Cleanup and fix issues

src/ImageSharp/Formats/Bmp/BmpDecoderCore.cs

@@ -6,6 +6,7 @@ using System.Buffers.Binary;
 using System.Diagnostics.CodeAnalysis;
 using System.Numerics;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Common.Helpers;
 using SixLabors.ImageSharp.IO;
 using SixLabors.ImageSharp.Memory;
@@ -1599,6 +1600,14 @@ internal sealed class BmpDecoderCore : IImageDecoderInternals
             }
         }
 
+        if (palette.Length > 0)
+        {
+            Color[] colorTable = new Color[palette.Length / Unsafe.SizeOf<Bgr24>()];
+            ReadOnlySpan<Bgr24> rgbTable = MemoryMarshal.Cast<byte, Bgr24>(palette);
+            Color.FromPixel(rgbTable, colorTable);
+            this.bmpMetadata.ColorTable = colorTable;
+        }
+
         int skipAmount = 0;
         if (this.fileHeader.HasValue)
         {

src/ImageSharp/Formats/Bmp/BmpEncoderCore.cs

@@ -109,6 +109,8 @@ internal sealed class BmpEncoderCore : IImageEncoderInternals
     {
         this.memoryAllocator = memoryAllocator;
         this.bitsPerPixel = encoder.BitsPerPixel;
+
+        // TODO: Use a palette quantizer if supplied.
         this.quantizer = encoder.Quantizer ?? KnownQuantizers.Octree;
         this.pixelSamplingStrategy = encoder.PixelSamplingStrategy;
         this.infoHeaderType = encoder.SupportTransparency ? BmpInfoHeaderType.WinVersion4 : BmpInfoHeaderType.WinVersion3;

src/ImageSharp/Formats/Bmp/BmpMetadata.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors.
+// Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
 namespace SixLabors.ImageSharp.Formats.Bmp;
@@ -23,6 +23,11 @@ public class BmpMetadata : IDeepCloneable
     {
         this.BitsPerPixel = other.BitsPerPixel;
         this.InfoHeaderType = other.InfoHeaderType;
+
+        if (other.ColorTable?.Length > 0)
+        {
+            this.ColorTable = other.ColorTable.Value.ToArray();
+        }
     }
 
     /// <summary>
@@ -35,8 +40,11 @@ public class BmpMetadata : IDeepCloneable
     /// </summary>
     public BmpBitsPerPixel BitsPerPixel { get; set; } = BmpBitsPerPixel.Pixel24;
 
+    /// <summary>
+    /// Gets or sets the color table, if any.
+    /// </summary>
+    public ReadOnlyMemory<Color>? ColorTable { get; set; }
+
     /// <inheritdoc/>
     public IDeepCloneable DeepClone() => new BmpMetadata(this);
-
-    // TODO: Colors used once we support encoding palette bmps.
 }

src/ImageSharp/Formats/Cur/CurConstants.cs

@@ -9,20 +9,31 @@ namespace SixLabors.ImageSharp.Formats.Cur;
 internal static class CurConstants
 {
     /// <summary>
-    /// The list of mimetypes that equate to a ico.
+    /// The list of mime types that equate to a cur.
     /// </summary>
     /// <remarks>
     /// See <see href="https://en.wikipedia.org/wiki/ICO_(file_format)#MIME_type"/>
     /// </remarks>
-    public static readonly IEnumerable<string> MimeTypes = new[]
-    {
-        "application/octet-stream",
-    };
+    public static readonly IEnumerable<string> MimeTypes =
+    [
+
+        // IANA-registered
+        "image/vnd.microsoft.icon",
+
+        // ICO & CUR types used by Windows
+        "image/x-icon",
+
+        // Erroneous types but have been used
+        "image/ico",
+        "image/icon",
+        "text/ico",
+        "application/ico",
+    ];
 
     /// <summary>
-    /// The list of file extensions that equate to a ico.
+    /// The list of file extensions that equate to a cur.
     /// </summary>
-    public static readonly IEnumerable<string> FileExtensions = new[] { "cur" };
+    public static readonly IEnumerable<string> FileExtensions = ["cur"];
 
     public const uint FileHeader = 0x00_02_00_00;
 }

src/ImageSharp/Formats/Cur/CurDecoderCore.cs

@@ -1,18 +1,24 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using SixLabors.ImageSharp.Formats.Bmp;
 using SixLabors.ImageSharp.Formats.Icon;
 using SixLabors.ImageSharp.Metadata;
 
 namespace SixLabors.ImageSharp.Formats.Cur;
 
