[Flutter GPU] Add missing MSAA stuff.

lib/gpu/context.cc

@@ -10,11 +10,18 @@
 #include "flutter/lib/ui/ui_dart_state.h"
 #include "fml/make_copyable.h"
 #include "impeller/core/platform.h"
+#include "impeller/renderer/context.h"
 #include "tonic/converter/dart_converter.h"
 
 namespace flutter {
 namespace gpu {
 
+bool SupportsNormalOffscreenMSAA(const impeller::Context& context) {
+  auto& capabilities = context.GetCapabilities();
+  return capabilities->SupportsOffscreenMSAA() &&
+         !capabilities->SupportsImplicitResolvingMSAA();
+}
+
 IMPLEMENT_WRAPPERTYPEINFO(flutter_gpu, Context);
 
 std::shared_ptr<impeller::Context> Context::default_context_;
@@ -111,3 +118,8 @@ extern int InternalFlutterGpu_Context_GetMinimumUniformByteAlignment(
     flutter::gpu::Context* wrapper) {
   return impeller::DefaultUniformAlignment();
 }
+
+extern bool InternalFlutterGpu_Context_GetSupportsOffscreenMSAA(
+    flutter::gpu::Context* wrapper) {
+  return flutter::gpu::SupportsNormalOffscreenMSAA(*wrapper->GetContext());
+}

lib/gpu/context.h

@@ -13,6 +13,8 @@
 namespace flutter {
 namespace gpu {
 
+bool SupportsNormalOffscreenMSAA(const impeller::Context& context);
+
 class Context : public RefCountedDartWrappable<Context> {
   DEFINE_WRAPPERTYPEINFO();
   FML_FRIEND_MAKE_REF_COUNTED(Context);
@@ -74,6 +76,10 @@ FLUTTER_GPU_EXPORT
 extern int InternalFlutterGpu_Context_GetMinimumUniformByteAlignment(
     flutter::gpu::Context* wrapper);
 
+FLUTTER_GPU_EXPORT
+extern bool InternalFlutterGpu_Context_GetSupportsOffscreenMSAA(
+    flutter::gpu::Context* wrapper);
+
 }  // extern "C"
 
 #endif  // FLUTTER_LIB_GPU_CONTEXT_H_

lib/gpu/lib/src/command_buffer.dart

@@ -9,13 +9,15 @@ part of flutter_gpu;
 typedef CompletionCallback<T> = void Function(bool success);
 
 base class CommandBuffer extends NativeFieldWrapperClass1 {
+  final GpuContext _gpuContext;
+
   /// Creates a new CommandBuffer.
-  CommandBuffer._(GpuContext gpuContext) {
-    _initialize(gpuContext);
+  CommandBuffer._(this._gpuContext) {
+    _initialize(_gpuContext);
   }
 
   RenderPass createRenderPass(RenderTarget renderTarget) {
-    return RenderPass._(this, renderTarget);
+    return RenderPass._(_gpuContext, this, renderTarget);
   }
 
   void submit({CompletionCallback? completionCallback}) {

lib/gpu/lib/src/context.dart

@@ -46,6 +46,18 @@ base class GpuContext extends NativeFieldWrapperClass1 {
     return _getMinimumUniformByteAlignment();
   }
 
+  /// Whether the backend supports multisample anti-aliasing for offscreen
+  /// color and stencil attachments. A subset of OpenGLES-only devices do not
+  /// support this functionality.
+  ///
+  /// Any texture created via [createTexture] is an offscreen texture.
+  /// There is currently no way to render directly against the "onscreen"
+  /// texture that the framework renders to, so all Flutter GPU textures are
+  /// "offscreen".
+  bool get doesSupportOffscreenMSAA {
+    return _getSupportsOffscreenMSAA();
+  }
+
   /// Allocates a new region of GPU-resident memory.
   ///
   /// The [storageMode] must be either [StorageMode.hostVisible] or
@@ -134,6 +146,10 @@ base class GpuContext extends NativeFieldWrapperClass1 {
   @Native<Int Function(Pointer<Void>)>(
       symbol: 'InternalFlutterGpu_Context_GetMinimumUniformByteAlignment')
   external int _getMinimumUniformByteAlignment();
+
+  @Native<Bool Function(Pointer<Void>)>(
+      symbol: 'InternalFlutterGpu_Context_GetSupportsOffscreenMSAA')
+  external bool _getSupportsOffscreenMSAA();
 }
 
 /// The default graphics context.

lib/gpu/lib/src/render_pass.dart

@@ -21,6 +21,37 @@ base class ColorAttachment {
 
   Texture texture;
   Texture? resolveTexture;
+
+  void _validate() {
+    if (resolveTexture != null) {
+      if (resolveTexture!.format != texture.format) {
+        throw Exception(
+            "ColorAttachment MSAA resolve texture must have the same format as the texture");
+      }
+      if (resolveTexture!.width != texture.width ||
+          resolveTexture!.height != texture.height) {
+        throw Exception(
+            "ColorAttachment MSAA resolve texture must have the same dimensions as the texture");
+      }
+      if (resolveTexture!.sampleCount != 1) {
+        throw Exception(
+            "ColorAttachment MSAA resolve texture must have a sample count of 1");
+      }
+      if (texture.sampleCount <= 1) {
+        throw Exception(
+            "ColorAttachment must have a sample count greater than 1 when a MSAA resolve texture is set");
+      }
+      if (storeAction != StoreAction.multisampleResolve &&
+          storeAction != StoreAction.storeAndMultisampleResolve) {
+        throw Exception(
+            "ColorAttachment StoreAction must be multisampleResolve or storeAndMultisampleResolve when a resolve texture is set");
+      }
+      if (resolveTexture!.storageMode == StorageMode.deviceTransient) {
+        throw Exception(
+            "ColorAttachment MSAA resolve texture must not have a storage mode of deviceTransient");
+      }
+    }
+  }
 }
 
 base class DepthStencilAttachment {
@@ -43,6 +74,19 @@ base class DepthStencilAttachment {
   int stencilClearValue;
 
