Added support to set scissor.

Author: AthulJoseph27

lib/gpu/lib/src/render_pass.dart

@@ -150,6 +150,12 @@ base class SamplerOptions {
   SamplerAddressMode heightAddressMode;
 }
 
+base class Scissor {
+  Scissor({this.x = 0, this.y = 0, this.width = 0, this.height = 0});
+
+  int x, y, width, height;
+}
+
 base class RenderTarget {
   const RenderTarget(
       {this.colorAttachments = const <ColorAttachment>[],
@@ -214,6 +220,9 @@ base class RenderPass extends NativeFieldWrapperClass1 {
       if (error != null) {
         throw Exception(error);
       }
+
+      final texture = renderTarget.depthStencilAttachment!.texture;
+      _setScissor(0, 0, texture.width, texture.height);
     }
     error = _begin(commandBuffer);
     if (error != null) {
@@ -326,6 +335,10 @@ base class RenderPass extends NativeFieldWrapperClass1 {
         targetFace.index);
   }
 
+  void setScissor(Scissor scissor) {
+    _setScissor(scissor.x, scissor.y, scissor.width, scissor.height);
+  }
+
   void setCullMode(CullMode cullMode) {
     _setCullMode(cullMode.index);
   }
@@ -477,6 +490,14 @@ base class RenderPass extends NativeFieldWrapperClass1 {
       int readMask,
       int writeMask,
       int target_face);
+  
+  @Native<Void Function(Pointer<Void>, Int, Int, Int, Int)>(
+      symbol: 'InternalFlutterGpu_RenderPass_SetScissor')
+  external void _setScissor(
+      int x,
+      int y,
+      int width,
+      int height);
 
   @Native<Void Function(Pointer<Void>, Int)>(
       symbol: 'InternalFlutterGpu_RenderPass_SetCullMode')

lib/gpu/render_pass.cc

@@ -206,6 +206,7 @@ bool RenderPass::Draw() {
   render_pass_->SetElementCount(element_count);
 
   render_pass_->SetStencilReference(stencil_reference);
+  render_pass_->SetScissor(scissor);
 
   bool result = render_pass_->Draw().ok();
 
@@ -536,6 +537,11 @@ void InternalFlutterGpu_RenderPass_SetStencilReference(
   wrapper->stencil_reference = static_cast<uint32_t>(stencil_reference);
 }
 
+void InternalFlutterGpu_RenderPass_SetScissor(
+    flutter::gpu::RenderPass* wrapper, int x, int y, int width, int height) {
+  wrapper->scissor = impeller::TRect<int64_t>::MakeXYWH(x, y, width, height);
+}
+
 void InternalFlutterGpu_RenderPass_SetStencilConfig(
     flutter::gpu::RenderPass* wrapper,
     int stencil_compare_operation,

lib/gpu/render_pass.h

@@ -74,6 +74,7 @@ class RenderPass : public RefCountedDartWrappable<RenderPass> {
   size_t element_count = 0;
 
   uint32_t stencil_reference = 0;
+  impeller::TRect<int64_t> scissor;
 
   // Helper flag to determine whether the vertex_count should override the
   // element count. The index count takes precedent.
@@ -234,6 +235,14 @@ extern void InternalFlutterGpu_RenderPass_SetStencilConfig(
     int write_mask,
     int target);
 
+FLUTTER_GPU_EXPORT
+extern void InternalFlutterGpu_RenderPass_SetScissor(
+    flutter::gpu::RenderPass* wrapper,
+    int x,
+    int y,
+    int width,
+    int height);
+
 FLUTTER_GPU_EXPORT
 extern void InternalFlutterGpu_RenderPass_SetCullMode(
     flutter::gpu::RenderPass* wrapper,

testing/dart/gpu_test.dart

@@ -736,4 +736,50 @@ void main() async {
     await comparer.addGoldenImage(
         image, 'flutter_gpu_test_hexgon_line_strip.png');
   }, skip: !impellerEnabled);
+
+  // Renders the middle part triangle using scissor.
+  test('Can render portion of the triangle using scissor', () 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 primitive type.
+    state.renderPass.setPrimitiveType(gpu.PrimitiveType.triangle);
+
+    // Set scissor.
+    state.renderPass.setScissor(gpu.Scissor(x: 25, y: 0, width: 50, height: 100));
+
+    final gpu.HostBuffer transients = gpu.gpuContext.createHostBuffer();
+    final gpu.BufferView vertices = transients.emplace(float32(<double>[
+      -1.0,
+      -1.0,
+      0.0,
+      1.0,
+      1.0,
+      -1.0]));
+    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_scissor.png');
+  }, skip: !impellerEnabled);
 }