Allow preservation of png color palette

src/ImageSharp/Formats/Png/PngDecoder.cs

@@ -61,24 +61,24 @@ public sealed class PngDecoder : ImageDecoder
             case PngColorType.Grayscale:
                 if (bits == PngBitDepth.Bit16)
                 {
-                    return !meta.HasTransparency
+                    return !meta.TransparentColor.HasValue
                         ? this.Decode<L16>(options, stream, cancellationToken)
                         : this.Decode<La32>(options, stream, cancellationToken);
                 }
 
-                return !meta.HasTransparency
+                return !meta.TransparentColor.HasValue
                     ? this.Decode<L8>(options, stream, cancellationToken)
                     : this.Decode<La16>(options, stream, cancellationToken);
 
             case PngColorType.Rgb:
                 if (bits == PngBitDepth.Bit16)
                 {
-                    return !meta.HasTransparency
+                    return !meta.TransparentColor.HasValue
                         ? this.Decode<Rgb48>(options, stream, cancellationToken)
                         : this.Decode<Rgba64>(options, stream, cancellationToken);
                 }
 
-                return !meta.HasTransparency
+                return !meta.TransparentColor.HasValue
                     ? this.Decode<Rgb24>(options, stream, cancellationToken)
                     : this.Decode<Rgba32>(options, stream, cancellationToken);
 

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -172,21 +172,20 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                             if (image is null)
                             {
                                 this.InitializeImage(metadata, out image);
+
+                                // Both PLTE and tRNS chunks, if present, have been read at this point as per spec.
+                                AssignColorPalette(this.palette, this.paletteAlpha, pngMetadata);
                             }
 
                             this.ReadScanlines(chunk, image.Frames.RootFrame, pngMetadata, cancellationToken);
 
                             break;
                         case PngChunkType.Palette:
-                            byte[] pal = new byte[chunk.Length];
-                            chunk.Data.GetSpan().CopyTo(pal);
-                            this.palette = pal;
+                            this.palette = chunk.Data.GetSpan().ToArray();
                             break;
                         case PngChunkType.Transparency:
-                            byte[] alpha = new byte[chunk.Length];
-                            chunk.Data.GetSpan().CopyTo(alpha);
-                            this.paletteAlpha = alpha;
-                            this.AssignTransparentMarkers(alpha, pngMetadata);
+                            this.paletteAlpha = chunk.Data.GetSpan().ToArray();
+                            this.AssignTransparentMarkers(this.paletteAlpha, pngMetadata);
                             break;
                         case PngChunkType.Text:
                             this.ReadTextChunk(metadata, pngMetadata, chunk.Data.GetSpan());
@@ -292,12 +291,15 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
 
                             this.SkipChunkDataAndCrc(chunk);
                             break;
+                        case PngChunkType.Palette:
+                            this.palette = chunk.Data.GetSpan().ToArray();
+                            break;
+
                         case PngChunkType.Transparency:
-                            byte[] alpha = new byte[chunk.Length];
-                            chunk.Data.GetSpan().CopyTo(alpha);
-                            this.paletteAlpha = alpha;
-                            this.AssignTransparentMarkers(alpha, pngMetadata);
+                            this.paletteAlpha = chunk.Data.GetSpan().ToArray();
+                            this.AssignTransparentMarkers(this.paletteAlpha, pngMetadata);
 
+                            // Spec says tRNS must be after PLTE so safe to exit.
                             if (this.colorMetadataOnly)
                             {
                                 goto EOF;
@@ -370,6 +372,9 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                 PngThrowHelper.ThrowNoHeader();
             }
 
+            // Both PLTE and tRNS chunks, if present, have been read at this point as per spec.
+            AssignColorPalette(this.palette, this.paletteAlpha, pngMetadata);
+
             return new ImageInfo(new PixelTypeInfo(this.CalculateBitsPerPixel()), new(this.header.Width, this.header.Height), metadata);
         }
         finally
@@ -766,9 +771,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                         this.header,
                         scanlineSpan,
                         rowSpan,
-                        pngMetadata.HasTransparency,
-                        pngMetadata.TransparentL16.GetValueOrDefault(),
-                        pngMetadata.TransparentL8.GetValueOrDefault());
+                        pngMetadata.TransparentColor);
 
                     break;
 
@@ -787,8 +790,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                         this.header,
                         scanlineSpan,
                         rowSpan,
-                        this.palette,
-                        this.paletteAlpha);
+                        pngMetadata.ColorTable);
 
                     break;
 
@@ -800,9 +802,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                         rowSpan,
                         this.bytesPerPixel,
                         this.bytesPerSample,
-                        pngMetadata.HasTransparency,
-                        pngMetadata.TransparentRgb48.GetValueOrDefault(),
-                        pngMetadata.TransparentRgb24.GetValueOrDefault());
+                        pngMetadata.TransparentColor);
 
                     break;
 
