Add WebGPU surface alpha mode option

Expose a configurable composite alpha mode for WebGPU windows and external surfaces, and pass it through to native surface configuration. This adds a public enum and option properties so callers can request automatic handling or force opaque presentation.

Author: JimBobSquarePants

src/ImageSharp.Drawing.WebGPU/WebGPUCompositeAlphaMode.cs

@@ -0,0 +1,20 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Drawing.Processing.Backends;
+
+/// <summary>
+/// Specifies how the native compositor interprets alpha in a WebGPU surface.
+/// </summary>
+public enum WebGPUCompositeAlphaMode
+{
+    /// <summary>
+    /// Allows WebGPU to select the surface composition mode.
+    /// </summary>
+    Auto,
+
+    /// <summary>
+    /// Ignores the surface alpha channel and presents every pixel as opaque.
+    /// </summary>
+    Opaque,
+}

src/ImageSharp.Drawing.WebGPU/WebGPUDrawingBackend.cs

@@ -4,9 +4,7 @@
 using System.Runtime.CompilerServices;
 using Silk.NET.WebGPU;
 using Silk.NET.WebGPU.Extensions.WGPU;
-using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
-using SixLabors.ImageSharp.Processing;
 
 namespace SixLabors.ImageSharp.Drawing.Processing.Backends;
 

src/ImageSharp.Drawing.WebGPU/WebGPUExternalSurface.cs

@@ -68,6 +68,7 @@ public WebGPUExternalSurface(
             configuration,
             new SilkNativeSurfaceAdapter(host),
             options.Format,
+            options.AlphaMode,
             this.presentMode,
             this.framebufferSize);
     }

src/ImageSharp.Drawing.WebGPU/WebGPUExternalSurfaceOptions.cs

@@ -27,4 +27,9 @@ public sealed class WebGPUExternalSurfaceOptions
     /// adapter reports it.
     /// </remarks>
     public WebGPUTextureFormat Format { get; set; } = WebGPUTextureFormat.Rgba8Unorm;
+
+    /// <summary>
+    /// Gets or sets how the native compositor interprets the surface alpha channel.
+    /// </summary>
+    public WebGPUCompositeAlphaMode AlphaMode { get; set; } = WebGPUCompositeAlphaMode.Auto;
 }

src/ImageSharp.Drawing.WebGPU/WebGPUSurfaceResources.cs

@@ -48,6 +48,7 @@ internal sealed unsafe class WebGPUSurfaceResources : IDisposable
     private FramePacingSignal? previousFrameWorkDone;
     private readonly FeatureName requiredFeature;
     private readonly Configuration configuration;
+    private readonly WebGPUCompositeAlphaMode alphaMode;
 
     // Mutable because the device-owned portion of the stack is replaced on device-loss recovery.
     private WebGPUDeviceContext deviceContext;
@@ -69,6 +70,7 @@ internal sealed unsafe class WebGPUSurfaceResources : IDisposable
     /// <param name="queueHandle">The owned default queue handle paired with <paramref name="deviceHandle"/>.</param>
     /// <param name="deviceContext">The device context bound to <paramref name="deviceHandle"/> and <paramref name="queueHandle"/>.</param>
     /// <param name="format">The negotiated swapchain texture format.</param>
+    /// <param name="alphaMode">How the native compositor interprets the swapchain alpha channel.</param>
     /// <param name="requiredFeature">The optional WebGPU feature required by the selected texture format.</param>
     private WebGPUSurfaceResources(
         WebGPU api,
@@ -80,6 +82,7 @@ private WebGPUSurfaceResources(
         WebGPUQueueHandle queueHandle,
         WebGPUDeviceContext deviceContext,
         WebGPUTextureFormat format,
+        WebGPUCompositeAlphaMode alphaMode,
         FeatureName requiredFeature)
     {
         this.Api = api;
@@ -91,6 +94,7 @@ private WebGPUSurfaceResources(
         this.QueueHandle = queueHandle;
         this.deviceContext = deviceContext;
         this.Format = format;
+        this.alphaMode = alphaMode;
         this.requiredFeature = requiredFeature;
     }
 
