Normalize Encoder Transparency Handling For Alpha Aware Image Formats

src/ImageSharp/Formats/AlphaAwareImageEncoder.cs

@@ -0,0 +1,15 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats;
+
+/// <summary>
+/// Acts as a base encoder for all formats that are aware of and can handle alpha transparency.
+/// </summary>
+public abstract class AlphaAwareImageEncoder : ImageEncoder
+{
+    /// <summary>
+    /// Gets or initializes the mode that determines how transparent pixels are handled during encoding.
+    /// </summary>
+    public TransparentColorMode TransparentColorMode { get; init; }
+}

src/ImageSharp/Formats/Cur/CurFrameMetadata.cs

@@ -48,13 +48,13 @@ private CurFrameMetadata(CurFrameMetadata other)
     /// Gets or sets the encoding width. <br />
     /// Can be any number between 0 and 255. Value 0 means a frame height of 256 pixels or greater.
     /// </summary>
-    public byte EncodingWidth { get; set; }
+    public byte? EncodingWidth { get; set; }
 
     /// <summary>
     /// Gets or sets the encoding height. <br />
     /// Can be any number between 0 and 255. Value 0 means a frame height of 256 pixels or greater.
     /// </summary>
-    public byte EncodingHeight { get; set; }
+    public byte? EncodingHeight { get; set; }
 
     /// <summary>
     /// Gets or sets the number of bits per pixel.<br/>
@@ -80,20 +80,6 @@ public static CurFrameMetadata FromFormatConnectingFrameMetadata(FormatConnectin
             };
         }
 
-        byte encodingWidth = metadata.EncodingWidth switch
-        {
-            > 255 => 0,
-            <= 255 and >= 1 => (byte)metadata.EncodingWidth,
-            _ => 0
-        };
-
-        byte encodingHeight = metadata.EncodingHeight switch
-        {
-            > 255 => 0,
-            <= 255 and >= 1 => (byte)metadata.EncodingHeight,
-            _ => 0
-        };
-
         int bpp = metadata.PixelTypeInfo.Value.BitsPerPixel;
         BmpBitsPerPixel bbpp = bpp switch
         {
@@ -116,8 +102,8 @@ public static CurFrameMetadata FromFormatConnectingFrameMetadata(FormatConnectin
         {
             BmpBitsPerPixel = bbpp,
             Compression = compression,
-            EncodingWidth = encodingWidth,
-            EncodingHeight = encodingHeight,
+            EncodingWidth = ClampEncodingDimension(metadata.EncodingWidth),
+            EncodingHeight = ClampEncodingDimension(metadata.EncodingHeight),
             ColorTable = compression == IconFrameCompression.Bmp ? metadata.ColorTable : null
         };
     }
@@ -138,8 +124,8 @@ public void AfterFrameApply<TPixel>(ImageFrame<TPixel> source, ImageFrame<TPixel
     {
         float ratioX = destination.Width / (float)source.Width;
         float ratioY = destination.Height / (float)source.Height;
-        this.EncodingWidth = Scale(this.EncodingWidth, destination.Width, ratioX);
-        this.EncodingHeight = Scale(this.EncodingHeight, destination.Height, ratioY);
+        this.EncodingWidth = ScaleEncodingDimension(this.EncodingWidth, destination.Width, ratioX);
+        this.EncodingHeight = ScaleEncodingDimension(this.EncodingHeight, destination.Height, ratioY);
     }
 
     /// <inheritdoc/>
@@ -156,16 +142,16 @@ internal void FromIconDirEntry(IconDirEntry entry)
         this.HotspotY = entry.BitCount;
     }
 
-    internal IconDirEntry ToIconDirEntry()
+    internal IconDirEntry ToIconDirEntry(Size size)
     {
         byte colorCount = this.Compression == IconFrameCompression.Png || this.BmpBitsPerPixel > BmpBitsPerPixel.Bit8
             ? (byte)0
             : (byte)ColorNumerics.GetColorCountForBitDepth((int)this.BmpBitsPerPixel);
 
         return new()
         {
-            Width = this.EncodingWidth,
-            Height = this.EncodingHeight,
+            Width = ClampEncodingDimension(this.EncodingWidth ?? size.Width),
+            Height = ClampEncodingDimension(this.EncodingHeight ?? size.Height),
             Planes = this.HotspotX,
             BitCount = this.HotspotY,
             ColorCount = colorCount
@@ -233,13 +219,22 @@ private PixelTypeInfo GetPixelTypeInfo()
         };
     }
 
-    private static byte Scale(byte? value, int destination, float ratio)
+    private static byte ScaleEncodingDimension(byte? value, int destination, float ratio)
     {
         if (value is null)
         {
-            return (byte)Math.Clamp(destination, 0, 255);
+            return ClampEncodingDimension(destination);
         }
 
