[Flutter GPU] Added support to set Scissor.

lib/gpu/lib/src/render_pass.dart

@@ -150,6 +150,18 @@ 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;
+
+  void _validate() {
+    if (x < 0 || y < 0 || width < 0 || height < 0) {
+      throw Exception("Invalid values for scissor. All values should be positive.");
+    }
+  }
+}
+
 base class RenderTarget {
   const RenderTarget(
       {this.colorAttachments = const <ColorAttachment>[],
@@ -326,6 +338,14 @@ base class RenderPass extends NativeFieldWrapperClass1 {
         targetFace.index);
   }
 
+  void setScissor(Scissor scissor) {
+    assert(() {
+      scissor._validate();
+      return true;
+    }());
+    _setScissor(scissor.x, scissor.y, scissor.width, scissor.height);
+  }
+
   void setCullMode(CullMode cullMode) {
     _setCullMode(cullMode.index);
   }
@@ -478,6 +498,14 @@ base class RenderPass extends NativeFieldWrapperClass1 {
       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')
   external void _setCullMode(int cullMode);

lib/gpu/render_pass.cc

@@ -207,6 +207,10 @@ bool RenderPass::Draw() {
 
   render_pass_->SetStencilReference(stencil_reference);
 
+  if (scissor.has_value()) {
+    render_pass_->SetScissor(scissor.value());
+  }
+
   bool result = render_pass_->Draw().ok();
 
   return result;
@@ -536,6 +540,14 @@ 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;
+  std::optional<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 +736,78 @@ 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, 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);
+
+    test('RenderPass.setScissor doesnt throw for valid values',
+      () async {
+    final state = createSimpleRenderPass();
+
+    state.renderPass.setScissor(gpu.Scissor(x: 25, width: 50, height: 100));
+    state.renderPass.setScissor(gpu.Scissor(width: 50, height: 100));
+  }, skip: !impellerEnabled);
+
+  test('RenderPass.setScissor throws for invalid values', () async {
+    final state = createSimpleRenderPass();
+
+    try {
+      state.renderPass.setScissor(gpu.Scissor(x: -1, width: 50, height: 100));
+      fail('Exception not thrown for invalid scissor.');
+    } catch (e) {
+      expect(e.toString(),
+          contains('Invalid values for scissor. All values should be positive.'));
+    }
+
+    try {
+      state.renderPass.setScissor(gpu.Scissor(width: 50, height: -100));
+      fail('Exception not thrown for invalid scissor.');
+    } catch (e) {
+      expect(e.toString(),
+          contains('Invalid values for scissor. All values should be positive.'));
+    }
+  }, skip: !impellerEnabled);
 }