@@ -860,9 +860,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                         rowSpan,
                         (uint)pixelOffset,
                         (uint)increment,
-                        pngMetadata.HasTransparency,
-                        pngMetadata.TransparentL16.GetValueOrDefault(),
-                        pngMetadata.TransparentL8.GetValueOrDefault());
+                        pngMetadata.TransparentColor);
 
                     break;
 
@@ -885,8 +883,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                         rowSpan,
                         (uint)pixelOffset,
                         (uint)increment,
-                        this.palette,
-                        this.paletteAlpha);
+                        pngMetadata.ColorTable);
 
                     break;
 
@@ -899,9 +896,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                         (uint)increment,
                         this.bytesPerPixel,
                         this.bytesPerSample,
-                        pngMetadata.HasTransparency,
-                        pngMetadata.TransparentRgb48.GetValueOrDefault(),
-                        pngMetadata.TransparentRgb24.GetValueOrDefault());
+                        pngMetadata.TransparentColor);
 
                     break;
 
@@ -924,10 +919,44 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
         }
     }
 
+    /// <summary>
+    /// Decodes and assigns the color palette to the metadata
+    /// </summary>
+    /// <param name="palette">The palette buffer.</param>
+    /// <param name="alpha">The alpha palette buffer.</param>
+    /// <param name="pngMetadata">The png metadata.</param>
+    private static void AssignColorPalette(ReadOnlySpan<byte> palette, ReadOnlySpan<byte> alpha, PngMetadata pngMetadata)
+    {
+        if (palette.Length == 0)
+        {
+            return;
+        }
+
+        Color[] colorTable = new Color[palette.Length / Unsafe.SizeOf<Rgb24>()];
+        ref Rgb24 paletteBase = ref MemoryMarshal.GetReference(MemoryMarshal.Cast<byte, Rgb24>(palette));
+        for (int i = 0; i < colorTable.Length; i++)
+        {
+            colorTable[i] = new Color(Unsafe.Add(ref paletteBase, (uint)i));
+        }
+
+        if (alpha.Length > 0)
+        {
+            // The alpha chunk may contain as many transparency entries as there are palette entries
+            // (more than that would not make any sense) or as few as one.
+            for (int i = 0; i < alpha.Length; i++)
+            {
+                ref Color color = ref colorTable[i];
+                color = color.WithAlpha(alpha[i] / 255F);
+            }
+        }
+
+        pngMetadata.ColorTable = colorTable;
+    }
+
     /// <summary>
     /// Decodes and assigns marker colors that identify transparent pixels in non indexed images.
     /// </summary>
-    /// <param name="alpha">The alpha tRNS array.</param>
+    /// <param name="alpha">The alpha tRNS buffer.</param>
     /// <param name="pngMetadata">The png metadata.</param>
     private void AssignTransparentMarkers(ReadOnlySpan<byte> alpha, PngMetadata pngMetadata)
     {
@@ -941,16 +970,14 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
                     ushort gc = BinaryPrimitives.ReadUInt16LittleEndian(alpha.Slice(2, 2));
                     ushort bc = BinaryPrimitives.ReadUInt16LittleEndian(alpha.Slice(4, 2));
 
-                    pngMetadata.TransparentRgb48 = new Rgb48(rc, gc, bc);
-                    pngMetadata.HasTransparency = true;
+                    pngMetadata.TransparentColor = new(new Rgb48(rc, gc, bc));
                     return;
                 }
 
                 byte r = ReadByteLittleEndian(alpha, 0);
                 byte g = ReadByteLittleEndian(alpha, 2);
                 byte b = ReadByteLittleEndian(alpha, 4);
-                pngMetadata.TransparentRgb24 = new Rgb24(r, g, b);
-                pngMetadata.HasTransparency = true;
+                pngMetadata.TransparentColor = new(new Rgb24(r, g, b));
             }
         }
         else if (this.pngColorType == PngColorType.Grayscale)
@@ -959,20 +986,14 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
             {
                 if (this.header.BitDepth == 16)
                 {
-                    pngMetadata.TransparentL16 = new L16(BinaryPrimitives.ReadUInt16LittleEndian(alpha[..2]));
+                    pngMetadata.TransparentColor = Color.FromPixel(new L16(BinaryPrimitives.ReadUInt16LittleEndian(alpha[..2])));
                 }
                 else
                 {
-                    pngMetadata.TransparentL8 = new L8(ReadByteLittleEndian(alpha, 0));
+                    pngMetadata.TransparentColor = Color.FromPixel(new L8(ReadByteLittleEndian(alpha, 0)));
                 }
-
-                pngMetadata.HasTransparency = true;
             }
         }
-        else if (this.pngColorType == PngColorType.Palette && alpha.Length > 0)
-        {
-            pngMetadata.HasTransparency = true;
-        }
     }
 
     /// <summary>
@@ -1461,7 +1482,7 @@ internal sealed class PngDecoderCore : IImageDecoderInternals
 
         // If we're reading color metadata only we're only interested in the IHDR and tRNS chunks.
         // We can skip all other chunk data in the stream for better performance.
-        if (this.colorMetadataOnly && type != PngChunkType.Header && type != PngChunkType.Transparency)
+        if (this.colorMetadataOnly && type != PngChunkType.Header && type != PngChunkType.Transparency && type != PngChunkType.Palette)
         {
             chunk = new PngChunk(length, type);
 

