From 8fd9b7224cff177789f30ba14c2235f91ca64f8d Mon Sep 17 00:00:00 2001
From: James Jackson-South <james_south@hotmail.com>
Date: Thu, 2 Dec 2021 00:37:05 +1100
Subject: [PATCH] Fix transparency chunk identification

---
 src/ImageSharp/Formats/Png/PngDecoder.cs     | 120 +++++++++++++------
 src/ImageSharp/Formats/Png/PngDecoderCore.cs |   6 +
 2 files changed, 88 insertions(+), 38 deletions(-)

diff --git a/src/ImageSharp/Formats/Png/PngDecoder.cs b/src/ImageSharp/Formats/Png/PngDecoder.cs
index 5637d5c7bf..04e70c51d0 100644
--- a/src/ImageSharp/Formats/Png/PngDecoder.cs
+++ b/src/ImageSharp/Formats/Png/PngDecoder.cs
@@ -34,26 +34,48 @@ namespace SixLabors.ImageSharp.Formats.Png
             PngMetadata meta = info.Metadata.GetPngMetadata();
             PngColorType color = meta.ColorType.GetValueOrDefault();
             PngBitDepth bits = meta.BitDepth.GetValueOrDefault();
-            return color switch
+            switch (color)
             {
-                PngColorType.Grayscale => (bits == PngBitDepth.Bit16)
-                ? this.Decode<L16>(configuration, stream)
-                : this.Decode<L8>(configuration, stream),
-
-                PngColorType.Rgb => this.Decode<Rgb24>(configuration, stream),
-
-                PngColorType.Palette => this.Decode<Rgba32>(configuration, stream),
-
-                PngColorType.GrayscaleWithAlpha => (bits == PngBitDepth.Bit16)
-                ? this.Decode<La32>(configuration, stream)
-                : this.Decode<La16>(configuration, stream),
-
-                PngColorType.RgbWithAlpha => (bits == PngBitDepth.Bit16)
-                ? this.Decode<Rgba64>(configuration, stream)
-                : this.Decode<Rgba32>(configuration, stream),
-
-                _ => this.Decode<Rgba32>(configuration, stream),
-            };
+                case PngColorType.Grayscale:
+                    if (bits == PngBitDepth.Bit16)
+                    {
+                        return !meta.HasTransparency
+                            ? this.Decode<L16>(configuration, stream)
+                            : this.Decode<La32>(configuration, stream);
+                    }
+
+                    return !meta.HasTransparency
+                        ? this.Decode<L8>(configuration, stream)
+                        : this.Decode<La16>(configuration, stream);
+
+                case PngColorType.Rgb:
+                    if (bits == PngBitDepth.Bit16)
+                    {
+                        return !meta.HasTransparency
+                            ? this.Decode<Rgb48>(configuration, stream)
+                            : this.Decode<Rgba64>(configuration, stream);
+                    }
+
+                    return !meta.HasTransparency
+                        ? this.Decode<Rgb24>(configuration, stream)
+                        : this.Decode<Rgba32>(configuration, stream);
+
+                case PngColorType.Palette:
+                    return this.Decode<Rgba32>(configuration, stream);
+
+                case PngColorType.GrayscaleWithAlpha:
+                    return (bits == PngBitDepth.Bit16)
+                    ? this.Decode<La32>(configuration, stream)
+                    : this.Decode<La16>(configuration, stream);
+
+                case PngColorType.RgbWithAlpha:
+                    return (bits == PngBitDepth.Bit16)
+                    ? this.Decode<Rgba64>(configuration, stream)
+                    : this.Decode<Rgba32>(configuration, stream);
+
+                default:
+                    return this.Decode<Rgba32>(configuration, stream);
+            }
         }
 
