Add AlphaFormat to ILockedFramebuffer (#20556)

* Add AlphaFormat to ILockedFramebuffer

* Update API suppressions

api/Avalonia.nupkg.xml

@@ -19,6 +19,12 @@
     <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0001</DiagnosticId>
+    <Target>T:Avalonia.Platform.IReadableBitmapWithAlphaImpl</Target>
+    <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0001</DiagnosticId>
     <Target>T:Avalonia.Utilities.StringTokenizer</Target>
@@ -55,6 +61,12 @@
     <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0001</DiagnosticId>
+    <Target>T:Avalonia.Platform.IReadableBitmapWithAlphaImpl</Target>
+    <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0001</DiagnosticId>
     <Target>T:Avalonia.Utilities.StringTokenizer</Target>
@@ -127,6 +139,12 @@
     <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>M:Avalonia.Media.Imaging.Bitmap.CopyPixels(Avalonia.Platform.ILockedFramebuffer,Avalonia.Platform.AlphaFormat)</Target>
+    <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0002</DiagnosticId>
     <Target>M:Avalonia.Media.StreamGeometryContext.ArcTo(Avalonia.Point,Avalonia.Size,System.Double,System.Boolean,Avalonia.Media.SweepDirection)</Target>
@@ -229,6 +247,12 @@
     <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>M:Avalonia.Platform.LockedFramebuffer.#ctor(System.IntPtr,Avalonia.PixelSize,System.Int32,Avalonia.Vector,Avalonia.Platform.PixelFormat,System.Action)</Target>
+    <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0002</DiagnosticId>
     <Target>M:Avalonia.Visuals.Platform.PathGeometryContext.ArcTo(Avalonia.Point,Avalonia.Size,System.Double,System.Boolean,Avalonia.Media.SweepDirection)</Target>
@@ -481,6 +505,12 @@
     <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>M:Avalonia.Media.Imaging.Bitmap.CopyPixels(Avalonia.Platform.ILockedFramebuffer,Avalonia.Platform.AlphaFormat)</Target>
+    <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0002</DiagnosticId>
     <Target>M:Avalonia.Media.StreamGeometryContext.ArcTo(Avalonia.Point,Avalonia.Size,System.Double,System.Boolean,Avalonia.Media.SweepDirection)</Target>
@@ -583,6 +613,12 @@
     <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>M:Avalonia.Platform.LockedFramebuffer.#ctor(System.IntPtr,Avalonia.PixelSize,System.Int32,Avalonia.Vector,Avalonia.Platform.PixelFormat,System.Action)</Target>
+    <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0002</DiagnosticId>
     <Target>M:Avalonia.Visuals.Platform.PathGeometryContext.ArcTo(Avalonia.Point,Avalonia.Size,System.Double,System.Boolean,Avalonia.Media.SweepDirection)</Target>
@@ -853,6 +889,18 @@
     <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0006</DiagnosticId>
+    <Target>P:Avalonia.Platform.ILockedFramebuffer.AlphaFormat</Target>
+    <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
+  </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0006</DiagnosticId>
+    <Target>P:Avalonia.Platform.IReadableBitmapImpl.AlphaFormat</Target>
+    <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0006</DiagnosticId>
     <Target>M:Avalonia.Input.Platform.IClipboard.SetDataAsync(Avalonia.Input.IAsyncDataTransfer)</Target>
@@ -1003,6 +1051,18 @@
     <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0006</DiagnosticId>
+    <Target>P:Avalonia.Platform.ILockedFramebuffer.AlphaFormat</Target>
+    <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
+  </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0006</DiagnosticId>
+    <Target>P:Avalonia.Platform.IReadableBitmapImpl.AlphaFormat</Target>
+    <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0006</DiagnosticId>
     <Target>M:Avalonia.OpenGL.IGlExternalSemaphore.SignalTimelineSemaphore(Avalonia.OpenGL.IGlExternalImageTexture,System.UInt64)</Target>
@@ -1087,12 +1147,24 @@
     <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0008</DiagnosticId>
+    <Target>T:Avalonia.Platform.IWriteableBitmapImpl</Target>
+    <Left>baseline/Avalonia/lib/net10.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net10.0/Avalonia.Base.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0008</DiagnosticId>
     <Target>T:Avalonia.Media.StreamGeometryContext</Target>
     <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
     <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0008</DiagnosticId>
+    <Target>T:Avalonia.Platform.IWriteableBitmapImpl</Target>
+    <Left>baseline/Avalonia/lib/net8.0/Avalonia.Base.dll</Left>
+    <Right>current/Avalonia/lib/net8.0/Avalonia.Base.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0009</DiagnosticId>
     <Target>T:Avalonia.Platform.Screen</Target>