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -875,7 +875,7 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
             // 4-byte unsigned integer of gamma * 100,000.
             uint gammaValue = (uint)(this.gamma * 100_000F);
 
-            BinaryPrimitives.WriteUInt32BigEndian(this.chunkDataBuffer.Span.Slice(0, 4), gammaValue);
+            BinaryPrimitives.WriteUInt32BigEndian(this.chunkDataBuffer.Span[..4], gammaValue);
 
             this.WriteChunk(stream, PngChunkType.Gamma, this.chunkDataBuffer.Span, 0, 4);
         }
@@ -889,7 +889,7 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
     /// <param name="pngMetadata">The image metadata.</param>
     private void WriteTransparencyChunk(Stream stream, PngMetadata pngMetadata)
     {
-        if (!pngMetadata.HasTransparency)
+        if (pngMetadata.TransparentColor is null)
         {
             return;
         }
@@ -897,19 +897,19 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
         Span<byte> alpha = this.chunkDataBuffer.Span;
         if (pngMetadata.ColorType == PngColorType.Rgb)
         {
-            if (pngMetadata.TransparentRgb48.HasValue && this.use16Bit)
+            if (this.use16Bit)
             {
-                Rgb48 rgb = pngMetadata.TransparentRgb48.Value;
+                Rgb48 rgb = pngMetadata.TransparentColor.Value.ToPixel<Rgb48>();
                 BinaryPrimitives.WriteUInt16LittleEndian(alpha, rgb.R);
                 BinaryPrimitives.WriteUInt16LittleEndian(alpha.Slice(2, 2), rgb.G);
                 BinaryPrimitives.WriteUInt16LittleEndian(alpha.Slice(4, 2), rgb.B);
 
                 this.WriteChunk(stream, PngChunkType.Transparency, this.chunkDataBuffer.Span, 0, 6);
             }
-            else if (pngMetadata.TransparentRgb24.HasValue)
+            else
             {
                 alpha.Clear();
-                Rgb24 rgb = pngMetadata.TransparentRgb24.Value;
+                Rgb24 rgb = pngMetadata.TransparentColor.Value.ToRgb24();
                 alpha[1] = rgb.R;
                 alpha[3] = rgb.G;
                 alpha[5] = rgb.B;
@@ -918,15 +918,17 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
         }
         else if (pngMetadata.ColorType == PngColorType.Grayscale)
         {
-            if (pngMetadata.TransparentL16.HasValue && this.use16Bit)
+            if (this.use16Bit)
             {
-                BinaryPrimitives.WriteUInt16LittleEndian(alpha, pngMetadata.TransparentL16.Value.PackedValue);
+                L16 l16 = pngMetadata.TransparentColor.Value.ToPixel<L16>();
+                BinaryPrimitives.WriteUInt16LittleEndian(alpha, l16.PackedValue);
                 this.WriteChunk(stream, PngChunkType.Transparency, this.chunkDataBuffer.Span, 0, 2);
             }
-            else if (pngMetadata.TransparentL8.HasValue)
+            else
             {
+                L8 l8 = pngMetadata.TransparentColor.Value.ToPixel<L8>();
                 alpha.Clear();
-                alpha[1] = pngMetadata.TransparentL8.Value.PackedValue;
+                alpha[1] = l8.PackedValue;
                 this.WriteChunk(stream, PngChunkType.Transparency, this.chunkDataBuffer.Span, 0, 2);
             }
         }
@@ -1175,7 +1177,7 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
 
         stream.Write(buffer);
 
-        uint crc = Crc32.Calculate(buffer.Slice(4)); // Write the type buffer
+        uint crc = Crc32.Calculate(buffer[4..]); // Write the type buffer
 
         if (data.Length > 0 && length > 0)
         {
@@ -1290,8 +1292,20 @@ internal sealed class PngEncoderCore : IImageEncoderInternals, IDisposable
         }
 
         // Use the metadata to determine what quantization depth to use if no quantizer has been set.
-        IQuantizer quantizer = encoder.Quantizer
-            ?? new WuQuantizer(new QuantizerOptions { MaxColors = ColorNumerics.GetColorCountForBitDepth(bitDepth) });
+        IQuantizer quantizer = encoder.Quantizer;
+        if (quantizer is null)
+        {
+            PngMetadata metadata = image.Metadata.GetPngMetadata();
+            if (metadata.ColorTable is not null)
+            {
+                // Use the provided palette in total. The caller is responsible for setting values.
+                quantizer = new PaletteQuantizer(metadata.ColorTable.Value);
+            }
+            else
+            {
+                quantizer = new WuQuantizer(new QuantizerOptions { MaxColors = ColorNumerics.GetColorCountForBitDepth(bitDepth) });
+            }
+        }
 