-internal sealed class CurDecoderCore(DecoderOptions options) : IconDecoderCore(options)
+internal sealed class CurDecoderCore : IconDecoderCore
 {
-    protected override void SetFrameMetadata(ImageFrameMetadata metadata, in IconDirEntry entry, IconFrameCompression compression, Bmp.BmpBitsPerPixel bitsPerPixel)
+    public CurDecoderCore(DecoderOptions options)
+        : base(options)
+    {
+    }
+
+    protected override void SetFrameMetadata(ImageFrameMetadata metadata, in IconDirEntry entry, IconFrameCompression compression, BmpBitsPerPixel bitsPerPixel)
     {
         CurFrameMetadata curFrameMetadata = metadata.GetCurMetadata();
         curFrameMetadata.FromIconDirEntry(entry);
         curFrameMetadata.Compression = compression;
-        curFrameMetadata.BitsPerPixel = bitsPerPixel;
+        curFrameMetadata.BmpBitsPerPixel = bitsPerPixel;
     }
 }

src/ImageSharp/Formats/Cur/CurEncoderCore.cs

@@ -5,6 +5,10 @@ using SixLabors.ImageSharp.Formats.Icon;
 
 namespace SixLabors.ImageSharp.Formats.Cur;
 
-internal sealed class CurEncoderCore() : IconEncoderCore(IconFileType.CUR)
+internal sealed class CurEncoderCore : IconEncoderCore
 {
+    public CurEncoderCore()
+        : base(IconFileType.CUR)
+    {
+    }
 }

src/ImageSharp/Formats/Cur/CurFrameMetadata.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using SixLabors.ImageSharp.Formats.Bmp;
 using SixLabors.ImageSharp.Formats.Icon;
 
 namespace SixLabors.ImageSharp.Formats.Cur;
@@ -17,70 +18,48 @@ public class CurFrameMetadata : IDeepCloneable<CurFrameMetadata>, IDeepCloneable
     {
     }
 
-    /// <summary>
-    /// Initializes a new instance of the <see cref="CurFrameMetadata"/> class.
-    /// </summary>
-    /// <param name="width">width</param>
-    /// <param name="height">height</param>
-    /// <param name="colorCount">colorCount</param>
-    /// <param name="hotspotX">hotspotX</param>
-    /// <param name="hotspotY">hotspotY</param>
-    public CurFrameMetadata(byte width, byte height, byte colorCount, ushort hotspotX, ushort hotspotY)
-    {
-        this.EncodingWidth = width;
-        this.EncodingHeight = height;
-        this.ColorCount = colorCount;
-        this.HotspotX = hotspotX;
-        this.HotspotY = hotspotY;
-    }
-
-    /// <inheritdoc cref="CurFrameMetadata()"/>
-    public CurFrameMetadata(CurFrameMetadata metadata)
+    private CurFrameMetadata(CurFrameMetadata metadata)
     {
-        this.EncodingWidth = metadata.EncodingWidth;
-        this.EncodingHeight = metadata.EncodingHeight;
-        this.ColorCount = metadata.ColorCount;
+        this.Compression = metadata.Compression;
         this.HotspotX = metadata.HotspotX;
         this.HotspotY = metadata.HotspotY;
-        this.Compression = metadata.Compression;
+        this.EncodingWidth = metadata.EncodingWidth;
+        this.EncodingHeight = metadata.EncodingHeight;
+        this.BmpBitsPerPixel = metadata.BmpBitsPerPixel;
     }
 
     /// <summary>
-    /// Gets or sets icoFrameCompression.
+    /// Gets or sets the frame compressions format.
     /// </summary>
     public IconFrameCompression Compression { get; set; }
 
     /// <summary>
-    /// Gets or sets ColorCount field. <br />
-    /// Specifies number of colors in the color palette. Should be 0 if the image does not use a color palette.
+    /// Gets or sets the horizontal coordinates of the hotspot in number of pixels from the left.
     /// </summary>
-    // TODO: BmpMetadata does not supported palette yet.
-    public byte ColorCount { get; set; }
+    public ushort HotspotX { get; set; }
 
     /// <summary>
-    /// Gets or sets Specifies the horizontal coordinates of the hotspot in number of pixels from the left.
+    /// Gets or sets the vertical coordinates of the hotspot in number of pixels from the top.
     /// </summary>
-    public ushort HotspotX { get; set; }
+    public ushort HotspotY { get; set; }
 
     /// <summary>
-    /// Gets or sets Specifies the vertical coordinates of the hotspot in number of pixels from the top.
+    /// Gets or sets the encoding width. <br />
+    /// Can be any number between 0 and 255. Value 0 means a frame height of 256 pixels.
     /// </summary>
-    public ushort HotspotY { get; set; }
+    public byte EncodingWidth { get; set; }
 
     /// <summary>
-    /// Gets or sets Height field. <br />
-    /// Specifies image height in pixels. Can be any number between 0 and 255. Value 0 means image height is 256 pixels.
+    /// Gets or sets the encoding height. <br />
+    /// Can be any number between 0 and 255. Value 0 means a frame height of 256 pixels.
     /// </summary>
     public byte EncodingHeight { get; set; }
 