src/Android/Avalonia.Android/Platform/SkiaPlatform/AndroidFramebuffer.cs

@@ -25,10 +25,10 @@ namespace Avalonia.Android.Platform.SkiaPlatform
             Size = new PixelSize(rc.right, rc.bottom);
             ANativeWindow_lock(_window, &buffer, &rc);
 
-            Format = buffer.format == AndroidPixelFormat.WINDOW_FORMAT_RGB_565
-                ? PixelFormat.Rgb565 : PixelFormat.Rgba8888;
+            (Format, AlphaFormat, RowBytes) = buffer.format == AndroidPixelFormat.WINDOW_FORMAT_RGB_565 ?
+                (PixelFormat.Rgb565, AlphaFormat.Opaque, buffer.stride * 2) :
+                (PixelFormat.Rgba8888, AlphaFormat.Premul, buffer.stride * 4);
 
-            RowBytes = buffer.stride * (Format == PixelFormat.Rgb565 ? 2 : 4);
             Address = buffer.bits;
 
             Dpi = new Vector(96, 96) * scaling;
@@ -46,6 +46,7 @@ namespace Avalonia.Android.Platform.SkiaPlatform
         public int RowBytes { get; }
         public Vector Dpi { get; }
         public PixelFormat Format { get; }
+        public AlphaFormat AlphaFormat { get; }
 
         [DllImport("android")]
         internal static extern IntPtr ANativeWindow_fromSurface(IntPtr jniEnv, IntPtr handle);

src/Avalonia.Base/Media/Imaging/Bitmap.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Diagnostics.CodeAnalysis;
 using System.IO;
 using System.Runtime.CompilerServices;
 using Avalonia.Platform;
@@ -177,7 +176,7 @@ namespace Avalonia.Media.Imaging
 
         public virtual PixelFormat? Format => (PlatformImpl.Item as IReadableBitmapImpl)?.Format;
 
-        public virtual AlphaFormat? AlphaFormat => (PlatformImpl.Item as IReadableBitmapWithAlphaImpl)?.AlphaFormat;
+        public virtual AlphaFormat? AlphaFormat => (PlatformImpl.Item as IReadableBitmapImpl)?.AlphaFormat;
 
         private protected unsafe void CopyPixelsCore(PixelRect sourceRect, IntPtr buffer, int bufferSize, int stride,
             ILockedFramebuffer fb)
@@ -237,16 +236,15 @@ namespace Avalonia.Media.Imaging
         /// Copies pixels to the target buffer and transcodes the pixel and alpha format if needed.
         /// </summary>
         /// <param name="buffer">The target buffer.</param>
-        /// <param name="alphaFormat">The alpha format.</param>
         /// <exception cref="NotSupportedException"></exception>
-        public void CopyPixels(ILockedFramebuffer buffer, AlphaFormat alphaFormat)
+        public void CopyPixels(ILockedFramebuffer buffer)
         {
-            if (PlatformImpl.Item is not IReadableBitmapWithAlphaImpl readable || readable.Format == null || readable.AlphaFormat == null)
+            if (PlatformImpl.Item is not IReadableBitmapImpl readable || readable.Format == null || readable.AlphaFormat == null)
             {
                 throw new NotSupportedException("CopyPixels is not supported for this bitmap type");
             }
 
