Manually setup FBOs for layers

src/Skia/Avalonia.Skia/DrawingContextImpl.cs

@@ -31,6 +31,7 @@ namespace Avalonia.Skia
         private bool _disposed;
         private GRContext _grContext;
         public GRContext GrContext => _grContext;
+        private ISkiaGpu _gpu;
         private readonly SKPaint _strokePaint = new SKPaint();
         private readonly SKPaint _fillPaint = new SKPaint();
         private readonly SKPaint _boxShadowPaint = new SKPaint();
@@ -65,6 +66,11 @@ namespace Avalonia.Skia
             /// GPU-accelerated context (optional)
             /// </summary>
             public GRContext GrContext;
+
+            /// <summary>
+            /// Skia GPU provider context (optional)
+            /// </summary>
+            public ISkiaGpu Gpu;
         }
 
         /// <summary>
@@ -79,6 +85,7 @@ namespace Avalonia.Skia
             _disposables = disposables;
             _canTextUseLcdRendering = !createInfo.DisableTextLcdRendering;
             _grContext = createInfo.GrContext;
+            _gpu = createInfo.Gpu;
             if (_grContext != null)
                 Monitor.Enter(_grContext);
             Surface = createInfo.Surface;
@@ -417,7 +424,7 @@ namespace Avalonia.Skia
         /// <inheritdoc />
         public IRenderTargetBitmapImpl CreateLayer(Size size)
         {
-            return CreateRenderTarget(size);
+            return CreateRenderTarget( size);
         }
 
         /// <inheritdoc />
@@ -925,7 +932,8 @@ namespace Avalonia.Skia
                 Dpi = _dpi,
                 Format = format,
                 DisableTextLcdRendering = !_canTextUseLcdRendering,
-                GrContext = _grContext
+                GrContext = _grContext,
+                Gpu = _gpu
             };
 
             return new SurfaceRenderTarget(createInfo);

src/Skia/Avalonia.Skia/Gpu/ISkiaGpu.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using Avalonia.OpenGL.Imaging;
 using SkiaSharp;
@@ -15,6 +16,19 @@ namespace Avalonia.Skia
         /// <param name="surfaces">Surfaces.</param>
         /// <returns>Created render target or <see langword="null"/> if it fails.</returns>
         ISkiaGpuRenderTarget TryCreateRenderTarget(IEnumerable<object> surfaces);
+        
+        /// <summary>
+        /// Creates an offscreen render target surface
+        /// </summary>
+        /// <param name="size">size in pixels</param>
+        ISkiaSurface TryCreateSurface(PixelSize size);
+    }
+    
+    public interface ISkiaSurface : IDisposable
+    {
+        SKSurface WriteSurface { get; }
+        SKSurface ReadSurface { get; }
+        
     }
 