     /// <summary>
-    /// Gets or sets Width field. <br />
-    /// Specifies image width in pixels. Can be any number between 0 and 255. Value 0 means image width is 256 pixels.
+    /// Gets or sets the number of bits per pixel.<br/>
+    /// Used when <see cref="Compression"/> is <see cref="IconFrameCompression.Bmp"/>
     /// </summary>
-    public byte EncodingWidth { get; set; }
-
-    /// <inheritdoc cref="Bmp.BmpMetadata.BitsPerPixel" />
-    public Bmp.BmpBitsPerPixel BitsPerPixel { get; set; } = Bmp.BmpBitsPerPixel.Pixel24;
+    public BmpBitsPerPixel BmpBitsPerPixel { get; set; } = BmpBitsPerPixel.Pixel32;
 
     /// <inheritdoc/>
     public CurFrameMetadata DeepClone() => new(this);
@@ -88,21 +67,27 @@ public class CurFrameMetadata : IDeepCloneable<CurFrameMetadata>, IDeepCloneable
     /// <inheritdoc/>
     IDeepCloneable IDeepCloneable.DeepClone() => this.DeepClone();
 
-    internal void FromIconDirEntry(in IconDirEntry entry)
+    internal void FromIconDirEntry(IconDirEntry entry)
     {
         this.EncodingWidth = entry.Width;
         this.EncodingHeight = entry.Height;
-        this.ColorCount = entry.ColorCount;
         this.HotspotX = entry.Planes;
         this.HotspotY = entry.BitCount;
     }
 
-    internal IconDirEntry ToIconDirEntry() => new()
+    internal IconDirEntry ToIconDirEntry()
     {
-        Width = this.EncodingWidth,
-        Height = this.EncodingHeight,
-        ColorCount = this.ColorCount,
-        Planes = this.HotspotX,
-        BitCount = this.HotspotY,
-    };
+        byte colorCount = this.Compression == IconFrameCompression.Png || this.BmpBitsPerPixel > BmpBitsPerPixel.Pixel8
+            ? (byte)0
+            : (byte)ColorNumerics.GetColorCountForBitDepth((int)this.BmpBitsPerPixel);
+
+        return new()
+        {
+            Width = this.EncodingWidth,
+            Height = this.EncodingHeight,
+            Planes = this.HotspotX,
+            BitCount = this.HotspotY,
+            ColorCount = colorCount
+        };
+    }
 }

src/ImageSharp/Formats/Ico/IcoConstants.cs

@@ -9,13 +9,14 @@ namespace SixLabors.ImageSharp.Formats.Ico;
 internal static class IcoConstants
 {
     /// <summary>
-    /// The list of mimetypes that equate to a ico.
+    /// The list of mime types that equate to a ico.
     /// </summary>
     /// <remarks>
     /// See <see href="https://en.wikipedia.org/wiki/ICO_(file_format)#MIME_type"/>
     /// </remarks>
-    public static readonly IEnumerable<string> MimeTypes = new[]
-    {
+    public static readonly IEnumerable<string> MimeTypes =
+    [
+
         // IANA-registered
         "image/vnd.microsoft.icon",
 
@@ -27,12 +28,12 @@ internal static class IcoConstants
         "image/icon",
         "text/ico",
         "application/ico",
-    };
+    ];
 
     /// <summary>
     /// The list of file extensions that equate to a ico.
     /// </summary>
-    public static readonly IEnumerable<string> FileExtensions = new[] { "ico" };
+    public static readonly IEnumerable<string> FileExtensions = ["ico"];
 
     public const uint FileHeader = 0x00_01_00_00;
 }

src/ImageSharp/Formats/Ico/IcoDecoderCore.cs

@@ -1,18 +1,24 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using SixLabors.ImageSharp.Formats.Bmp;
 using SixLabors.ImageSharp.Formats.Icon;
 using SixLabors.ImageSharp.Metadata;
 
 namespace SixLabors.ImageSharp.Formats.Ico;
 
-internal sealed class IcoDecoderCore(DecoderOptions options) : IconDecoderCore(options)
+internal sealed class IcoDecoderCore : IconDecoderCore
 {
-    protected override void SetFrameMetadata(ImageFrameMetadata metadata, in IconDirEntry entry, IconFrameCompression compression, Bmp.BmpBitsPerPixel bitsPerPixel)
+    public IcoDecoderCore(DecoderOptions options)
+        : base(options)
+    {
+    }
+
+    protected override void SetFrameMetadata(ImageFrameMetadata metadata, in IconDirEntry entry, IconFrameCompression compression, BmpBitsPerPixel bitsPerPixel)
     {
         IcoFrameMetadata icoFrameMetadata = metadata.GetIcoMetadata();
         icoFrameMetadata.FromIconDirEntry(entry);
         icoFrameMetadata.Compression = compression;
-        icoFrameMetadata.BitsPerPixel = bitsPerPixel;
+        icoFrameMetadata.BmpBitsPerPixel = bitsPerPixel;
     }
 }