-            if (buffer.Format != readable.Format || alphaFormat != readable.AlphaFormat)
+            if (buffer.Format != readable.Format || buffer.AlphaFormat != readable.AlphaFormat)
             {
                 using (var fb = readable.Lock())
                 {
@@ -255,11 +253,11 @@ namespace Avalonia.Media.Imaging
                         fb.Size,
                         fb.RowBytes,
                         fb.Format,
-                        readable.AlphaFormat.Value, 
+                        fb.AlphaFormat,
                         buffer.Address, 
                         buffer.RowBytes,
                         buffer.Format,
-                        alphaFormat);
+                        buffer.AlphaFormat);
                 }
             }
             else

src/Avalonia.Base/Media/Imaging/WriteableBitmap.cs

@@ -71,10 +71,10 @@ namespace Avalonia.Media.Imaging
             
             return new LockedFramebuffer(_pixelFormatMemory.Address, _pixelFormatMemory.Size,
                 _pixelFormatMemory.RowBytes,
-                Dpi, _pixelFormatMemory.Format, () =>
+                Dpi, _pixelFormatMemory.Format, _pixelFormatMemory.AlphaFormat, () =>
                 {
                     using var inner = ((IWriteableBitmapImpl)PlatformImpl.Item).Lock();
-                    _pixelFormatMemory.CopyToRgba(Platform.AlphaFormat.Unpremul, inner.Address, inner.RowBytes);
+                    _pixelFormatMemory.CopyToRgba(inner.AlphaFormat, inner.Address, inner.RowBytes);
                 });
         }
 

src/Avalonia.Base/Platform/ILockedFramebuffer.cs

@@ -29,6 +29,9 @@ namespace Avalonia.Platform
         /// </summary>
         PixelFormat Format { get; }
 
-        //TODO12: Add AlphaFormat
+        /// <summary>
+        /// Gets the alpha format.
+        /// </summary>
+        AlphaFormat AlphaFormat { get; }
     }
 }

src/Avalonia.Base/Platform/IReadableBitmapImpl.cs

@@ -2,15 +2,10 @@ using Avalonia.Metadata;
 
 namespace Avalonia.Platform;
 
-public interface IReadableBitmapImpl
+[PrivateApi]
+public interface IReadableBitmapImpl : IBitmapImpl
 {
     PixelFormat? Format { get; }
-    ILockedFramebuffer Lock();
-}
-
-//TODO12: Remove me once we can change IReadableBitmapImpl
-[Unstable]
-public interface IReadableBitmapWithAlphaImpl : IReadableBitmapImpl
-{
     AlphaFormat? AlphaFormat { get; }
+    ILockedFramebuffer Lock();
 }

src/Avalonia.Base/Platform/IWriteableBitmapImpl.cs

@@ -5,8 +5,8 @@ namespace Avalonia.Platform
     /// <summary>
     /// Defines the platform-specific interface for a <see cref="Avalonia.Media.Imaging.WriteableBitmap"/>.
     /// </summary>
-    [Unstable]
-    public interface IWriteableBitmapImpl : IBitmapImpl, IReadableBitmapWithAlphaImpl
+    [PrivateApi]
+    public interface IWriteableBitmapImpl : IBitmapImpl, IReadableBitmapImpl
     {
     }
 }

src/Avalonia.Base/Platform/LockedFramebuffer.cs

@@ -7,7 +7,7 @@ namespace Avalonia.Platform
         private readonly Action? _onDispose;
 
         public LockedFramebuffer(IntPtr address, PixelSize size, int rowBytes, Vector dpi, PixelFormat format,
-            Action? onDispose)
+            AlphaFormat alphaFormat, Action? onDispose)
         {
             _onDispose = onDispose;
             Address = address;
@@ -15,6 +15,7 @@ namespace Avalonia.Platform
             RowBytes = rowBytes;
             Dpi = dpi;
             Format = format;
+            AlphaFormat = alphaFormat;
         }
 
         public IntPtr Address { get; }
@@ -22,6 +23,7 @@ namespace Avalonia.Platform
         public int RowBytes { get; }
         public Vector Dpi { get; }
         public PixelFormat Format { get; }
+        public AlphaFormat AlphaFormat { get; }
 
         public void Dispose()
         {

src/Avalonia.Base/Platform/RetainedFramebuffer.cs

@@ -1,6 +1,4 @@
 using System;
-using System.Runtime.InteropServices;
-using Avalonia.Metadata;
 using Avalonia.Platform.Internal;
 
 namespace Avalonia.Platform;
@@ -10,6 +8,7 @@ internal class RetainedFramebuffer : IDisposable
     public PixelSize Size { get; }
     public  int RowBytes { get; }
     public PixelFormat Format { get; }
+    public AlphaFormat AlphaFormat { get; }
     public IntPtr Address => _blob?.Address ?? throw new ObjectDisposedException(nameof(RetainedFramebuffer));
     private UnmanagedBlob? _blob;
 