     public interface IOpenGlAwareSkiaGpu : ISkiaGpu

src/Skia/Avalonia.Skia/Gpu/OpenGl/FboSkiaSurface.cs

@@ -0,0 +1,124 @@
+using System;
+using Avalonia.OpenGL;
+using SkiaSharp;
+using static Avalonia.OpenGL.GlConsts;
+namespace Avalonia.Skia
+{
+    public class FboSkiaSurface : ISkiaSurface
+    {
+        private readonly GRContext _grContext;
+        private readonly IGlContext _glContext;
+        private readonly PixelSize _pixelSize;
+        private int _fbo;
+        private int _depthStencil;
+        private int _texture;
+
+        private static readonly bool[] TrueFalse = new[] { true, false };
+        public FboSkiaSurface(GRContext grContext, IGlContext glContext, PixelSize pixelSize)
+        {
+            _grContext = grContext;
+            _glContext = glContext;
+            _pixelSize = pixelSize;
+            var InternalFormat = glContext.Version.Type == GlProfileType.OpenGLES ? GL_RGBA : GL_RGBA8;
+            var gl = glContext.GlInterface;
+            
+            // Save old bindings
+            gl.GetIntegerv(GL_FRAMEBUFFER_BINDING, out var oldFbo);
+            gl.GetIntegerv(GL_RENDERBUFFER_BINDING, out var oldRenderbuffer);
+            gl.GetIntegerv(GL_TEXTURE_BINDING_2D, out var oldTexture);
+
+            var arr = new int[2];
+
+            // Generate FBO
+            gl.GenFramebuffers(1, arr);
+            _fbo = arr[0];
+            gl.BindFramebuffer(GL_FRAMEBUFFER, _fbo);
+
+            // Create a texture to render into
+            gl.GenTextures(1, arr);
+            _texture = arr[0];
+            gl.BindTexture(GL_TEXTURE_2D, _texture);
+            gl.TexImage2D(GL_TEXTURE_2D, 0,
+                InternalFormat, pixelSize.Width, pixelSize.Height,
+                0, GL_RGBA, GL_UNSIGNED_BYTE, IntPtr.Zero);
+            gl.TexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
+            gl.TexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
+            gl.FramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, _texture, 0);
+
+            var success = false;
+            foreach (var useStencil8 in TrueFalse)
+            {
+                gl.GenRenderbuffers(1, arr);
+                _depthStencil = arr[0];
+                gl.BindRenderbuffer(GL_RENDERBUFFER, _depthStencil);
+
+                if (useStencil8)
+                {
+                    gl.RenderbufferStorage(GL_RENDERBUFFER, GL_STENCIL_INDEX8, pixelSize.Width, pixelSize.Height);
+                    gl.FramebufferRenderbuffer(GL_FRAMEBUFFER, GL_STENCIL_ATTACHMENT, GL_RENDERBUFFER, _depthStencil);
+                }
+                else
+                {
+                    gl.RenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH24_STENCIL8, pixelSize.Width, pixelSize.Height);
+                    gl.FramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, _depthStencil);
+                    gl.FramebufferRenderbuffer(GL_FRAMEBUFFER, GL_STENCIL_ATTACHMENT, GL_RENDERBUFFER, _depthStencil);
+                }
+
+                var status = gl.CheckFramebufferStatus(GL_FRAMEBUFFER);
+                if (status == GL_FRAMEBUFFER_COMPLETE)
+                {
+                    success = true;
+                    break;
+                }
+                else
+                {
+                    gl.BindRenderbuffer(GL_RENDERBUFFER, oldRenderbuffer);
+                    gl.DeleteRenderbuffers(1, arr);
+                }
+            }
+            
+            gl.BindRenderbuffer(GL_RENDERBUFFER, oldRenderbuffer);
+            gl.BindTexture(GL_TEXTURE_2D, oldTexture);
+            gl.BindFramebuffer(GL_FRAMEBUFFER, oldFbo);
+
+            if (!success)
+            {
+                arr[0] = _fbo;
+                gl.DeleteFramebuffers(1, arr);
+                arr[0] = _texture;
+                gl.DeleteTextures(1, arr);
+                throw new OpenGlException("Unable to create FBO with stencil");
+            }
+
+            ReadSurface = SKSurface.Create(_grContext, new GRBackendTexture(pixelSize.Width, pixelSize.Height, false,
+                new GRGlTextureInfo(GL_TEXTURE_2D, (uint)_texture, (uint)InternalFormat)), SKColorType.Rgba8888);
+
+            var target = new GRBackendRenderTarget(pixelSize.Width, pixelSize.Height, 0, 0,
+                new GRGlFramebufferInfo((uint)_fbo, SKColorType.Rgba8888.ToGlSizedFormat()));
+            WriteSurface = SKSurface.Create(_grContext, target,
+                GRSurfaceOrigin.BottomLeft, SKColorType.Rgba8888);
+        }
+        
+        public void Dispose()
+        {
+            using (_glContext.EnsureCurrent())
+            {
+                WriteSurface.Dispose();
+                WriteSurface = null;
+                ReadSurface.Dispose();
+                ReadSurface = null;
+                var gl = _glContext.GlInterface;
+                if (_fbo != 0)
+                {
+                    gl.DeleteFramebuffers(1, new[] { _fbo });
+                    gl.DeleteTextures(1, new[] { _texture });
+                    gl.DeleteRenderbuffers(1, new[] { _depthStencil });
+                    _fbo = _texture = _depthStencil = 0;
+                }
+            }
+        }
+
+        public SKSurface WriteSurface { get; private set; }
+        public SKSurface ReadSurface { get; private set; }
+    }
+}

src/Skia/Avalonia.Skia/Gpu/OpenGl/GlSkiaGpu.cs

@@ -43,6 +43,11 @@ namespace Avalonia.Skia
             return null;
         }
 
+        public ISkiaSurface TryCreateSurface(PixelSize size)
+        {
+            return new FboSkiaSurface(_grContext, _glContext, size);
+        }
+
         public IOpenGlBitmapImpl CreateOpenGlBitmap(PixelSize size, Vector dpi) => new GlOpenGlBitmapImpl(_glContext, size, dpi);
     }
 }

src/Skia/Avalonia.Skia/Gpu/SkiaGpuRenderTarget.cs

@@ -8,10 +8,12 @@ namespace Avalonia.Skia
     /// </summary>
     internal class SkiaGpuRenderTarget : IRenderTargetWithCorruptionInfo
     {
+        private readonly ISkiaGpu _skiaGpu;
         private readonly ISkiaGpuRenderTarget _renderTarget;
 
