Add default alpha and pixel formats to render interfaces.

src/Avalonia.Headless/HeadlessPlatformRenderInterface.cs

@@ -21,7 +21,11 @@ namespace Avalonia.Headless
 
         public IEnumerable<string> InstalledFontNames { get; } = new[] { "Tahoma" };
 
-        public bool SupportsIndividualRoundRects => throw new NotImplementedException();
+        public bool SupportsIndividualRoundRects => false;
+
+        public AlphaFormat DefaultAlphaFormat => AlphaFormat.Premul;
+
+        public PixelFormat DefaultPixelFormat => PixelFormat.Rgba8888;
 
         public IFormattedTextImpl CreateFormattedText(string text, Typeface typeface, double fontSize, TextAlignment textAlignment, TextWrapping wrapping, Size constraint, IReadOnlyList<FormattedTextStyleSpan> spans)
         {
@@ -51,7 +55,7 @@ namespace Avalonia.Headless
             return new HeadlessBitmapStub(size, dpi);
         }
 
-        public IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat? format = null, AlphaFormat? alphaFormat = null)
+        public IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat format, AlphaFormat alphaFormat)
         {
             return new HeadlessBitmapStub(size, dpi);
         }

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

@@ -95,6 +95,24 @@ namespace Avalonia.Media.Imaging
             PlatformImpl.Dispose();
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Bitmap"/> class.
+        /// </summary>
+        /// <param name="format">The pixel format.</param>
+        /// <param name="alphaFormat">The alpha format.</param>
+        /// <param name="data">The pointer to the source bytes.</param>
+        /// <param name="size">The size of the bitmap in device pixels.</param>
+        /// <param name="dpi">The DPI of the bitmap.</param>
+        /// <param name="stride">The number of bytes per row.</param>
+        [Obsolete("Use overload taking an AlphaFormat.")]
+        public Bitmap(PixelFormat format, IntPtr data, PixelSize size, Vector dpi, int stride)
+        {
+            var ri = AvaloniaLocator.Current.GetService<IPlatformRenderInterface>();
+
+            PlatformImpl = RefCountable.Create(AvaloniaLocator.Current.GetService<IPlatformRenderInterface>()
+                .LoadBitmap(format, ri.DefaultAlphaFormat, data, size, dpi, stride));
+        }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="Bitmap"/> class.
         /// </summary>

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

@@ -1,4 +1,5 @@
-using Avalonia.Platform;
+using System;
+using Avalonia.Platform;
 
 namespace Avalonia.Media.Imaging
 {
@@ -7,6 +8,19 @@ namespace Avalonia.Media.Imaging
     /// </summary>
     public class WriteableBitmap : Bitmap
     {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="WriteableBitmap"/> class.
+        /// </summary>
+        /// <param name="size">The size of the bitmap in device pixels.</param>
+        /// <param name="dpi">The DPI of the bitmap.</param>
+        /// <param name="format">The pixel format (optional).</param>
+        /// <returns>An <see cref="IWriteableBitmapImpl"/>.</returns>
+        [Obsolete("Use overload taking an AlphaFormat.")]
+        public WriteableBitmap(PixelSize size, Vector dpi, PixelFormat? format = null)
+            : base(CreatePlatformImpl(size, dpi, format, null))
+        {
+        }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="WriteableBitmap"/> class.
         /// </summary>
@@ -15,11 +29,21 @@ namespace Avalonia.Media.Imaging
         /// <param name="format">The pixel format (optional).</param>
         /// <param name="alphaFormat">The alpha format (optional).</param>
         /// <returns>An <see cref="IWriteableBitmapImpl"/>.</returns>
-        public WriteableBitmap(PixelSize size, Vector dpi, PixelFormat? format = null, AlphaFormat? alphaFormat = null) 
-            : base(AvaloniaLocator.Current.GetService<IPlatformRenderInterface>().CreateWriteableBitmap(size, dpi, format, alphaFormat))
+        public WriteableBitmap(PixelSize size, Vector dpi, PixelFormat format, AlphaFormat alphaFormat) 
+            : base(CreatePlatformImpl(size, dpi, format, alphaFormat))
         {
         }
-        
+
         public ILockedFramebuffer Lock() => ((IWriteableBitmapImpl) PlatformImpl.Item).Lock();
+
+        private static IBitmapImpl CreatePlatformImpl(PixelSize size, in Vector dpi, PixelFormat? format, AlphaFormat? alphaFormat)
+        {
+            var ri = AvaloniaLocator.Current.GetService<IPlatformRenderInterface>();
+
+            PixelFormat finalFormat = format ?? ri.DefaultPixelFormat;
+            AlphaFormat finalAlphaFormat = alphaFormat ?? ri.DefaultAlphaFormat;
+
+            return ri.CreateWriteableBitmap(size, dpi, finalFormat, finalAlphaFormat);
+        }
     }
 }