         // Create quantized frame returning the palette and set the bit depth.
         using IQuantizer<TPixel> frameQuantizer = quantizer.CreatePixelSpecificQuantizer<TPixel>(image.GetConfiguration());

src/ImageSharp/Formats/Png/PngMetadata.cs

@@ -1,8 +1,6 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
-using SixLabors.ImageSharp.PixelFormats;
-
 namespace SixLabors.ImageSharp.Formats.Png;
 
 /// <summary>
@@ -27,11 +25,12 @@ public class PngMetadata : IDeepCloneable
         this.ColorType = other.ColorType;
         this.Gamma = other.Gamma;
         this.InterlaceMethod = other.InterlaceMethod;
-        this.HasTransparency = other.HasTransparency;
-        this.TransparentL8 = other.TransparentL8;
-        this.TransparentL16 = other.TransparentL16;
-        this.TransparentRgb24 = other.TransparentRgb24;
-        this.TransparentRgb48 = other.TransparentRgb48;
+        this.TransparentColor = other.TransparentColor;
+
+        if (other.ColorTable?.Length > 0)
+        {
+            this.ColorTable = other.ColorTable.Value.ToArray();
+        }
 
         for (int i = 0; i < other.TextData.Count; i++)
         {
@@ -61,33 +60,14 @@ public class PngMetadata : IDeepCloneable
     public float Gamma { get; set; }
 
     /// <summary>
-    /// Gets or sets the Rgb24 transparent color.
-    /// This represents any color in an 8 bit Rgb24 encoded png that should be transparent.
-    /// </summary>
-    public Rgb24? TransparentRgb24 { get; set; }
-
-    /// <summary>
-    /// Gets or sets the Rgb48 transparent color.
-    /// This represents any color in a 16 bit Rgb24 encoded png that should be transparent.
-    /// </summary>
-    public Rgb48? TransparentRgb48 { get; set; }
-
-    /// <summary>
-    /// Gets or sets the 8 bit grayscale transparent color.
-    /// This represents any color in an 8 bit grayscale encoded png that should be transparent.
-    /// </summary>
-    public L8? TransparentL8 { get; set; }
-
-    /// <summary>
-    /// Gets or sets the 16 bit grayscale transparent color.
-    /// This represents any color in a 16 bit grayscale encoded png that should be transparent.
+    /// Gets or sets the color table, if any.
     /// </summary>
-    public L16? TransparentL16 { get; set; }
+    public ReadOnlyMemory<Color>? ColorTable { get; set; }
 
     /// <summary>
-    /// Gets or sets a value indicating whether the image contains a transparency chunk and markers were decoded.
+    /// Gets or sets the transparent color used with non palette based images, if a transparency chunk and markers were decoded.
     /// </summary>
-    public bool HasTransparency { get; set; }
+    public Color? TransparentColor { get; set; }
 
     /// <summary>
     /// Gets or sets the collection of text data stored within the iTXt, tEXt, and zTXt chunks.

src/ImageSharp/Formats/Png/PngScanlineProcessor.cs

@@ -18,9 +18,7 @@ internal static class PngScanlineProcessor
         in PngHeader header,
         ReadOnlySpan<byte> scanlineSpan,
         Span<TPixel> rowSpan,
-        bool hasTrans,
-        L16 luminance16Trans,
-        L8 luminanceTrans)
+        Color? transparentColor)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         TPixel pixel = default;
@@ -28,7 +26,7 @@ internal static class PngScanlineProcessor
         ref TPixel rowSpanRef = ref MemoryMarshal.GetReference(rowSpan);
         int scaleFactor = 255 / (ColorNumerics.GetColorCountForBitDepth(header.BitDepth) - 1);
 
-        if (!hasTrans)
+        if (transparentColor is null)
         {
             if (header.BitDepth == 16)
             {
@@ -55,13 +53,14 @@ internal static class PngScanlineProcessor
 
         if (header.BitDepth == 16)
         {
+            L16 transparent = transparentColor.Value.ToPixel<L16>();
             La32 source = default;
             int o = 0;
             for (nuint x = 0; x < (uint)header.Width; x++, o += 2)
             {
                 ushort luminance = BinaryPrimitives.ReadUInt16BigEndian(scanlineSpan.Slice(o, 2));
                 source.L = luminance;
-                source.A = luminance.Equals(luminance16Trans.PackedValue) ? ushort.MinValue : ushort.MaxValue;
+                source.A = luminance.Equals(transparent.PackedValue) ? ushort.MinValue : ushort.MaxValue;
 
                 pixel.FromLa32(source);
                 Unsafe.Add(ref rowSpanRef, x) = pixel;
@@ -69,13 +68,13 @@ internal static class PngScanlineProcessor
         }
         else
         {
+            byte transparent = (byte)(transparentColor.Value.ToPixel<L8>().PackedValue * scaleFactor);
             La16 source = default;
-            byte scaledLuminanceTrans = (byte)(luminanceTrans.PackedValue * scaleFactor);
             for (nuint x = 0; x < (uint)header.Width; x++)
             {
                 byte luminance = (byte)(Unsafe.Add(ref scanlineSpanRef, x) * scaleFactor);
                 source.L = luminance;
-                source.A = luminance.Equals(scaledLuminanceTrans) ? byte.MinValue : byte.MaxValue;
+                source.A = luminance.Equals(transparent) ? byte.MinValue : byte.MaxValue;
 