         /// <inheritdoc/>
@@ -74,26 +96,48 @@ namespace SixLabors.ImageSharp.Formats.Png
             PngMetadata meta = info.Metadata.GetPngMetadata();
             PngColorType color = meta.ColorType.GetValueOrDefault();
             PngBitDepth bits = meta.BitDepth.GetValueOrDefault();
-            return color switch
+            switch (color)
             {
-                PngColorType.Grayscale => (bits == PngBitDepth.Bit16)
-                ? await this.DecodeAsync<L16>(configuration, stream, cancellationToken).ConfigureAwait(false)
-                : await this.DecodeAsync<L8>(configuration, stream, cancellationToken).ConfigureAwait(false),
-
-                PngColorType.Rgb => await this.DecodeAsync<Rgb24>(configuration, stream, cancellationToken).ConfigureAwait(false),
-
-                PngColorType.Palette => await this.DecodeAsync<Rgba32>(configuration, stream, cancellationToken).ConfigureAwait(false),
-
-                PngColorType.GrayscaleWithAlpha => (bits == PngBitDepth.Bit16)
-                ? await this.DecodeAsync<La32>(configuration, stream, cancellationToken).ConfigureAwait(false)
-                : await this.DecodeAsync<La16>(configuration, stream, cancellationToken).ConfigureAwait(false),
-
-                PngColorType.RgbWithAlpha => (bits == PngBitDepth.Bit16)
-                ? await this.DecodeAsync<Rgba64>(configuration, stream, cancellationToken).ConfigureAwait(false)
-                : await this.DecodeAsync<Rgba32>(configuration, stream, cancellationToken).ConfigureAwait(false),
-
-                _ => await this.DecodeAsync<Rgba32>(configuration, stream, cancellationToken).ConfigureAwait(false),
-            };
+                case PngColorType.Grayscale:
+                    if (bits == PngBitDepth.Bit16)
+                    {
+                        return !meta.HasTransparency
+                            ? await this.DecodeAsync<L16>(configuration, stream, cancellationToken).ConfigureAwait(false)
+                            : await this.DecodeAsync<La32>(configuration, stream, cancellationToken).ConfigureAwait(false);
+                    }
+
+                    return !meta.HasTransparency
+                        ? await this.DecodeAsync<L8>(configuration, stream, cancellationToken).ConfigureAwait(false)
+                        : await this.DecodeAsync<La16>(configuration, stream, cancellationToken).ConfigureAwait(false);
+
+                case PngColorType.Rgb:
+                    if (bits == PngBitDepth.Bit16)
+                    {
+                        return !meta.HasTransparency
+                            ? await this.DecodeAsync<Rgb48>(configuration, stream, cancellationToken).ConfigureAwait(false)
+                            : await this.DecodeAsync<Rgba64>(configuration, stream, cancellationToken).ConfigureAwait(false);
+                    }
+
+                    return !meta.HasTransparency
+                        ? await this.DecodeAsync<Rgb24>(configuration, stream, cancellationToken).ConfigureAwait(false)
+                        : await this.DecodeAsync<Rgba32>(configuration, stream, cancellationToken).ConfigureAwait(false);
+
+                case PngColorType.Palette:
+                    return await this.DecodeAsync<Rgba32>(configuration, stream, cancellationToken).ConfigureAwait(false);
+
+                case PngColorType.GrayscaleWithAlpha:
+                    return (bits == PngBitDepth.Bit16)
+                        ? await this.DecodeAsync<La32>(configuration, stream, cancellationToken).ConfigureAwait(false)
+                        : await this.DecodeAsync<La16>(configuration, stream, cancellationToken).ConfigureAwait(false);
+
+                case PngColorType.RgbWithAlpha:
+                    return (bits == PngBitDepth.Bit16)
+                        ? await this.DecodeAsync<Rgba64>(configuration, stream, cancellationToken).ConfigureAwait(false)
+                        : await this.DecodeAsync<Rgba32>(configuration, stream, cancellationToken).ConfigureAwait(false);
+
+                default:
+                    return await this.DecodeAsync<Rgba32>(configuration, stream, cancellationToken).ConfigureAwait(false);
+            }
         }
 
         /// <inheritdoc/>
diff --git a/src/ImageSharp/Formats/Png/PngDecoderCore.cs b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
index 333b00f23d..ba737cb42b 100644
--- a/src/ImageSharp/Formats/Png/PngDecoderCore.cs
+++ b/src/ImageSharp/Formats/Png/PngDecoderCore.cs
@@ -258,6 +258,12 @@ namespace SixLabors.ImageSharp.Formats.Png
                             case PngChunkType.Data:
                                 this.SkipChunkDataAndCrc(chunk);
                                 break;
+                            case PngChunkType.Transparency:
+                                byte[] alpha = new byte[chunk.Length];
+                                chunk.Data.GetSpan().CopyTo(alpha);
+                                this.paletteAlpha = alpha;
+                                this.AssignTransparentMarkers(alpha, pngMetadata);
+                                break;
                             case PngChunkType.Text:
                                 this.ReadTextChunk(pngMetadata, chunk.Data.GetSpan());
                                 break;