src/Avalonia.Visuals/Platform/IPlatformRenderInterface.cs

@@ -81,10 +81,10 @@ namespace Avalonia.Platform
         /// </summary>
         /// <param name="size">The size of the bitmap in device pixels.</param>
         /// <param name="dpi">The DPI of the bitmap.</param>
-        /// <param name="format">Pixel format (optional).</param>
-        /// <param name="alphaFormat">Alpha format (optional).</param>
+        /// <param name="format">Pixel format.</param>
+        /// <param name="alphaFormat">Alpha format .</param>
         /// <returns>An <see cref="IWriteableBitmapImpl"/>.</returns>
-        IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat? format = null, AlphaFormat? alphaFormat = null);
+        IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat format, AlphaFormat alphaFormat);
 
         /// <summary>
         /// Loads a bitmap implementation from a file..
@@ -141,5 +141,15 @@ namespace Avalonia.Platform
         IGlyphRunImpl CreateGlyphRun(GlyphRun glyphRun, out double width);
 
         bool SupportsIndividualRoundRects { get; }
+
+        /// <summary>
+        /// Default <see cref="AlphaFormat"/> used on this platform.
+        /// </summary>
+        public AlphaFormat DefaultAlphaFormat { get; }
+
+        /// <summary>
+        /// Default <see cref="PixelFormat"/> used on this platform.
+        /// </summary>
+        public PixelFormat DefaultPixelFormat { get; }
     }
 }

src/Skia/Avalonia.Skia/PlatformRenderInterface.cs

@@ -22,6 +22,8 @@ namespace Avalonia.Skia
 
         public PlatformRenderInterface(ISkiaGpu skiaGpu, long? maxResourceBytes = null)
         {
+            DefaultPixelFormat = SKImageInfo.PlatformColorType.ToPixelFormat();
+
             if (skiaGpu != null)
             {
                 _skiaGpu = skiaGpu;
@@ -150,7 +152,7 @@ namespace Avalonia.Skia
         }
 
         /// <inheritdoc />
-        public IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat? format = null, AlphaFormat? alphaFormat = null)
+        public IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat format, AlphaFormat alphaFormat)
         {
             return new WriteableBitmapImpl(size, dpi, format, alphaFormat);
         }
@@ -265,5 +267,9 @@ namespace Avalonia.Skia
         }
 
         public bool SupportsIndividualRoundRects => true;
+
+        public AlphaFormat DefaultAlphaFormat => AlphaFormat.Premul;
+
+        public PixelFormat DefaultPixelFormat { get; }
     }
 }

src/Skia/Avalonia.Skia/WriteableBitmapImpl.cs

@@ -23,14 +23,13 @@ namespace Avalonia.Skia
         /// <param name="dpi">The DPI of the bitmap.</param>
         /// <param name="format">The pixel format.</param>
         /// <param name="alphaFormat">The alpha format.</param>
-        public WriteableBitmapImpl(PixelSize size, Vector dpi, PixelFormat? format = null, AlphaFormat? alphaFormat = null)
+        public WriteableBitmapImpl(PixelSize size, Vector dpi, PixelFormat format, AlphaFormat alphaFormat)
         {
             PixelSize = size;
             Dpi = dpi;
 