                 pixel.FromLa16(source);
                 Unsafe.Add(ref rowSpanRef, x) = pixel;
@@ -89,9 +88,7 @@ internal static class PngScanlineProcessor
         Span<TPixel> rowSpan,
         uint pixelOffset,
         uint increment,
-        bool hasTrans,
-        L16 luminance16Trans,
-        L8 luminanceTrans)
+        Color? transparentColor)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         TPixel pixel = default;
@@ -99,7 +96,7 @@ internal static class PngScanlineProcessor
         ref TPixel rowSpanRef = ref MemoryMarshal.GetReference(rowSpan);
         int scaleFactor = 255 / (ColorNumerics.GetColorCountForBitDepth(header.BitDepth) - 1);
 
-        if (!hasTrans)
+        if (transparentColor is null)
         {
             if (header.BitDepth == 16)
             {
@@ -126,13 +123,14 @@ internal static class PngScanlineProcessor
 
         if (header.BitDepth == 16)
         {
+            L16 transparent = transparentColor.Value.ToPixel<L16>();
             La32 source = default;
             int o = 0;
             for (nuint x = pixelOffset; x < (uint)header.Width; x += increment, o += 2)
             {
                 ushort luminance = BinaryPrimitives.ReadUInt16BigEndian(scanlineSpan.Slice(o, 2));
                 source.L = luminance;
-                source.A = luminance.Equals(luminance16Trans.PackedValue) ? ushort.MinValue : ushort.MaxValue;
+                source.A = luminance.Equals(transparent.PackedValue) ? ushort.MinValue : ushort.MaxValue;
 
                 pixel.FromLa32(source);
                 Unsafe.Add(ref rowSpanRef, x) = pixel;
@@ -140,13 +138,13 @@ internal static class PngScanlineProcessor
         }
         else
         {
+            byte transparent = (byte)(transparentColor.Value.ToPixel<L8>().PackedValue * scaleFactor);
             La16 source = default;
-            byte scaledLuminanceTrans = (byte)(luminanceTrans.PackedValue * scaleFactor);
             for (nuint x = pixelOffset, o = 0; x < (uint)header.Width; x += increment, o++)
             {
                 byte luminance = (byte)(Unsafe.Add(ref scanlineSpanRef, o) * scaleFactor);
                 source.L = luminance;
-                source.A = luminance.Equals(scaledLuminanceTrans) ? byte.MinValue : byte.MaxValue;
+                source.A = luminance.Equals(transparent) ? byte.MinValue : byte.MaxValue;
 
                 pixel.FromLa16(source);
                 Unsafe.Add(ref rowSpanRef, x) = pixel;
@@ -241,11 +239,10 @@ internal static class PngScanlineProcessor
         in PngHeader header,
         ReadOnlySpan<byte> scanlineSpan,
         Span<TPixel> rowSpan,
-        ReadOnlySpan<byte> palette,
-        byte[] paletteAlpha)
+        ReadOnlyMemory<Color>? palette)
         where TPixel : unmanaged, IPixel<TPixel>
     {
-        if (palette.IsEmpty)
+        if (palette is null)
         {
             PngThrowHelper.ThrowMissingPalette();
         }
@@ -253,36 +250,13 @@ internal static class PngScanlineProcessor
         TPixel pixel = default;
         ref byte scanlineSpanRef = ref MemoryMarshal.GetReference(scanlineSpan);
         ref TPixel rowSpanRef = ref MemoryMarshal.GetReference(rowSpan);
-        ReadOnlySpan<Rgb24> palettePixels = MemoryMarshal.Cast<byte, Rgb24>(palette);
-        ref Rgb24 palettePixelsRef = ref MemoryMarshal.GetReference(palettePixels);
+        ref Color paletteBase = ref MemoryMarshal.GetReference(palette.Value.Span);
 
-        if (paletteAlpha?.Length > 0)
+        for (nuint x = 0; x < (uint)header.Width; x++)
         {
-            // If the alpha palette is not null and has one or more entries, this means, that the image contains an alpha
-            // channel and we should try to read it.
-            Rgba32 rgba = default;
-            ref byte paletteAlphaRef = ref MemoryMarshal.GetArrayDataReference(paletteAlpha);
-
-            for (nuint x = 0; x < (uint)header.Width; x++)
-            {
-                uint index = Unsafe.Add(ref scanlineSpanRef, x);
-                rgba.Rgb = Unsafe.Add(ref palettePixelsRef, index);
-                rgba.A = paletteAlpha.Length > index ? Unsafe.Add(ref paletteAlphaRef, index) : byte.MaxValue;
-
-                pixel.FromRgba32(rgba);
-                Unsafe.Add(ref rowSpanRef, x) = pixel;
-            }
-        }
-        else
-        {
-            for (nuint x = 0; x < (uint)header.Width; x++)
-            {
-                int index = Unsafe.Add(ref scanlineSpanRef, x);
-                Rgb24 rgb = Unsafe.Add(ref palettePixelsRef, index);
-
-                pixel.FromRgb24(rgb);
-                Unsafe.Add(ref rowSpanRef, x) = pixel;
-            }
+            uint index = Unsafe.Add(ref scanlineSpanRef, x);
+            pixel.FromRgba32(Unsafe.Add(ref paletteBase, index).ToRgba32());
+            Unsafe.Add(ref rowSpanRef, x) = pixel;
         }
     }
 