@@ -17,13 +16,13 @@ internal class RetainedFramebuffer : IDisposable
         ? format
         : throw new ArgumentOutOfRangeException(nameof(format));
 
-    public RetainedFramebuffer(PixelSize size, PixelFormat format) : this(size, ValidateKnownFormat(format),
-        format.BitsPerPixel / 8 * size.Width)
+    public RetainedFramebuffer(PixelSize size, PixelFormat format, AlphaFormat alphaFormat)
+        : this(size, ValidateKnownFormat(format), alphaFormat, format.BitsPerPixel / 8 * size.Width)
     {
         
     }
     
-    public RetainedFramebuffer(PixelSize size, PixelFormat format, int rowBytes)
+    public RetainedFramebuffer(PixelSize size, PixelFormat format, AlphaFormat alphaFormat, int rowBytes)
     {
         if (size.Width <= 0 || size.Height <= 0)
             throw new ArgumentOutOfRangeException(nameof(size));
@@ -32,6 +31,7 @@ internal class RetainedFramebuffer : IDisposable
         Size = size;
         RowBytes = rowBytes;
         Format = format;
+        AlphaFormat = alphaFormat;
         _blob = new UnmanagedBlob(RowBytes * size.Height);
     }
 
@@ -39,7 +39,7 @@ internal class RetainedFramebuffer : IDisposable
     {
         if (_blob == null)
             throw new ObjectDisposedException(nameof(RetainedFramebuffer));
-        return  new LockedFramebuffer(_blob.Address, Size, RowBytes, dpi, Format, () =>
+        return  new LockedFramebuffer(_blob.Address, Size, RowBytes, dpi, Format, AlphaFormat, () =>
         {
             blit(this);
             GC.KeepAlive(this);
@@ -51,4 +51,4 @@ internal class RetainedFramebuffer : IDisposable
         _blob?.Dispose();
         _blob = null;
     }
-}
+}

src/Avalonia.Controls/Remote/Server/RemoteServerTopLevelImpl.Framebuffer.cs

@@ -76,6 +76,7 @@ namespace Avalonia.Controls.Remote.Server
                         Stride,
                         new Vector(_dpi, _dpi),
                         new PlatformPixelFormat((PixelFormatEnum)Format),
+                        Format == ProtocolPixelFormat.Rgb565 ? AlphaFormat.Opaque : AlphaFormat.Premul,
                         () =>
                         {
                             handle.Free();

src/Avalonia.Native/DeferredFramebuffer.cs

@@ -29,6 +29,7 @@ namespace Avalonia.Native
         public int RowBytes { get; set; }
         public Vector Dpi { get; set; }
         public PixelFormat Format { get; set; }
+        public AlphaFormat AlphaFormat { get; set; }
 
         public void Dispose()
         {

src/Avalonia.X11/X11CursorFactory.cs

@@ -133,7 +133,7 @@ namespace Avalonia.X11
                 return new LockedFramebuffer(
                     _blob.Address + Marshal.SizeOf<XcursorImage>(),
                     _pixelSize, _pixelSize.Width * 4,
-                    new Vector(96, 96), PixelFormat.Bgra8888, null);
+                    new Vector(96, 96), PixelFormat.Bgra8888, AlphaFormat.Premul, null);
             }
             
             public IFramebufferRenderTarget CreateFramebufferRenderTarget() => new FuncFramebufferRenderTarget(Lock);

src/Avalonia.X11/X11FramebufferSurface.cs

@@ -61,7 +61,7 @@ namespace Avalonia.X11
             {
                 _fb?.Dispose();
                 _fb = null;
-                _fb = new RetainedFramebuffer(new PixelSize(width, height), PixelFormat.Bgra8888);
+                _fb = new RetainedFramebuffer(new PixelSize(width, height), PixelFormat.Bgra8888, AlphaFormat.Premul);
             }
 
             properties = new FramebufferLockProperties(framebufferValid);

src/Avalonia.X11/X11IconLoader.cs

@@ -83,7 +83,7 @@ namespace Avalonia.X11
         {
             var h = GCHandle.Alloc(_bdata, GCHandleType.Pinned);
             return new LockedFramebuffer(h.AddrOfPinnedObject(), new PixelSize(_width, _height), _width * 4,
-                new Vector(96, 96), PixelFormat.Bgra8888,
+                new Vector(96, 96), PixelFormat.Bgra8888, AlphaFormat.Premul,
                 () => h.Free());
         }
         