src/ImageSharp/Formats/Ico/IcoEncoder.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors.
+// Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
 namespace SixLabors.ImageSharp.Formats.Ico;
@@ -6,7 +6,7 @@ namespace SixLabors.ImageSharp.Formats.Ico;
 /// <summary>
 /// Image encoder for writing an image to a stream as a Windows Icon.
 /// </summary>
-public sealed class IcoEncoder : QuantizingImageEncoder
+public sealed class IcoEncoder : ImageEncoder
 {
     /// <inheritdoc/>
     protected override void Encode<TPixel>(Image<TPixel> image, Stream stream, CancellationToken cancellationToken)

src/ImageSharp/Formats/Ico/IcoEncoderCore.cs

@@ -5,6 +5,10 @@ using SixLabors.ImageSharp.Formats.Icon;
 
 namespace SixLabors.ImageSharp.Formats.Ico;
 
-internal sealed class IcoEncoderCore() : IconEncoderCore(IconFileType.ICO)
+internal sealed class IcoEncoderCore : IconEncoderCore
 {
+    public IcoEncoderCore()
+        : base(IconFileType.ICO)
+    {
+    }
 }

src/ImageSharp/Formats/Ico/IcoFrameMetadata.cs

@@ -1,12 +1,13 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
+using SixLabors.ImageSharp.Formats.Bmp;
 using SixLabors.ImageSharp.Formats.Icon;
 
 namespace SixLabors.ImageSharp.Formats.Ico;
 
 /// <summary>
-/// IcoFrameMetadata. TODO: Remove base class and merge into this class.
+/// Provides Ico specific metadata information for the image frame.
 /// </summary>
 public class IcoFrameMetadata : IDeepCloneable<IcoFrameMetadata>, IDeepCloneable
 {
@@ -17,54 +18,36 @@ public class IcoFrameMetadata : IDeepCloneable<IcoFrameMetadata>, IDeepCloneable
     {
     }
 
-    /// <summary>
-    /// Initializes a new instance of the <see cref="IcoFrameMetadata"/> class.
-    /// </summary>
-    /// <param name="width">width</param>
-    /// <param name="height">height</param>
-    /// <param name="colorCount">colorCount</param>
-    public IcoFrameMetadata(byte width, byte height, byte colorCount)
-    {
-        this.EncodingWidth = width;
-        this.EncodingHeight = height;
-        this.ColorCount = colorCount;
-    }
-
-    /// <inheritdoc cref="IcoFrameMetadata()"/>
-    public IcoFrameMetadata(IcoFrameMetadata metadata)
+    private IcoFrameMetadata(IcoFrameMetadata metadata)
     {
+        this.Compression = metadata.Compression;
         this.EncodingWidth = metadata.EncodingWidth;
         this.EncodingHeight = metadata.EncodingHeight;
-        this.ColorCount = metadata.ColorCount;
-        this.Compression = metadata.Compression;
+        this.BmpBitsPerPixel = metadata.BmpBitsPerPixel;
     }
 
     /// <summary>
-    /// Gets or sets icoFrameCompression.
+    /// Gets or sets the frame compressions format.
     /// </summary>
     public IconFrameCompression Compression { get; set; }
 
     /// <summary>
-    /// Gets or sets ColorCount field. <br />
-    /// Specifies number of colors in the color palette. Should be 0 if the image does not use a color palette.
+    /// Gets or sets the encoding width. <br />
+    /// Can be any number between 0 and 255. Value 0 means a frame height of 256 pixels.
     /// </summary>
-    // TODO: BmpMetadata does not supported palette yet.
-    public byte ColorCount { get; set; }
+    public byte EncodingWidth { get; set; }
 
     /// <summary>
-    /// Gets or sets Height field. <br />
-    /// Specifies image height in pixels. Can be any number between 0 and 255. Value 0 means image height is 256 pixels.
+    /// Gets or sets the encoding height. <br />
+    /// Can be any number between 0 and 255. Value 0 means a frame height of 256 pixels.
     /// </summary>
     public byte EncodingHeight { get; set; }
 
     /// <summary>
-    /// Gets or sets Width field. <br />
-    /// Specifies image width in pixels. Can be any number between 0 and 255. Value 0 means image width is 256 pixels.
+    /// Gets or sets the number of bits per pixel.<br/>
+    /// Used when <see cref="Compression"/> is <see cref="IconFrameCompression.Bmp"/>
     /// </summary>
-    public byte EncodingWidth { get; set; }
-
-    /// <inheritdoc cref="Bmp.BmpMetadata.BitsPerPixel" />
-    public Bmp.BmpBitsPerPixel BitsPerPixel { get; set; } = Bmp.BmpBitsPerPixel.Pixel24;
+    public BmpBitsPerPixel BmpBitsPerPixel { get; set; } = BmpBitsPerPixel.Pixel32;
 