   Texture texture;
+
+  void _validate() {
+    if (texture.storageMode == StorageMode.deviceTransient) {
+      if (depthLoadAction == LoadAction.load) {
+        throw Exception(
+            "DepthStencilAttachment depthLoadAction must not be load when the texture has a storage mode of deviceTransient");
+      }
+      if (stencilLoadAction == LoadAction.load) {
+        throw Exception(
+            "DepthStencilAttachment stencilLoadAction must not be load when the texture has a storage mode of deviceTransient");
+      }
+    }
+  }
 }
 
 base class StencilConfig {
@@ -117,17 +161,33 @@ base class RenderTarget {
             colorAttachments: [colorAttachment],
             depthStencilAttachment: depthStencilAttachment);
 
+  _validate() {
+    for (final color in colorAttachments) {
+      color._validate();
+    }
+    if (depthStencilAttachment != null) {
+      depthStencilAttachment!._validate();
+    }
+  }
+
   final List<ColorAttachment> colorAttachments;
   final DepthStencilAttachment? depthStencilAttachment;
 }
 
 base class RenderPass extends NativeFieldWrapperClass1 {
   /// Creates a new RenderPass.
-  RenderPass._(CommandBuffer commandBuffer, RenderTarget renderTarget) {
+  RenderPass._(GpuContext gpuContext, CommandBuffer commandBuffer,
+      RenderTarget renderTarget) {
+    assert(() {
+      renderTarget._validate();
+      return true;
+    }());
+
     _initialize();
     String? error;
     for (final (index, color) in renderTarget.colorAttachments.indexed) {
       error = _setColorAttachment(
+          gpuContext,
           index,
           color.loadAction.index,
           color.storeAction.index,
@@ -287,6 +347,7 @@ base class RenderPass extends NativeFieldWrapperClass1 {
 
   @Native<
       Handle Function(
+          Pointer<Void>,
           Pointer<Void>,
           Int,
           Int,
@@ -298,6 +359,7 @@ base class RenderPass extends NativeFieldWrapperClass1 {
           Pointer<Void>,
           Handle)>(symbol: 'InternalFlutterGpu_RenderPass_SetColorAttachment')
   external String? _setColorAttachment(
+      GpuContext context,
       int colorAttachmentIndex,
       int loadAction,
       int storeAction,
@@ -432,7 +494,7 @@ base class RenderPass extends NativeFieldWrapperClass1 {
       symbol: 'InternalFlutterGpu_RenderPass_SetPrimitiveType')
   external void _setPrimitiveType(int primitiveType);
 
-    @Native<Void Function(Pointer<Void>, Int)>(
+  @Native<Void Function(Pointer<Void>, Int)>(
       symbol: 'InternalFlutterGpu_RenderPass_SetWindingOrder')
   external void _setWindingOrder(int windingOrder);
 

lib/gpu/render_pass.cc

@@ -20,6 +20,7 @@
 #include "impeller/renderer/pipeline.h"
 #include "impeller/renderer/pipeline_descriptor.h"
 #include "impeller/renderer/pipeline_library.h"
+#include "lib/gpu/context.h"
 #include "lib/ui/ui_dart_state.h"
 #include "tonic/converter/dart_converter.h"
 
@@ -222,6 +223,7 @@ void InternalFlutterGpu_RenderPass_Initialize(Dart_Handle wrapper) {
 
 Dart_Handle InternalFlutterGpu_RenderPass_SetColorAttachment(
     flutter::gpu::RenderPass* wrapper,
+    flutter::gpu::Context* context,
     int color_attachment_index,
     int load_action,
     int store_action,
@@ -242,6 +244,14 @@ Dart_Handle InternalFlutterGpu_RenderPass_SetColorAttachment(
         tonic::DartConverter<flutter::gpu::Texture*>::FromDart(
             resolve_texture_wrapper);
     desc.resolve_texture = resolve_texture->GetTexture();
+
+    // If the backend doesn't support normal MSAA, gracefully fallback to
+    // rendering without MSAA.
+    if (!flutter::gpu::SupportsNormalOffscreenMSAA(*context->GetContext())) {
+      desc.texture = desc.resolve_texture;
+      desc.resolve_texture = nullptr;
+      desc.store_action = impeller::StoreAction::kStore;
+    }
   }
   wrapper->GetRenderTarget().SetColorAttachment(desc, color_attachment_index);
   return Dart_Null();

lib/gpu/render_pass.h

@@ -118,6 +118,7 @@ extern void InternalFlutterGpu_RenderPass_Initialize(Dart_Handle wrapper);
 FLUTTER_GPU_EXPORT
 extern Dart_Handle InternalFlutterGpu_RenderPass_SetColorAttachment(
     flutter::gpu::RenderPass* wrapper,
+    flutter::gpu::Context* context,
     int color_attachment_index,
     int load_action,
     int store_action,