src/Browser/Avalonia.Browser/Rendering/BrowserSoftwareRenderTarget.cs

@@ -47,7 +47,7 @@ partial class BrowserSoftwareRenderTarget : BrowserRenderTarget, IFramebufferPla
             {
                 _fb?.Dispose();
                 _fb = null;
-                _fb = new RetainedFramebuffer(size, PixelFormat.Rgba8888);
+                _fb = new RetainedFramebuffer(size, PixelFormat.Rgba8888, AlphaFormat.Premul);
             }
 
             return _fb.Lock(new Vector(scaling * 96, scaling * 96), _parent._blit);
@@ -66,4 +66,4 @@ partial class BrowserSoftwareRenderTarget : BrowserRenderTarget, IFramebufferPla
     {
         PutPixelData(Js, fb.Address.ToInt32(), fb.Size.Width * fb.Size.Height * 4, fb.Size.Width, fb.Size.Height);
     }
-}
+}

src/Headless/Avalonia.Headless/HeadlessPlatformRenderInterface.cs

@@ -414,8 +414,8 @@ namespace Avalonia.Headless
 
             public Vector Dpi { get; }
             public PixelSize PixelSize { get; }
-            public PixelFormat? Format { get; }
-            public AlphaFormat? AlphaFormat { get; }
+            public PixelFormat? Format => PixelFormat.Rgba8888;
+            public AlphaFormat? AlphaFormat => Platform.AlphaFormat.Premul;
             public int Version { get; set; }
 
             public void Save(string fileName, int? quality = null)
@@ -434,7 +434,7 @@ namespace Avalonia.Headless
                 Version++;
                 var mem = Marshal.AllocHGlobal(PixelSize.Width * PixelSize.Height * 4);
                 return new LockedFramebuffer(mem, PixelSize, PixelSize.Width * 4, Dpi, PixelFormat.Rgba8888,
-                    () => Marshal.FreeHGlobal(mem));
+                    Platform.AlphaFormat.Premul, () => Marshal.FreeHGlobal(mem));
             }
         }
 

src/Headless/Avalonia.Headless/HeadlessWindowImpl.cs

@@ -200,6 +200,7 @@ namespace Avalonia.Headless
             public int RowBytes => _fb.RowBytes;
             public Vector Dpi => _fb.Dpi;
             public PixelFormat Format => _fb.Format;
+            public AlphaFormat AlphaFormat => _fb.AlphaFormat;
         }
 
         public ILockedFramebuffer Lock()

src/Linux/Avalonia.LinuxFramebuffer/Output/FbDevBackBuffer.cs

@@ -101,12 +101,17 @@ namespace Avalonia.LinuxFramebuffer.Output
         public static LockedFramebuffer LockFb(IntPtr address, fb_var_screeninfo varInfo,
             fb_fix_screeninfo fixedInfo, Vector dpi, Action? dispose)
         {
+            var (format, alphaFormat) = varInfo switch
+            {
+                { bits_per_pixel: 16 } => (PixelFormat.Rgb565, AlphaFormat.Opaque),
+                { bits_per_pixel: 32, blue.offset: 16 } => (PixelFormat.Rgba8888, AlphaFormat.Premul),
+                _ => (PixelFormat.Bgra8888, AlphaFormat.Premul)
+            };
+
             return new LockedFramebuffer(address,
                 new PixelSize((int)varInfo.xres, (int)varInfo.yres),
                 (int)fixedInfo.line_length, dpi,
-                varInfo.bits_per_pixel == 16 ? PixelFormat.Rgb565
-                : varInfo.blue.offset == 16 ? PixelFormat.Rgba8888
-                : PixelFormat.Bgra8888, dispose);
+                format, alphaFormat, dispose);
         }
 
         private void BlitToDevice()

src/Skia/Avalonia.Skia/FramebufferRenderTarget.cs

@@ -67,7 +67,7 @@ namespace Avalonia.Skia
             var framebuffer = _renderTargetWithProperties?.Lock(out lockProperties) ?? _renderTarget.Lock();
             var framebufferImageInfo = new SKImageInfo(framebuffer.Size.Width, framebuffer.Size.Height,
                 framebuffer.Format.ToSkColorType(),
-                framebuffer.Format == PixelFormat.Rgb565 ? SKAlphaType.Opaque : SKAlphaType.Premul);
+                framebuffer.AlphaFormat.ToSkAlphaType());
 