-            var colorType = PixelFormatHelper.ResolveColorType(format);
-
-            SKAlphaType alphaType = alphaFormat?.ToSkAlphaType() ?? SKAlphaType.Premul;
+            SKColorType colorType = format.ToSkColorType();
+            SKAlphaType alphaType = alphaFormat.ToSkAlphaType();
             
             var runtimePlatform = AvaloniaLocator.Current?.GetService<IRuntimePlatform>();
             

src/Windows/Avalonia.Direct2D1/Direct2D1Platform.cs

@@ -166,7 +166,7 @@ namespace Avalonia.Direct2D1
             return new WicRenderTargetBitmapImpl(size, dpi);
         }
 
-        public IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat? format = null, AlphaFormat? alphaFormat = null)
+        public IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat format, AlphaFormat alphaFormat)
         {
             return new WriteableWicBitmapImpl(size, dpi, format, alphaFormat);
         }
@@ -261,5 +261,9 @@ namespace Avalonia.Direct2D1
         }
 
         public bool SupportsIndividualRoundRects => false;
+
+        public AlphaFormat DefaultAlphaFormat => AlphaFormat.Premul;
+
+        public PixelFormat DefaultPixelFormat => PixelFormat.Bgra8888;
     }
 }

tests/Avalonia.Benchmarks/NullRenderingPlatform.cs

@@ -46,7 +46,7 @@ namespace Avalonia.Benchmarks
             throw new NotImplementedException();
         }
 
-        public IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat? format = null, AlphaFormat? alphaFormat = null)
+        public IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat format, AlphaFormat alphaFormat)
         {
             throw new NotImplementedException();
         }
@@ -94,5 +94,9 @@ namespace Avalonia.Benchmarks
         }
 
         public bool SupportsIndividualRoundRects => true;
+
+        public AlphaFormat DefaultAlphaFormat => AlphaFormat.Premul;
+
+        public PixelFormat DefaultPixelFormat => PixelFormat.Rgba8888;
     }
 }

tests/Avalonia.UnitTests/MockPlatformRenderInterface.cs

@@ -55,8 +55,8 @@ namespace Avalonia.UnitTests
         public IWriteableBitmapImpl CreateWriteableBitmap(
             PixelSize size,
             Vector dpi,
-            PixelFormat? format = null,
-            AlphaFormat? alphaFormat = null)
+            PixelFormat format,
+            AlphaFormat alphaFormat)
         {
             throw new NotImplementedException();
         }
@@ -104,5 +104,9 @@ namespace Avalonia.UnitTests
         }
 
         public bool SupportsIndividualRoundRects { get; set; }
+
+        public AlphaFormat DefaultAlphaFormat => AlphaFormat.Premul;
+
+        public PixelFormat DefaultPixelFormat => PixelFormat.Rgba8888;
     }
 }

tests/Avalonia.Visuals.UnitTests/VisualTree/MockRenderInterface.cs

@@ -58,13 +58,15 @@ namespace Avalonia.Visuals.UnitTests.VisualTree
         }
 
         public bool SupportsIndividualRoundRects { get; set; }
+        public AlphaFormat DefaultAlphaFormat { get; }
+        public PixelFormat DefaultPixelFormat { get; }
 
         public IFontManagerImpl CreateFontManager()
         {
             return new MockFontManagerImpl();
         }
 
-        public IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat? fmt, AlphaFormat? alphaFormat)
+        public IWriteableBitmapImpl CreateWriteableBitmap(PixelSize size, Vector dpi, PixelFormat fmt, AlphaFormat alphaFormat)
         {
             throw new NotImplementedException();
         }