-        return Math.Min((byte)MathF.Ceiling(value.Value * ratio), (byte)Math.Clamp(destination, 0, 255));
+        return ClampEncodingDimension(MathF.Ceiling(value.Value * ratio));
     }
+
+    private static byte ClampEncodingDimension(float? dimension)
+        => dimension switch
+        {
+            // Encoding dimensions can be between 0-256 where 0 means 256 or greater.
+            > 255 => 0,
+            <= 255 and >= 1 => (byte)dimension,
+            _ => 0
+        };
 }

src/ImageSharp/Formats/EncodingUtilities.cs

@@ -0,0 +1,97 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Buffers;
+using System.Numerics;
+using System.Runtime.Intrinsics;
+using SixLabors.ImageSharp.Memory;
+using SixLabors.ImageSharp.PixelFormats;
+
+namespace SixLabors.ImageSharp.Formats;
+
+/// <summary>
+/// Provides utilities for encoding images.
+/// </summary>
+internal static class EncodingUtilities
+{
+    public static bool ShouldClearTransparentPixels<TPixel>(TransparentColorMode mode)
+        where TPixel : unmanaged, IPixel<TPixel>
+        => mode == TransparentColorMode.Clear &&
+                   TPixel.GetPixelTypeInfo().AlphaRepresentation == PixelAlphaRepresentation.Unassociated;
+
+    /// <summary>
+    /// Convert transparent pixels, to pixels represented by <paramref name="color"/>, which can yield
+    /// to better compression in some cases.
+    /// </summary>
+    /// <typeparam name="TPixel">The type of the pixel.</typeparam>
+    /// <param name="clone">The cloned <see cref="ImageFrame{TPixel}"/> where the transparent pixels will be changed.</param>
+    /// <param name="color">The color to replace transparent pixels with.</param>
+    public static void ClearTransparentPixels<TPixel>(ImageFrame<TPixel> clone, Color color)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        Buffer2DRegion<TPixel> buffer = clone.PixelBuffer.GetRegion();
+        ClearTransparentPixels(clone.Configuration, ref buffer, color);
+    }
+
+    /// <summary>
+    /// Convert transparent pixels, to pixels represented by <paramref name="color"/>, which can yield
+    /// to better compression in some cases.
+    /// </summary>
+    /// <typeparam name="TPixel">The type of the pixel.</typeparam>
+    /// <param name="configuration">The configuration.</param>
+    /// <param name="clone">The cloned <see cref="Buffer2DRegion{T}"/> where the transparent pixels will be changed.</param>
+    /// <param name="color">The color to replace transparent pixels with.</param>
+    public static void ClearTransparentPixels<TPixel>(
+        Configuration configuration,
+        ref Buffer2DRegion<TPixel> clone,
+        Color color)
+        where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using IMemoryOwner<Vector4> vectors = configuration.MemoryAllocator.Allocate<Vector4>(clone.Width);
+        Span<Vector4> vectorsSpan = vectors.GetSpan();
+        Vector4 replacement = color.ToScaledVector4();
+        for (int y = 0; y < clone.Height; y++)
+        {
+            Span<TPixel> span = clone.DangerousGetRowSpan(y);
+            PixelOperations<TPixel>.Instance.ToVector4(configuration, span, vectorsSpan, PixelConversionModifiers.Scale);
+            ClearTransparentPixelRow(vectorsSpan, replacement);
+            PixelOperations<TPixel>.Instance.FromVector4Destructive(configuration, vectorsSpan, span, PixelConversionModifiers.Scale);
+        }
+    }
+
+    private static void ClearTransparentPixelRow(
+        Span<Vector4> vectorsSpan,
+        Vector4 replacement)
+    {
+        if (Vector128.IsHardwareAccelerated)
+        {
+            Vector128<float> replacement128 = replacement.AsVector128();
+
+            for (int i = 0; i < vectorsSpan.Length; i++)
+            {
+                ref Vector4 v = ref vectorsSpan[i];
+                Vector128<float> v128 = v.AsVector128();
+
+                // Do `vector == 0`
+                Vector128<float> mask = Vector128.Equals(v128, Vector128<float>.Zero);
+
+                // Replicate the result for W to all elements (is AllBitsSet if the W was 0 and Zero otherwise)
+                mask = Vector128.Shuffle(mask, Vector128.Create(3, 3, 3, 3));
+
+                // Use the mask to select the replacement vector
+                // (replacement & mask) | (v128 & ~mask)
+                v = Vector128.ConditionalSelect(mask, replacement128, v128).AsVector4();
+            }
+        }
+        else
+        {
+            for (int i = 0; i < vectorsSpan.Length; i++)
+            {
+                if (vectorsSpan[i].W == 0F)
+                {
+                    vectorsSpan[i] = replacement;
+                }
+            }
+        }
+    }
+}

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -665,7 +665,7 @@ private void RestoreToBackground<TPixel>(ImageFrame<TPixel> frame)
             return;
         }
 
-        Rectangle interest = Rectangle.Intersect(frame.Bounds(), this.restoreArea.Value);
+        Rectangle interest = Rectangle.Intersect(frame.Bounds, this.restoreArea.Value);
         Buffer2DRegion<TPixel> pixelRegion = frame.PixelBuffer.GetRegion(interest);
         pixelRegion.Clear();