             CreateSurface(framebufferImageInfo, framebuffer);
             _hadConversionShim |= _conversionShim != null;

src/Skia/Avalonia.Skia/ImmutableBitmap.cs

@@ -10,7 +10,7 @@ namespace Avalonia.Skia
     /// <summary>
     /// Immutable Skia bitmap.
     /// </summary>
-    internal class ImmutableBitmap : IDrawableBitmapImpl, IReadableBitmapWithAlphaImpl
+    internal class ImmutableBitmap : IDrawableBitmapImpl, IReadableBitmapImpl
     {
         private readonly SKImage _image;
         private readonly SKBitmap? _bitmap;
@@ -195,7 +195,9 @@ namespace Avalonia.Skia
             if (_bitmap.ColorType.ToAvalonia() is not { } format)
                 throw new NotSupportedException($"Unsupported format {_bitmap.ColorType}");
 
-            return new LockedFramebuffer(_bitmap.GetPixels(), PixelSize, _bitmap.RowBytes, Dpi, format, null);
+            var alphaFormat = _bitmap.AlphaType.ToAlphaFormat();
+
+            return new LockedFramebuffer(_bitmap.GetPixels(), PixelSize, _bitmap.RowBytes, Dpi, format, alphaFormat, null);
         }
     }
 }

src/Skia/Avalonia.Skia/WriteableBitmapImpl.cs

@@ -238,6 +238,8 @@ namespace Avalonia.Skia
             public Vector Dpi => _parent.Dpi;
             /// <inheritdoc />
             public PixelFormat Format => _bitmap.ColorType.ToPixelFormat();
+
+            public AlphaFormat AlphaFormat => _bitmap.AlphaType.ToAlphaFormat();
         }
     }
 }

src/Windows/Avalonia.Win32/FramebufferManager.cs

@@ -49,7 +49,7 @@ namespace Avalonia.Win32
 
                 return fb = new LockedFramebuffer(
                     framebufferData.Data.Address, framebufferData.Size, framebufferData.RowBytes,
-                    GetCurrentDpi(), s_format, _onDisposeAction);
+                    GetCurrentDpi(), s_format, AlphaFormat.Premul, _onDisposeAction);
             }
             finally
             {

src/Windows/Avalonia.Win32/Interop/Win32Icon.cs

@@ -96,14 +96,14 @@ internal class Win32Icon : IDisposable
 
     static IntPtr CreateHBitmap(Bitmap source)
     {
-        using var fb = AllocFramebuffer(source.PixelSize, PixelFormats.Bgra8888);
-        source.CopyPixels(fb, AlphaFormat.Unpremul);
+        using var fb = AllocFramebuffer(source.PixelSize, PixelFormats.Bgra8888, AlphaFormat.Unpremul);
+        source.CopyPixels(fb);
         return UnmanagedMethods.CreateBitmap(source.PixelSize.Width, source.PixelSize.Height, 1, 32, fb.Address);
     }
 
     static unsafe IntPtr AlphaToMask(Bitmap source)
     {
-        using var alphaMaskBuffer = AllocFramebuffer(source.PixelSize, PixelFormats.BlackWhite);
+        using var alphaMaskBuffer = AllocFramebuffer(source.PixelSize, PixelFormats.BlackWhite, AlphaFormat.Opaque);
         var height = alphaMaskBuffer.Size.Height;
         var width = alphaMaskBuffer.Size.Width;
 
@@ -114,8 +114,8 @@ internal class Win32Icon : IDisposable
         }
         else
         {
-            using var argbBuffer = AllocFramebuffer(source.PixelSize, PixelFormat.Bgra8888);
-            source.CopyPixels(argbBuffer, AlphaFormat.Unpremul);
+            using var argbBuffer = AllocFramebuffer(source.PixelSize, PixelFormat.Bgra8888, AlphaFormat.Unpremul);
+            source.CopyPixels(argbBuffer);
             var pSource = (byte*)argbBuffer.Address;
             var pDest = (byte*)alphaMaskBuffer.Address;
 