@@ -292,42 +266,24 @@ internal static class PngScanlineProcessor
         Span<TPixel> rowSpan,
         uint pixelOffset,
         uint increment,
-        ReadOnlySpan<byte> palette,
-        byte[] paletteAlpha)
+        ReadOnlyMemory<Color>? palette)
         where TPixel : unmanaged, IPixel<TPixel>
     {
+        if (palette is null)
+        {
+            PngThrowHelper.ThrowMissingPalette();
+        }
+
         TPixel pixel = default;
         ref byte scanlineSpanRef = ref MemoryMarshal.GetReference(scanlineSpan);
         ref TPixel rowSpanRef = ref MemoryMarshal.GetReference(rowSpan);
-        ReadOnlySpan<Rgb24> palettePixels = MemoryMarshal.Cast<byte, Rgb24>(palette);
-        ref Rgb24 palettePixelsRef = ref MemoryMarshal.GetReference(palettePixels);
+        ref Color paletteBase = ref MemoryMarshal.GetReference(palette.Value.Span);
 
-        if (paletteAlpha?.Length > 0)
+        for (nuint x = pixelOffset, o = 0; x < (uint)header.Width; x += increment, o++)
         {
-            // If the alpha palette is not null and has one or more entries, this means, that the image contains an alpha
-            // channel and we should try to read it.
-            Rgba32 rgba = default;
-            ref byte paletteAlphaRef = ref MemoryMarshal.GetArrayDataReference(paletteAlpha);
-            for (nuint x = pixelOffset, o = 0; x < (uint)header.Width; x += increment, o++)
-            {
-                uint index = Unsafe.Add(ref scanlineSpanRef, o);
-                rgba.A = paletteAlpha.Length > index ? Unsafe.Add(ref paletteAlphaRef, index) : byte.MaxValue;
-                rgba.Rgb = Unsafe.Add(ref palettePixelsRef, index);
-
-                pixel.FromRgba32(rgba);
-                Unsafe.Add(ref rowSpanRef, x) = pixel;
-            }
-        }
-        else
-        {
-            for (nuint x = pixelOffset, o = 0; x < (uint)header.Width; x += increment, o++)
-            {
-                int index = Unsafe.Add(ref scanlineSpanRef, o);
-                Rgb24 rgb = Unsafe.Add(ref palettePixelsRef, index);
-
-                pixel.FromRgb24(rgb);
-                Unsafe.Add(ref rowSpanRef, x) = pixel;
-            }
+            uint index = Unsafe.Add(ref scanlineSpanRef, o);
+            pixel.FromRgba32(Unsafe.Add(ref paletteBase, index).ToRgba32());
+            Unsafe.Add(ref rowSpanRef, x) = pixel;
         }
     }
 
@@ -338,15 +294,13 @@ internal static class PngScanlineProcessor
         Span<TPixel> rowSpan,
         int bytesPerPixel,
         int bytesPerSample,
-        bool hasTrans,
-        Rgb48 rgb48Trans,
-        Rgb24 rgb24Trans)
+        Color? transparentColor)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         TPixel pixel = default;
         ref TPixel rowSpanRef = ref MemoryMarshal.GetReference(rowSpan);
 
-        if (!hasTrans)
+        if (transparentColor is null)
         {
             if (header.BitDepth == 16)
             {
@@ -372,6 +326,8 @@ internal static class PngScanlineProcessor
 
         if (header.BitDepth == 16)
         {
+            Rgb48 transparent = transparentColor.Value.ToPixel<Rgb48>();
+
             Rgb48 rgb48 = default;
             Rgba64 rgba64 = default;
             int o = 0;
@@ -382,7 +338,7 @@ internal static class PngScanlineProcessor
                 rgb48.B = BinaryPrimitives.ReadUInt16BigEndian(scanlineSpan.Slice(o + (2 * bytesPerSample), bytesPerSample));
 
                 rgba64.Rgb = rgb48;
-                rgba64.A = rgb48.Equals(rgb48Trans) ? ushort.MinValue : ushort.MaxValue;
+                rgba64.A = rgb48.Equals(transparent) ? ushort.MinValue : ushort.MaxValue;
 