     /// <inheritdoc/>
     public IcoFrameMetadata DeepClone() => new(this);
@@ -72,24 +55,29 @@ public class IcoFrameMetadata : IDeepCloneable<IcoFrameMetadata>, IDeepCloneable
     /// <inheritdoc/>
     IDeepCloneable IDeepCloneable.DeepClone() => this.DeepClone();
 
-    internal void FromIconDirEntry(in IconDirEntry entry)
+    internal void FromIconDirEntry(IconDirEntry entry)
     {
         this.EncodingWidth = entry.Width;
         this.EncodingHeight = entry.Height;
-        this.ColorCount = entry.ColorCount;
     }
 
-    internal IconDirEntry ToIconDirEntry() => new()
+    internal IconDirEntry ToIconDirEntry()
     {
-        Width = this.EncodingWidth,
-        Height = this.EncodingHeight,
-        ColorCount = this.ColorCount,
-        Planes = 1,
-        BitCount = this.Compression switch
+        byte colorCount = this.Compression == IconFrameCompression.Png || this.BmpBitsPerPixel > BmpBitsPerPixel.Pixel8
+            ? (byte)0
+            : (byte)ColorNumerics.GetColorCountForBitDepth((int)this.BmpBitsPerPixel);
+
+        return new()
         {
-            IconFrameCompression.Bmp => (ushort)this.BitsPerPixel,
-            IconFrameCompression.Png => 32,
-            _ => throw new NotSupportedException($"Value: {this.Compression}"),
-        },
-    };
+            Width = this.EncodingWidth,
+            Height = this.EncodingHeight,
+            Planes = 1,
+            ColorCount = colorCount,
+            BitCount = this.Compression switch
+            {
+                IconFrameCompression.Bmp => (ushort)this.BmpBitsPerPixel,
+                IconFrameCompression.Png or _ => 32,
+            },
+        };
+    }
 }

src/ImageSharp/Formats/Icon/IconAssert.cs

@@ -5,14 +5,6 @@ namespace SixLabors.ImageSharp.Formats.Icon;
 
 internal class IconAssert
 {
-    internal static void CanSeek(Stream stream)
-    {
-        if (!stream.CanSeek)
-        {
-            throw new NotSupportedException("This stream cannot support seek");
-        }
-    }
-
     internal static int EndOfStream(int v, int length)
     {
         if (v != length)
@@ -22,28 +14,4 @@ internal class IconAssert
 
         return v;
     }
-
-    internal static long EndOfStream(long v, long length)
-    {
-        if (v != length)
-        {
-            throw new EndOfStreamException();
-        }
-
-        return v;
-    }
-
-    internal static byte Is1ByteSize(int i)
-    {
-        if (i is 256)
-        {
-            return 0;
-        }
-        else if (i > byte.MaxValue)
-        {
-            throw new FormatException("Image size Too Large.");
-        }
-
-        return (byte)i;
-    }
 }

src/ImageSharp/Formats/Icon/IconDecoderCore.cs

@@ -10,12 +10,15 @@ using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp.Formats.Icon;
 
-internal abstract class IconDecoderCore(DecoderOptions options) : IImageDecoderInternals
+internal abstract class IconDecoderCore : IImageDecoderInternals
 {
     private IconDir fileHeader;
     private IconDirEntry[]? entries;
 
-    public DecoderOptions Options { get; } = options;
+    protected IconDecoderCore(DecoderOptions options)
+        => this.Options = options;
+
+    public DecoderOptions Options { get; }
 
     public Size Dimensions { get; private set; }
 
@@ -61,10 +64,11 @@ internal abstract class IconDecoderCore(DecoderOptions options) : IImageDecoderI
         }
 
         ImageMetadata metadata = new();
+        BmpMetadata? bmpMetadata = null;
         PngMetadata? pngMetadata = null;
         Image<TPixel> result = new(this.Options.Configuration, metadata, decodedEntries.Select(x =>
         {
-            BmpBitsPerPixel bitsPerPixel = default;
+            BmpBitsPerPixel bitsPerPixel = BmpBitsPerPixel.Pixel32;
             ImageFrame<TPixel> target = new(this.Options.Configuration, this.Dimensions);
             ImageFrame<TPixel> source = x.Image.Frames.RootFrameUnsafe;
             for (int y = 0; y < source.Height; y++)
@@ -80,12 +84,12 @@ internal abstract class IconDecoderCore(DecoderOptions options) : IImageDecoderI
                     pngMetadata = x.Image.Metadata.GetPngMetadata();
                 }
 
-                // Bmp does not contain frame specific metadata.
                 target.Metadata.SetFormatMetadata(PngFormat.Instance, target.Metadata.GetPngMetadata());
             }
             else
             {
-                bitsPerPixel = x.Image.Metadata.GetBmpMetadata().BitsPerPixel;
+                bmpMetadata = x.Image.Metadata.GetBmpMetadata();
+                bitsPerPixel = bmpMetadata.BitsPerPixel;
             }
 
             this.SetFrameMetadata(target.Metadata, this.entries[x.Index], x.Compression, bitsPerPixel);
@@ -96,6 +100,11 @@ internal abstract class IconDecoderCore(DecoderOptions options) : IImageDecoderI
         }).ToArray());
 
