Merge pull request #2215 from SixLabors/bp/pngindexedtrans

Identify transparency for indexed PNG correctly

Author: JimBobSquarePants

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Buffers;
 using System.Buffers.Binary;
-using System.Collections.Generic;
 using System.IO;
 using System.IO.Compression;
 using System.Runtime.CompilerServices;
@@ -380,8 +379,8 @@ public IImageInfo Identify(BufferedReadStream stream, CancellationToken cancella
         /// <summary>
         /// Reads the least significant bits from the byte pair with the others set to 0.
         /// </summary>
-        /// <param name="buffer">The source buffer</param>
-        /// <param name="offset">THe offset</param>
+        /// <param name="buffer">The source buffer.</param>
+        /// <param name="offset">THe offset.</param>
         /// <returns>The <see cref="int"/></returns>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private static byte ReadByteLittleEndian(ReadOnlySpan<byte> buffer, int offset)
@@ -392,7 +391,7 @@ private static byte ReadByteLittleEndian(ReadOnlySpan<byte> buffer, int offset)
         /// specified number of bits.
         /// </summary>
         /// <param name="source">The bytes to convert from. Cannot be empty.</param>
-        /// <param name="bytesPerScanline">The number of bytes per scanline</param>
+        /// <param name="bytesPerScanline">The number of bytes per scanline.</param>
         /// <param name="bits">The number of bits per value.</param>
         /// <param name="buffer">The new array.</param>
         /// <returns>The resulting <see cref="ReadOnlySpan{Byte}"/> array.</returns>
@@ -973,6 +972,10 @@ private void AssignTransparentMarkers(ReadOnlySpan<byte> alpha, PngMetadata pngM
                     pngMetadata.HasTransparency = true;
                 }
             }
+            else if (this.pngColorType == PngColorType.Palette && alpha.Length > 0)
+            {
+                pngMetadata.HasTransparency = true;
+            }
         }
 
         /// <summary>

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

@@ -470,6 +470,30 @@ public void Issue1765<TPixel>(TestImageProvider<TPixel> provider)
             Assert.Null(ex);
         }
 
+        // https://github.com/SixLabors/ImageSharp/issues/2209
+        [Theory]
+        [WithFile(TestImages.Png.Issue2209IndexedWithTransparency, PixelTypes.Rgba32)]
+        public void Issue2209_Decode_HasTransparencyIsTrue<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using Image<TPixel> image = provider.GetImage(PngDecoder);
+            image.DebugSave(provider);
+            PngMetadata metadata = image.Metadata.GetPngMetadata();
+            Assert.True(metadata.HasTransparency);
+        }
+
+        // https://github.com/SixLabors/ImageSharp/issues/2209
+        [Theory]
+        [InlineData(TestImages.Png.Issue2209IndexedWithTransparency)]
+        public void Issue2209_Identify_HasTransparencyIsTrue(string imagePath)
+        {
+            var testFile = TestFile.Create(imagePath);
+            using var stream = new MemoryStream(testFile.Bytes, false);
+            IImageInfo imageInfo = Image.Identify(stream);
+            PngMetadata metadata = imageInfo.Metadata.GetPngMetadata();
+            Assert.True(metadata.HasTransparency);
+        }
+
         // https://github.com/SixLabors/ImageSharp/issues/410
         [Theory]
         [WithFile(TestImages.Png.Bad.Issue410_MalformedApplePng, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/TestImages.cs

@@ -125,6 +125,9 @@ public static class Png
             // Discussion 1875: https://github.com/SixLabors/ImageSharp/discussions/1875
             public const string Issue1875 = "Png/raw-profile-type-exif.png";
 
+            // Issue 2209: https://github.com/SixLabors/ImageSharp/issues/2209
+            public const string Issue2209IndexedWithTransparency = "Png/issues/Issue_2209.png";
+
             public static class Bad
             {
                 public const string MissingDataChunk = "Png/xdtn0g01.png";