                 pixel.FromRgba64(rgba64);
                 Unsafe.Add(ref rowSpanRef, x) = pixel;
@@ -390,6 +346,8 @@ internal static class PngScanlineProcessor
         }
         else
         {
+            Rgb24 transparent = transparentColor.Value.ToPixel<Rgb24>();
+
             Rgba32 rgba32 = default;
             ReadOnlySpan<Rgb24> rgb24Span = MemoryMarshal.Cast<byte, Rgb24>(scanlineSpan);
             ref Rgb24 rgb24SpanRef = ref MemoryMarshal.GetReference(rgb24Span);
@@ -397,7 +355,7 @@ internal static class PngScanlineProcessor
             {
                 ref readonly Rgb24 rgb24 = ref Unsafe.Add(ref rgb24SpanRef, x);
                 rgba32.Rgb = rgb24;
-                rgba32.A = rgb24.Equals(rgb24Trans) ? byte.MinValue : byte.MaxValue;
+                rgba32.A = rgb24.Equals(transparent) ? byte.MinValue : byte.MaxValue;
 
                 pixel.FromRgba32(rgba32);
                 Unsafe.Add(ref rowSpanRef, x) = pixel;
@@ -413,21 +371,19 @@ internal static class PngScanlineProcessor
         uint increment,
         int bytesPerPixel,
         int bytesPerSample,
-        bool hasTrans,
-        Rgb48 rgb48Trans,
-        Rgb24 rgb24Trans)
+        Color? transparentColor)
         where TPixel : unmanaged, IPixel<TPixel>
     {
         TPixel pixel = default;
         ref byte scanlineSpanRef = ref MemoryMarshal.GetReference(scanlineSpan);
         ref TPixel rowSpanRef = ref MemoryMarshal.GetReference(rowSpan);
+        bool hasTransparency = transparentColor is not null;
 
-        if (header.BitDepth == 16)
+        if (transparentColor is null)
         {
-            if (hasTrans)
+            if (header.BitDepth == 16)
             {
                 Rgb48 rgb48 = default;
-                Rgba64 rgba64 = default;
                 int o = 0;
                 for (nuint x = pixelOffset; x < (uint)header.Width; x += increment, o += bytesPerPixel)
                 {
@@ -435,24 +391,21 @@ internal static class PngScanlineProcessor
                     rgb48.G = BinaryPrimitives.ReadUInt16BigEndian(scanlineSpan.Slice(o + bytesPerSample, bytesPerSample));
                     rgb48.B = BinaryPrimitives.ReadUInt16BigEndian(scanlineSpan.Slice(o + (2 * bytesPerSample), bytesPerSample));
 
-                    rgba64.Rgb = rgb48;
-                    rgba64.A = rgb48.Equals(rgb48Trans) ? ushort.MinValue : ushort.MaxValue;
-
-                    pixel.FromRgba64(rgba64);
+                    pixel.FromRgb48(rgb48);
                     Unsafe.Add(ref rowSpanRef, x) = pixel;
                 }
             }
             else
             {
-                Rgb48 rgb48 = default;
+                Rgb24 rgb = default;
                 int o = 0;
                 for (nuint x = pixelOffset; x < (uint)header.Width; x += increment, o += bytesPerPixel)
                 {
-                    rgb48.R = BinaryPrimitives.ReadUInt16BigEndian(scanlineSpan.Slice(o, bytesPerSample));
-                    rgb48.G = BinaryPrimitives.ReadUInt16BigEndian(scanlineSpan.Slice(o + bytesPerSample, bytesPerSample));
-                    rgb48.B = BinaryPrimitives.ReadUInt16BigEndian(scanlineSpan.Slice(o + (2 * bytesPerSample), bytesPerSample));
+                    rgb.R = Unsafe.Add(ref scanlineSpanRef, (uint)o);
+                    rgb.G = Unsafe.Add(ref scanlineSpanRef, (uint)(o + bytesPerSample));
+                    rgb.B = Unsafe.Add(ref scanlineSpanRef, (uint)(o + (2 * bytesPerSample)));
 
-                    pixel.FromRgb48(rgb48);
+                    pixel.FromRgb24(rgb);
                     Unsafe.Add(ref rowSpanRef, x) = pixel;
                 }
             }
@@ -460,32 +413,40 @@ internal static class PngScanlineProcessor
             return;
         }
 
-        if (hasTrans)
+        if (header.BitDepth == 16)
         {
-            Rgba32 rgba = default;
+            Rgb48 transparent = transparentColor.Value.ToPixel<Rgb48>();
+
+            Rgb48 rgb48 = default;
+            Rgba64 rgba64 = default;
             int o = 0;
             for (nuint x = pixelOffset; x < (uint)header.Width; x += increment, o += bytesPerPixel)
             {
-                rgba.R = Unsafe.Add(ref scanlineSpanRef, (uint)o);
-                rgba.G = Unsafe.Add(ref scanlineSpanRef, (uint)(o + bytesPerSample));
-                rgba.B = Unsafe.Add(ref scanlineSpanRef, (uint)(o + (2 * bytesPerSample)));
-                rgba.A = rgb24Trans.Equals(rgba.Rgb) ? byte.MinValue : byte.MaxValue;
+                rgb48.R = BinaryPrimitives.ReadUInt16BigEndian(scanlineSpan.Slice(o, bytesPerSample));
+                rgb48.G = BinaryPrimitives.ReadUInt16BigEndian(scanlineSpan.Slice(o + bytesPerSample, bytesPerSample));
+                rgb48.B = BinaryPrimitives.ReadUInt16BigEndian(scanlineSpan.Slice(o + (2 * bytesPerSample), bytesPerSample));
 