         // Copy the format specific metadata to the image.
+        if (bmpMetadata != null)
+        {
+            result.Metadata.SetFormatMetadata(BmpFormat.Instance, bmpMetadata);
+        }
+
         if (pngMetadata != null)
         {
             result.Metadata.SetFormatMetadata(PngFormat.Instance, pngMetadata);
@@ -114,9 +123,10 @@ internal abstract class IconDecoderCore(DecoderOptions options) : IImageDecoderI
 
         ImageMetadata metadata = new();
         ImageFrameMetadata[] frames = new ImageFrameMetadata[this.fileHeader.Count];
+        int bpp = 0;
         for (int i = 0; i < frames.Length; i++)
         {
-            BmpBitsPerPixel bitsPerPixel = default;
+            BmpBitsPerPixel bitsPerPixel = BmpBitsPerPixel.Pixel32;
             ref IconDirEntry entry = ref this.entries[i];
 
             // If we hit the end of the stream we should break.
@@ -140,11 +150,13 @@ internal abstract class IconDecoderCore(DecoderOptions options) : IImageDecoderI
             ImageInfo temp = this.GetDecoder(isPng).Identify(stream, cancellationToken);
 
             frames[i] = new();
-            if (isPng)
+            if (!isPng)
             {
                 bitsPerPixel = temp.Metadata.GetBmpMetadata().BitsPerPixel;
             }
 
+            bpp = Math.Max(bpp, (int)bitsPerPixel);
+
             this.SetFrameMetadata(frames[i], this.entries[i], isPng ? IconFrameCompression.Png : IconFrameCompression.Bmp, bitsPerPixel);
 
             // Since Windows Vista, the size of an image is determined from the BITMAPINFOHEADER structure or PNG image data
@@ -152,7 +164,7 @@ internal abstract class IconDecoderCore(DecoderOptions options) : IImageDecoderI
             this.Dimensions = new(Math.Max(this.Dimensions.Width, temp.Size.Width), Math.Max(this.Dimensions.Height, temp.Size.Height));
         }
 
-        return new(new(32), this.Dimensions, metadata, frames);
+        return new(new(bpp), this.Dimensions, metadata, frames);
     }
 
     protected abstract void SetFrameMetadata(ImageFrameMetadata metadata, in IconDirEntry entry, IconFrameCompression compression, BmpBitsPerPixel bitsPerPixel);
@@ -211,15 +223,13 @@ internal abstract class IconDecoderCore(DecoderOptions options) : IImageDecoderI
                 GeneralOptions = this.Options,
             });
         }
-        else
+
+        return new BmpDecoderCore(new()
         {
-            return new BmpDecoderCore(new()
-            {
-                GeneralOptions = this.Options,
-                ProcessedAlphaMask = true,
-                SkipFileHeader = true,
-                UseDoubleHeight = true,
-            });
-        }
+            GeneralOptions = this.Options,
+            ProcessedAlphaMask = true,
+            SkipFileHeader = true,
+            UseDoubleHeight = true,
+        });
     }
 }

src/ImageSharp/Formats/Icon/IconDir.cs

@@ -9,8 +9,20 @@ namespace SixLabors.ImageSharp.Formats.Icon;
 internal struct IconDir(ushort reserved, IconFileType type, ushort count)
 {
     public const int Size = 3 * sizeof(ushort);
+
+    /// <summary>
+    /// Reserved. Must always be 0.
+    /// </summary>
     public ushort Reserved = reserved;
+
+    /// <summary>
+    /// Specifies image type: 1 for icon (.ICO) image, 2 for cursor (.CUR) image. Other values are invalid.
+    /// </summary>
     public IconFileType Type = type;
+
+    /// <summary>
+    /// Specifies number of images in the file.
+    /// </summary>
     public ushort Count = count;
 
     public IconDir(IconFileType type)
@@ -23,9 +35,9 @@ internal struct IconDir(ushort reserved, IconFileType type, ushort count)
     {
     }
 
-    public static IconDir Parse(in ReadOnlySpan<byte> data)
+    public static IconDir Parse(ReadOnlySpan<byte> data)
         => MemoryMarshal.Cast<byte, IconDir>(data)[0];
 
-    public unsafe void WriteTo(in Stream stream)
+    public readonly unsafe void WriteTo(Stream stream)
         => stream.Write(MemoryMarshal.Cast<IconDir, byte>([this]));
 }

src/ImageSharp/Formats/Icon/IconDirEntry.cs

@@ -10,25 +10,51 @@ internal struct IconDirEntry
 {
     public const int Size = (4 * sizeof(byte)) + (2 * sizeof(ushort)) + (2 * sizeof(uint));
 