@@ -140,7 +140,7 @@ internal class Win32Icon : IDisposable
         return UnmanagedMethods.CreateBitmap(width, height, 1, 1, alphaMaskBuffer.Address);
     }
 
-    static LockedFramebuffer AllocFramebuffer(PixelSize size, PixelFormat format)
+    static LockedFramebuffer AllocFramebuffer(PixelSize size, PixelFormat format, AlphaFormat alphaFormat)
     {
         if (size.Width < 1 || size.Height < 1)
             throw new ArgumentOutOfRangeException();
@@ -149,7 +149,7 @@ internal class Win32Icon : IDisposable
         var data = Marshal.AllocHGlobal(size.Height * stride);
         if (data == IntPtr.Zero)
             throw new OutOfMemoryException();
-        return new LockedFramebuffer(data, size, stride, new Vector(96, 96), format,
+        return new LockedFramebuffer(data, size, stride, new Vector(96, 96), format, alphaFormat,
             () => Marshal.FreeHGlobal(data));
     }
     

tests/Avalonia.RenderTests/Composition/DirectFbCompositionTests.cs

@@ -83,7 +83,7 @@ public class DirectFbCompositionTests : TestBase
         SKBitmap fb = new SKBitmap(200, 200, SKColorType.Rgba8888, SKAlphaType.Premul);
 
         ILockedFramebuffer LockFb() => new LockedFramebuffer(fb.GetAddress(0, 0), new(fb.Width, fb.Height),
-            fb.RowBytes, new Vector(96, 96), PixelFormat.Rgba8888, null);
+            fb.RowBytes, new Vector(96, 96), PixelFormat.Rgba8888, AlphaFormat.Premul, null);
 
         bool previousFrameIsRetained = false;
         IFramebufferRenderTarget rt = advertised

tests/Avalonia.RenderTests/Media/BitmapTests.cs

@@ -25,10 +25,11 @@ namespace Avalonia.Skia.RenderTests
 
         class Framebuffer : ILockedFramebuffer, IFramebufferPlatformSurface
         {
-            public Framebuffer(PixelFormat fmt, PixelSize size)
+            public Framebuffer(PixelFormat fmt, AlphaFormat alphaFormat, PixelSize size)
             {
                 Format = fmt;
                 var bpp = fmt == PixelFormat.Rgb565 ? 2 : 4;
+                AlphaFormat = alphaFormat;
                 Size = size;
                 RowBytes = bpp * size.Width;
                 Address = Marshal.AllocHGlobal(size.Height * RowBytes);
@@ -40,6 +41,8 @@ namespace Avalonia.Skia.RenderTests
 
             public PixelFormat Format { get; }
 
+            public AlphaFormat AlphaFormat { get; }
+
             public PixelSize Size { get; }
 
             public int RowBytes { get; }
@@ -64,7 +67,7 @@ namespace Avalonia.Skia.RenderTests
         {
             var fmt = new PixelFormat(fmte);
             var testName = nameof(FramebufferRenderResultsShouldBeUsableAsBitmap) + "_" + fmt;
-            var fb = new Framebuffer(fmt, new PixelSize(80, 80));
+            var fb = new Framebuffer(fmt, AlphaFormat.Premul, new PixelSize(80, 80));
             var r = AvaloniaLocator.Current.GetRequiredService<IPlatformRenderInterface>();
             using(var cpuContext = r.CreateBackendContext(null))
             using (var target = cpuContext.CreateRenderTarget(new object[] { fb }))

tests/Avalonia.UnitTests/CompositorTestServices.cs

@@ -174,7 +174,7 @@ public class CompositorTestServices : IDisposable
             {
                 var ptr = Marshal.AllocHGlobal(128);
                 return new LockedFramebuffer(ptr, new PixelSize(1, 1), 4, new Vector(96, 96),
-                    PixelFormat.Rgba8888, () => Marshal.FreeHGlobal(ptr));
+                    PixelFormat.Rgba8888, AlphaFormat.Premul, () => Marshal.FreeHGlobal(ptr));
             }
             
             public IFramebufferRenderTarget CreateFramebufferRenderTarget() => new FuncFramebufferRenderTarget(Lock);