@@ -138,6 +142,7 @@ private WebGPUSurfaceResources(
     /// <param name="configuration">The ImageSharp configuration the device context will use for its rendering backend.</param>
     /// <param name="nativeSource">The native surface source that provides the platform handles for surface creation.</param>
     /// <param name="format">The swapchain texture format.</param>
+    /// <param name="alphaMode">How the native compositor interprets the swapchain alpha channel.</param>
     /// <param name="initialPresentMode">The present mode to apply to the first surface configuration.</param>
     /// <param name="initialFramebufferSize">The framebuffer size to apply to the first surface configuration. Zero-area
     /// sizes are permitted and leave the surface unconfigured until the caller invokes <see cref="ConfigureSurface"/>
@@ -152,6 +157,7 @@ public static WebGPUSurfaceResources Create(
         Configuration configuration,
         INativeWindowSource nativeSource,
         WebGPUTextureFormat format,
+        WebGPUCompositeAlphaMode alphaMode,
         WebGPUPresentMode initialPresentMode,
         Size initialFramebufferSize)
     {
@@ -191,6 +197,7 @@ public static WebGPUSurfaceResources Create(
                 deviceResources.QueueHandle,
                 deviceResources.DeviceContext,
                 format,
+                alphaMode,
                 requiredFeature);
 
             resources.ConfigureSurface(initialPresentMode, initialFramebufferSize);
@@ -250,6 +257,7 @@ private void ConfigureSurfaceCore(WebGPUPresentMode presentMode, Size framebuffe
         {
             Usage = TextureUsage.RenderAttachment | TextureUsage.CopySrc | TextureUsage.CopyDst | TextureUsage.TextureBinding,
             Format = WebGPUTextureFormatMapper.ToNative(this.Format),
+            AlphaMode = ToNative(this.alphaMode),
             PresentMode = ToNative(presentMode),
             Width = (uint)framebufferSize.Width,
             Height = (uint)framebufferSize.Height,
@@ -275,6 +283,19 @@ private static SilkPresentMode ToNative(WebGPUPresentMode presentMode)
             _ => throw new InvalidOperationException("The WebGPU present mode mapping is incomplete.")
         };
 
+    /// <summary>
+    /// Maps the public composite-alpha enumeration to the native Silk.NET value.
+    /// </summary>
+    /// <param name="alphaMode">The composite-alpha mode to map.</param>
+    /// <returns>The native composite-alpha mode.</returns>
+    private static CompositeAlphaMode ToNative(WebGPUCompositeAlphaMode alphaMode)
+        => alphaMode switch
+        {
+            WebGPUCompositeAlphaMode.Auto => CompositeAlphaMode.Auto,
+            WebGPUCompositeAlphaMode.Opaque => CompositeAlphaMode.Opaque,
+            _ => throw new InvalidOperationException("The WebGPU composite alpha mode mapping is incomplete.")
+        };
+
     /// <summary>
     /// Acquires the next presentable frame from <see cref="SurfaceHandle"/> and wraps it as a
     /// <see cref="WebGPUSurfaceFrame"/> with a ready-to-use <see cref="DrawingCanvas"/>.

src/ImageSharp.Drawing.WebGPU/WebGPUWindow.cs

@@ -53,6 +53,7 @@ public WebGPUWindow(Configuration configuration, WebGPUWindowOptions options)
         this.window = Window.Create(CreateSilkOptions(options));
         this.Configuration = configuration;
         this.Format = options.Format;
+        this.AlphaMode = options.AlphaMode;
         this.presentMode = options.PresentMode;
 
         try
@@ -62,6 +63,7 @@ public WebGPUWindow(Configuration configuration, WebGPUWindowOptions options)
                 configuration,
                 this.window,
                 this.Format,
+                this.AlphaMode,
                 this.presentMode,
                 ToSize(this.window.FramebufferSize));
         }
@@ -272,6 +274,11 @@ public WebGPUPresentMode PresentMode
     /// </summary>
     public WebGPUTextureFormat Format { get; }
 
+    /// <summary>
+    /// Gets how the native compositor interprets the window surface alpha channel.
+    /// </summary>
+    public WebGPUCompositeAlphaMode AlphaMode { get; }
+
     /// <summary>
     /// Tries to acquire the next drawable frame using default drawing options.
     /// </summary>
@@ -558,6 +565,7 @@ private static WindowOptions CreateSilkOptions(WebGPUWindowOptions options)
         silkOptions.WindowState = ToNative(options.WindowState);
         silkOptions.WindowBorder = ToNative(options.WindowBorder);
         silkOptions.TopMost = options.IsTopMost;
+
         return silkOptions;
     }
 

src/ImageSharp.Drawing.WebGPU/WebGPUWindowOptions.cs

@@ -87,4 +87,9 @@ public sealed class WebGPUWindowOptions
     /// Gets or sets the swapchain texture format used by acquired frames.
     /// </summary>
     public WebGPUTextureFormat Format { get; set; } = WebGPUTextureFormat.Rgba8Unorm;
+
+    /// <summary>
+    /// Gets or sets how the native compositor interprets the window surface alpha channel.
+    /// </summary>
+    public WebGPUCompositeAlphaMode AlphaMode { get; set; } = WebGPUCompositeAlphaMode.Auto;
 }