+    /// <summary>
+    /// Specifies image width in pixels. Can be any number between 0 and 255. Value 0 means image width is 256 pixels.
+    /// </summary>
     public byte Width;
 
+    /// <summary>
+    /// Specifies image height in pixels. Can be any number between 0 and 255. Value 0 means image height is 256 pixels.[
+    /// </summary>
     public byte Height;
 
+    /// <summary>
+    /// Specifies number of colors in the color palette. Should be 0 if the image does not use a color palette.
+    /// </summary>
     public byte ColorCount;
 
+    /// <summary>
+    /// Reserved. Should be 0.
+    /// </summary>
     public byte Reserved;
 
+    /// <summary>
+    /// In ICO format: Specifies color planes. Should be 0 or 1.<br/>
+    /// In CUR format: Specifies the horizontal coordinates of the hotspot in number of pixels from the left.
+    /// </summary>
     public ushort Planes;
 
+    /// <summary>
+    /// In ICO format: Specifies bits per pixel.<br/>
+    /// In CUR format: Specifies the vertical coordinates of the hotspot in number of pixels from the top.
+    /// </summary>
     public ushort BitCount;
 
+    /// <summary>
+    /// Specifies the size of the image's data in bytes
+    /// </summary>
     public uint BytesInRes;
 
+    /// <summary>
+    /// Specifies the offset of BMP or PNG data from the beginning of the ICO/CUR file.
+    /// </summary>
     public uint ImageOffset;
 
     public static IconDirEntry Parse(in ReadOnlySpan<byte> data)
         => MemoryMarshal.Cast<byte, IconDirEntry>(data)[0];
 
-    public unsafe void WriteTo(in Stream stream)
+    public readonly unsafe void WriteTo(in Stream stream)
         => stream.Write(MemoryMarshal.Cast<IconDirEntry, byte>([this]));
 }

src/ImageSharp/Formats/Icon/IconEncoderCore.cs

@@ -2,18 +2,23 @@
 // Licensed under the Six Labors Split License.
 
 using System.Diagnostics.CodeAnalysis;
+using SixLabors.ImageSharp.Formats.Bmp;
 using SixLabors.ImageSharp.Formats.Cur;
 using SixLabors.ImageSharp.Formats.Ico;
+using SixLabors.ImageSharp.Formats.Png;
 using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing.Processors.Quantization;
 
 namespace SixLabors.ImageSharp.Formats.Icon;
 
-internal abstract class IconEncoderCore(IconFileType iconFileType)
-    : IImageEncoderInternals
+internal abstract class IconEncoderCore : IImageEncoderInternals
 {
+    private readonly IconFileType iconFileType;
     private IconDir fileHeader;
+    private EncodingFrameMetadata[]? entries;
 
-    private IconFrameMetadata[]? entries;
+    protected IconEncoderCore(IconFileType iconFileType)
+        => this.iconFileType = iconFileType;
 
     public void Encode<TPixel>(
         Image<TPixel> image,
@@ -24,17 +29,18 @@ internal abstract class IconEncoderCore(IconFileType iconFileType)
         Guard.NotNull(image, nameof(image));
         Guard.NotNull(stream, nameof(stream));
 
-        IconAssert.CanSeek(stream);
-
         // Stream may not at 0.
         long basePosition = stream.Position;
         this.InitHeader(image);
 
+        // We don't write the header and entries yet as we need to write the image data first.
         int dataOffset = IconDir.Size + (IconDirEntry.Size * this.entries.Length);
         _ = stream.Seek(dataOffset, SeekOrigin.Current);
 
         for (int i = 0; i < image.Frames.Count; i++)
         {
+            // Since Windows Vista, the size of an image is determined from the BITMAPINFOHEADER structure or PNG image data
+            // which technically allows storing icons with larger than 256 pixels, but such larger sizes are not recommended by Microsoft.
             ImageFrame<TPixel> frame = image.Frames[i];
             int width = this.entries[i].Entry.Width;
             if (width is 0)
@@ -50,36 +56,54 @@ internal abstract class IconEncoderCore(IconFileType iconFileType)
 
             this.entries[i].Entry.ImageOffset = (uint)stream.Position;
 
-            Image<TPixel> img = new(width, height);
+            // We crop the frame to the size specified in the metadata.
+            // TODO: we can optimize this by cropping the frame only if the new size is both required and different.
+            using Image<TPixel> encodingFrame = new(width, height);
             for (int y = 0; y < height; y++)
             {
-                frame.PixelBuffer.DangerousGetRowSpan(y)[..width].CopyTo(img.GetRootFramePixelBuffer().DangerousGetRowSpan(y));
+                frame.PixelBuffer.DangerousGetRowSpan(y)[..width]
+                     .CopyTo(encodingFrame.GetRootFramePixelBuffer().DangerousGetRowSpan(y));
             }
 