-                pixel.FromRgba32(rgba);
+                rgba64.Rgb = rgb48;
+                rgba64.A = rgb48.Equals(transparent) ? ushort.MinValue : ushort.MaxValue;
+
+                pixel.FromRgba64(rgba64);
                 Unsafe.Add(ref rowSpanRef, x) = pixel;
             }
         }
         else
         {
-            Rgb24 rgb = default;
+            Rgb24 transparent = transparentColor.Value.ToPixel<Rgb24>();
+
+            Rgba32 rgba = default;
             int o = 0;
             for (nuint x = pixelOffset; x < (uint)header.Width; x += increment, o += bytesPerPixel)
             {
-                rgb.R = Unsafe.Add(ref scanlineSpanRef, (uint)o);
-                rgb.G = Unsafe.Add(ref scanlineSpanRef, (uint)(o + bytesPerSample));
-                rgb.B = Unsafe.Add(ref scanlineSpanRef, (uint)(o + (2 * bytesPerSample)));
+                rgba.R = Unsafe.Add(ref scanlineSpanRef, (uint)o);
+                rgba.G = Unsafe.Add(ref scanlineSpanRef, (uint)(o + bytesPerSample));
+                rgba.B = Unsafe.Add(ref scanlineSpanRef, (uint)(o + (2 * bytesPerSample)));
+                rgba.A = transparent.Equals(rgba.Rgb) ? byte.MinValue : byte.MaxValue;
 
-                pixel.FromRgb24(rgb);
+                pixel.FromRgba32(rgba);
                 Unsafe.Add(ref rowSpanRef, x) = pixel;
             }
         }

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs

@@ -539,7 +539,8 @@ public partial class PngDecoderTests
     {
         using Image<TPixel> image = provider.GetImage(PngDecoder.Instance);
         PngMetadata metadata = image.Metadata.GetPngMetadata();
-        Assert.True(metadata.HasTransparency);
+        Assert.NotNull(metadata.ColorTable);
+        Assert.Contains(metadata.ColorTable.Value.ToArray(), x => x.ToRgba32().A < 255);
     }
 
     // https://github.com/SixLabors/ImageSharp/issues/2209
@@ -551,7 +552,8 @@ public partial class PngDecoderTests
         using MemoryStream stream = new(testFile.Bytes, false);
         ImageInfo imageInfo = Image.Identify(stream);
         PngMetadata metadata = imageInfo.Metadata.GetPngMetadata();
-        Assert.True(metadata.HasTransparency);
+        Assert.NotNull(metadata.ColorTable);
+        Assert.Contains(metadata.ColorTable.Value.ToArray(), x => x.ToRgba32().A < 255);
     }
 
     // https://github.com/SixLabors/ImageSharp/issues/410

tests/ImageSharp.Tests/Formats/Png/PngEncoderTests.cs

@@ -449,44 +449,15 @@ public partial class PngEncoderTests
         TestFile testFile = TestFile.Create(imagePath);
         using Image<Rgba32> input = testFile.CreateRgba32Image();
         PngMetadata inMeta = input.Metadata.GetPngMetadata();
-        Assert.True(inMeta.HasTransparency);
+        Assert.True(inMeta.TransparentColor.HasValue);
 
         using MemoryStream memStream = new();
         input.Save(memStream, PngEncoder);
         memStream.Position = 0;
         using Image<Rgba32> output = Image.Load<Rgba32>(memStream);
         PngMetadata outMeta = output.Metadata.GetPngMetadata();
-        Assert.True(outMeta.HasTransparency);
-
-        switch (pngColorType)
-        {
-            case PngColorType.Grayscale:
-                if (pngBitDepth.Equals(PngBitDepth.Bit16))
-                {
-                    Assert.True(outMeta.TransparentL16.HasValue);
-                    Assert.Equal(inMeta.TransparentL16, outMeta.TransparentL16);
-                }
-                else
-                {
-                    Assert.True(outMeta.TransparentL8.HasValue);
-                    Assert.Equal(inMeta.TransparentL8, outMeta.TransparentL8);
-                }
-
-                break;
-            case PngColorType.Rgb:
-                if (pngBitDepth.Equals(PngBitDepth.Bit16))
-                {
-                    Assert.True(outMeta.TransparentRgb48.HasValue);
-                    Assert.Equal(inMeta.TransparentRgb48, outMeta.TransparentRgb48);
-                }
-                else
-                {
-                    Assert.True(outMeta.TransparentRgb24.HasValue);
-                    Assert.Equal(inMeta.TransparentRgb24, outMeta.TransparentRgb24);
-                }
-
-                break;
-        }
+        Assert.True(outMeta.TransparentColor.HasValue);
+        Assert.Equal(inMeta.TransparentColor, outMeta.TransparentColor);
     }
 
     [Theory]