-        public SkiaGpuRenderTarget(ISkiaGpuRenderTarget renderTarget)
+        public SkiaGpuRenderTarget(ISkiaGpu skiaGpu, ISkiaGpuRenderTarget renderTarget)
         {
+            _skiaGpu = skiaGpu;
             _renderTarget = renderTarget;
         }
 
@@ -30,7 +32,8 @@ namespace Avalonia.Skia
                 Surface = session.SkSurface,
                 Dpi = SkiaPlatform.DefaultDpi * session.ScaleFactor,
                 VisualBrushRenderer = visualBrushRenderer,
-                DisableTextLcdRendering = true
+                DisableTextLcdRendering = true,
+                Gpu = _skiaGpu
             };
 
             return new DrawingContextImpl(nfo, session);

src/Skia/Avalonia.Skia/PlatformRenderInterface.cs

@@ -138,7 +138,7 @@ namespace Avalonia.Skia
             var gpuRenderTarget = _skiaGpu?.TryCreateRenderTarget(surfaces);
             if (gpuRenderTarget != null)
             {
-                return new SkiaGpuRenderTarget(gpuRenderTarget);
+                return new SkiaGpuRenderTarget(_skiaGpu, gpuRenderTarget);
             }
 
             foreach (var surface in surfaces)

src/Skia/Avalonia.Skia/SurfaceRenderTarget.cs

@@ -13,10 +13,29 @@ namespace Avalonia.Skia
     /// </summary>
     internal class SurfaceRenderTarget : IRenderTargetBitmapImpl, IDrawableBitmapImpl
     {
-        private readonly SKSurface _surface;
+        private readonly ISkiaSurface _surface;
         private readonly SKCanvas _canvas;
         private readonly bool _disableLcdRendering;
         private readonly GRContext _grContext;
+        private readonly ISkiaGpu _gpu;
+
+        class SkiaSurfaceWrapper : ISkiaSurface
+        {
+            public SKSurface WriteSurface { get; private set; }
+            public SKSurface ReadSurface { get; private set; }
+
+            public SkiaSurfaceWrapper(SKSurface surface)
+            {
+                WriteSurface = ReadSurface = surface;
+            }
+
+            public void Dispose()
+            {
+                WriteSurface?.Dispose();
+                WriteSurface = null;
+                ReadSurface = null;
+            }
+        }
         
         /// <summary>
         /// Create new surface render target.
@@ -29,9 +48,14 @@ namespace Avalonia.Skia
 
             _disableLcdRendering = createInfo.DisableTextLcdRendering;
             _grContext = createInfo.GrContext;
-            _surface = CreateSurface(createInfo.GrContext, PixelSize.Width, PixelSize.Height, createInfo.Format);
+            _gpu = createInfo.Gpu;
 
-            _canvas = _surface?.Canvas;
+            _surface = _gpu?.TryCreateSurface(PixelSize);
+            if (_surface == null)
+                _surface = new SkiaSurfaceWrapper(CreateSurface(createInfo.GrContext, PixelSize.Width, PixelSize.Height,
+                    createInfo.Format));
+
+            _canvas = _surface?.WriteSurface.Canvas;
 
             if (_surface == null || _canvas == null)
             {
@@ -70,11 +94,12 @@ namespace Avalonia.Skia
             
             var createInfo = new DrawingContextImpl.CreateInfo
             {
-                Surface = _surface,
+                Surface = _surface.WriteSurface,
                 Dpi = Dpi,
                 VisualBrushRenderer = visualBrushRenderer,
                 DisableTextLcdRendering = _disableLcdRendering,
-                GrContext = _grContext
+                GrContext = _grContext,
+                Gpu = _gpu
             };
 
             return new DrawingContextImpl(createInfo, Disposable.Create(() => Version++));
@@ -111,8 +136,8 @@ namespace Avalonia.Skia
         {
             if (sourceRect.Left == 0 && sourceRect.Top == 0 && sourceRect.Size == destRect.Size)
             {
-                _surface.Canvas.Flush();
+                _surface.ReadSurface.Canvas.Flush();
-                _surface.Draw(context.Canvas, destRect.Left, destRect.Top, paint);
+                _surface.ReadSurface.Draw(context.Canvas, destRect.Left, destRect.Top, paint);
             }
             else
                 using (var image = SnapshotImage())
@@ -127,7 +152,7 @@ namespace Avalonia.Skia
         /// <returns>Image snapshot.</returns>
         public SKImage SnapshotImage()
         {
-            return _surface.Snapshot();
+            return _surface.ReadSurface.Snapshot();
         }
 
         /// <summary>
@@ -178,6 +203,8 @@ namespace Avalonia.Skia
             /// GPU-accelerated context (optional)
             /// </summary>
             public GRContext GrContext;
+
+            public ISkiaGpu Gpu;
         }
     }
 }