }
 
+enum class FlutterGPUWindingOrder {
+  kClockwise,
+  kCounterClockwise,
+};
+
+constexpr impeller::WindingOrder ToImpellerWindingOrder(
+    FlutterGPUWindingOrder value) {
+  switch (value) {
+    case FlutterGPUWindingOrder::kClockwise:
+      return impeller::WindingOrder::kClockwise;
+    case FlutterGPUWindingOrder::kCounterClockwise:
+      return impeller::WindingOrder::kCounterClockwise;
+  }
+}
+
+constexpr impeller::WindingOrder ToImpellerWindingOrder(int value) {
+  return ToImpellerWindingOrder(static_cast<FlutterGPUWindingOrder>(value));
+}
+
 }  // namespace gpu
 }  // namespace flutter
 

lib/gpu/lib/src/formats.dart

@@ -141,6 +141,11 @@ enum CullMode {
   backFace,
 }
 
+enum WindingOrder {
+  clockwise,
+  counterClockwise,
+}
+
 enum CompareFunction {
   /// Comparison test never passes.
   never,

lib/gpu/lib/src/render_pass.dart

@@ -268,6 +268,10 @@ base class RenderPass extends NativeFieldWrapperClass1 {
     _setCullMode(cullMode.index);
   }
 
+  void setWindingOrder(WindingOrder windingOrder) {
+    _setWindingOrder(windingOrder.index);
+  }
+
   void draw() {
     if (!_draw()) {
       throw Exception("Failed to append draw");
@@ -410,6 +414,10 @@ base class RenderPass extends NativeFieldWrapperClass1 {
       symbol: 'InternalFlutterGpu_RenderPass_SetCullMode')
   external void _setCullMode(int cullMode);
 
+  @Native<Void Function(Pointer<Void>, Int)>(
+      symbol: 'InternalFlutterGpu_RenderPass_SetWindingOrder')
+  external void _setWindingOrder(int windingOrder);
+
   @Native<Bool Function(Pointer<Void>)>(
       symbol: 'InternalFlutterGpu_RenderPass_Draw')
   external bool _draw();

lib/gpu/render_pass.cc

@@ -570,6 +570,15 @@ void InternalFlutterGpu_RenderPass_SetCullMode(
   pipeline_descriptor.SetCullMode(flutter::gpu::ToImpellerCullMode(cull_mode));
 }
 
+void InternalFlutterGpu_RenderPass_SetWindingOrder(
+    flutter::gpu::RenderPass* wrapper,
+    int winding_order) {
+  impeller::PipelineDescriptor& pipeline_descriptor =
+      wrapper->GetPipelineDescriptor();
+  pipeline_descriptor.SetWindingOrder(
+      flutter::gpu::ToImpellerWindingOrder(winding_order));
+}
+
 bool InternalFlutterGpu_RenderPass_Draw(flutter::gpu::RenderPass* wrapper) {
   return wrapper->Draw();
 }

lib/gpu/render_pass.h

@@ -248,6 +248,11 @@ extern void InternalFlutterGpu_RenderPass_SetCullMode(
     flutter::gpu::RenderPass* wrapper,
     int cull_mode);
 
+FLUTTER_GPU_EXPORT
+extern void InternalFlutterGpu_RenderPass_SetWindingOrder(
+    flutter::gpu::RenderPass* wrapper,
+    int winding_order);
+
 FLUTTER_GPU_EXPORT
 extern bool InternalFlutterGpu_RenderPass_Draw(
     flutter::gpu::RenderPass* wrapper);

testing/dart/gpu_test.dart

@@ -469,11 +469,21 @@ void main() async {
       state.renderPass.draw();
     }
 
+    // Draw the green rectangle.
+    // Defaults to clockwise winding order. So frontface culling should not
+    // impact the green triangle.
     state.renderPass.setCullMode(gpu.CullMode.frontFace);
     drawTriangle(Colors.lime);
+
+    // Backface cull a red triangle.
     state.renderPass.setCullMode(gpu.CullMode.backFace);
     drawTriangle(Colors.red);
 
+    // Invert the winding mode and frontface cull a red rectangle.
+    state.renderPass.setWindingOrder(gpu.WindingOrder.counterClockwise);
+    state.renderPass.setCullMode(gpu.CullMode.frontFace);
+    drawTriangle(Colors.red);
+
     state.commandBuffer.submit();
 
     final ui.Image image = state.renderTexture.asImage();