-            QuantizingImageEncoder encoder = this.entries[i].Compression switch
+            ref EncodingFrameMetadata encodingMetadata = ref this.entries[i];
+
+            QuantizingImageEncoder encoder = encodingMetadata.Compression switch
             {
-                IconFrameCompression.Bmp => new Bmp.BmpEncoder()
+                IconFrameCompression.Bmp => new BmpEncoder()
                 {
+                    // We don't have access to the palette in the metadata so we need to quantize the image
+                    // using a new one generated from the pixel data.
+                    Quantizer = encodingMetadata.Entry.BitCount <= 8
+                    ? new WuQuantizer(new()
+                    {
+                        MaxColors = encodingMetadata.Entry.ColorCount
+                    })
+                    : null,
                     ProcessedAlphaMask = true,
                     UseDoubleHeight = true,
                     SkipFileHeader = true,
                     SupportTransparency = false,
-                    BitsPerPixel = iconFileType is IconFileType.ICO
-                        ? (Bmp.BmpBitsPerPixel?)this.entries[i].Entry.BitCount
-                        : Bmp.BmpBitsPerPixel.Pixel24 // TODO: Here you need to switch to selecting the corresponding value according to the size of the image
+                    BitsPerPixel = encodingMetadata.BmpBitsPerPixel
+                },
+                IconFrameCompression.Png => new PngEncoder()
+                {
+                    // Only 32bit Png supported.
+                    // https://devblogs.microsoft.com/oldnewthing/20101022-00/?p=12473
+                    BitDepth = PngBitDepth.Bit8,
+                    ColorType = PngColorType.RgbWithAlpha
                 },
-                IconFrameCompression.Png => new Png.PngEncoder(),
                 _ => throw new NotSupportedException(),
             };
 
-            encoder.Encode(img, stream);
-            this.entries[i].Entry.BytesInRes = (uint)stream.Position - this.entries[i].Entry.ImageOffset;
+            encoder.Encode(encodingFrame, stream);
+            encodingMetadata.Entry.BytesInRes = (uint)stream.Position - encodingMetadata.Entry.ImageOffset;
         }
 
+        // We now need to rewind the stream and write the header and the entries.
         long endPosition = stream.Position;
         _ = stream.Seek(basePosition, SeekOrigin.Begin);
         this.fileHeader.WriteTo(stream);
-        foreach (IconFrameMetadata frame in this.entries)
+        foreach (EncodingFrameMetadata frame in this.entries)
         {
             frame.Entry.WriteTo(stream);
         }
@@ -88,33 +112,38 @@ internal abstract class IconEncoderCore(IconFileType iconFileType)
     }
 
     [MemberNotNull(nameof(entries))]
-    private void InitHeader(in Image image)
+    private void InitHeader(Image image)
     {
-        this.fileHeader = new(iconFileType, (ushort)image.Frames.Count);
-        this.entries = iconFileType switch
+        this.fileHeader = new(this.iconFileType, (ushort)image.Frames.Count);
+        this.entries = this.iconFileType switch
         {
             IconFileType.ICO =>
             image.Frames.Select(i =>
             {
                 IcoFrameMetadata metadata = i.Metadata.GetIcoMetadata();
-                return new IconFrameMetadata(metadata.Compression, metadata.ToIconDirEntry());
+                return new EncodingFrameMetadata(metadata.Compression, metadata.BmpBitsPerPixel, metadata.ToIconDirEntry());
             }).ToArray(),
             IconFileType.CUR =>
             image.Frames.Select(i =>
             {
                 CurFrameMetadata metadata = i.Metadata.GetCurMetadata();
-                return new IconFrameMetadata(metadata.Compression, metadata.ToIconDirEntry());
+                return new EncodingFrameMetadata(metadata.Compression, metadata.BmpBitsPerPixel, metadata.ToIconDirEntry());
             }).ToArray(),
             _ => throw new NotSupportedException(),
         };
     }
 
-    internal sealed class IconFrameMetadata(IconFrameCompression compression, IconDirEntry iconDirEntry)
+    internal sealed class EncodingFrameMetadata(
+        IconFrameCompression compression,
+        BmpBitsPerPixel bmpBitsPerPixel,
+        IconDirEntry iconDirEntry)
     {
         private IconDirEntry iconDirEntry = iconDirEntry;
 
         public IconFrameCompression Compression { get; set; } = compression;
 
+        public BmpBitsPerPixel BmpBitsPerPixel { get; set; } = bmpBitsPerPixel;
+
         public ref IconDirEntry Entry => ref this.iconDirEntry;
     }
 }

src/ImageSharp/Formats/Icon/IconImageFormatDetector.cs

@@ -60,9 +60,7 @@ public class IconImageFormatDetector : IImageFormatDetector
 
             return true;
         }
-        else
-        {
-            return false;
-        }
+
+        return false;
     }
 }