Fix trns preservation and add tests

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -760,9 +760,9 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <param name="pngMetaData">The image meta data.</param>
         private void WriteTransparencyChunk(Stream stream, PngMetaData pngMetaData)
         {
+            Span<byte> alpha = this.chunkDataBuffer.AsSpan();
             if (pngMetaData.ColorType.Equals(PngColorType.Rgb))
             {
-                Span<byte> alpha = this.buffer.AsSpan();
                 if (pngMetaData.TransparentRgb48.HasValue && this.use16Bit)
                 {
                     Rgb48 rgb = pngMetaData.TransparentRgb48.Value;
@@ -770,7 +770,7 @@ namespace SixLabors.ImageSharp.Formats.Png
                     BinaryPrimitives.WriteUInt16LittleEndian(alpha.Slice(2, 2), rgb.G);
                     BinaryPrimitives.WriteUInt16LittleEndian(alpha.Slice(4, 2), rgb.B);
 
-                    this.WriteChunk(stream, PngChunkType.Transparency, this.buffer, 0, 6);
+                    this.WriteChunk(stream, PngChunkType.Transparency, this.chunkDataBuffer, 0, 6);
                 }
                 else if (pngMetaData.TransparentRgb24.HasValue)
                 {
@@ -779,22 +779,21 @@ namespace SixLabors.ImageSharp.Formats.Png
                     alpha[1] = rgb.R;
                     alpha[3] = rgb.G;
                     alpha[5] = rgb.B;
-                    this.WriteChunk(stream, PngChunkType.Transparency, this.buffer, 0, 6);
+                    this.WriteChunk(stream, PngChunkType.Transparency, this.chunkDataBuffer, 0, 6);
                 }
             }
             else if (pngMetaData.ColorType.Equals(PngColorType.Grayscale))
             {
-                Span<byte> alpha = this.buffer.AsSpan();
                 if (pngMetaData.TransparentGray16.HasValue && this.use16Bit)
                 {
                     BinaryPrimitives.WriteUInt16LittleEndian(alpha, pngMetaData.TransparentGray16.Value.PackedValue);
-                    this.WriteChunk(stream, PngChunkType.Transparency, this.buffer, 0, 2);
+                    this.WriteChunk(stream, PngChunkType.Transparency, this.chunkDataBuffer, 0, 2);
                 }
                 else if (pngMetaData.TransparentGray8.HasValue)
                 {
                     alpha.Clear();
                     alpha[1] = pngMetaData.TransparentGray8.Value.PackedValue;
-                    this.WriteChunk(stream, PngChunkType.Transparency, this.buffer, 0, 2);
+                    this.WriteChunk(stream, PngChunkType.Transparency, this.chunkDataBuffer, 0, 2);
                 }
             }
         }

src/ImageSharp/Formats/Png/PngMetaData.cs

@@ -26,6 +26,11 @@ namespace SixLabors.ImageSharp.Formats.Png
             this.BitDepth = other.BitDepth;
             this.ColorType = other.ColorType;
             this.Gamma = other.Gamma;
+            this.HasTrans = other.HasTrans;
+            this.TransparentGray8 = other.TransparentGray8;
+            this.TransparentGray16 = other.TransparentGray16;
+            this.TransparentRgb24 = other.TransparentRgb24;
+            this.TransparentRgb48 = other.TransparentRgb48;
         }
 
         /// <summary>

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

@@ -25,6 +25,18 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             { TestImages.Png.Bpp1, PngBitDepth.Bit1 }
         };
 
+        public static readonly TheoryData<string, PngBitDepth, PngColorType> PngTrnsFiles =
+        new TheoryData<string, PngBitDepth, PngColorType>
+        {
+            { TestImages.Png.Gray1BitTrans, PngBitDepth.Bit1, PngColorType.Grayscale },
+            { TestImages.Png.Gray2BitTrans, PngBitDepth.Bit2, PngColorType.Grayscale },
+            { TestImages.Png.Gray4BitTrans, PngBitDepth.Bit4, PngColorType.Grayscale },
+            { TestImages.Png.Gray8BitTrans, PngBitDepth.Bit8, PngColorType.Grayscale },
+            { TestImages.Png.GrayTrns16BitInterlaced, PngBitDepth.Bit16, PngColorType.Grayscale },
+            { TestImages.Png.Rgb24BppTrans, PngBitDepth.Bit8, PngColorType.Rgb },
+            { TestImages.Png.Rgb48BppTrans, PngBitDepth.Bit16, PngColorType.Rgb }
+        };
+
         /// <summary>
         /// All types except Palette
         /// </summary>
@@ -249,6 +261,61 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             }
         }
 
+        [Theory]
+        [MemberData(nameof(PngTrnsFiles))]
+        public void Encode_PreserveTrns(string imagePath, PngBitDepth pngBitDepth, PngColorType pngColorType)
+        {
+            var options = new PngEncoder();
+
+            var testFile = TestFile.Create(imagePath);
+            using (Image<Rgba32> input = testFile.CreateImage())
+            {
+                PngMetaData inMeta = input.MetaData.GetFormatMetaData(PngFormat.Instance);
+                Assert.True(inMeta.HasTrans);
+
+                using (var memStream = new MemoryStream())
+                {
+                    input.Save(memStream, options);
+                    memStream.Position = 0;
+                    using (var output = Image.Load<Rgba32>(memStream))
+                    {
+                        PngMetaData outMeta = output.MetaData.GetFormatMetaData(PngFormat.Instance);
+                        Assert.True(outMeta.HasTrans);
+
+                        switch (pngColorType)
+                        {
+                            case PngColorType.Grayscale:
+                                if (pngBitDepth.Equals(PngBitDepth.Bit16))
+                                {
+                                    Assert.True(outMeta.TransparentGray16.HasValue);
+                                    Assert.Equal(inMeta.TransparentGray16, outMeta.TransparentGray16);
+                                }
+                                else
+                                {
+                                    Assert.True(outMeta.TransparentGray8.HasValue);
+                                    Assert.Equal(inMeta.TransparentGray8, outMeta.TransparentGray8);
+                                }
+
+                                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;
+                        }
+                    }
+                }
+            }
+        }
+
         private static void TestPngEncoderCore<TPixel>(
             TestImageProvider<TPixel> provider,
             PngColorType pngColorType,

tests/ImageSharp.Tests/TestImages.cs

@@ -46,6 +46,9 @@ namespace SixLabors.ImageSharp.Tests
             public const string PDSrc = "Png/pd-source.png";
             public const string PDDest = "Png/pd-dest.png";
             public const string Gray1BitTrans = "Png/gray-1-trns.png";
+            public const string Gray2BitTrans = "Png/gray-2-tRNS.png";
+            public const string Gray4BitTrans = "Png/gray-4-tRNS.png";
+            public const string Gray8BitTrans = "Png/gray-8-tRNS.png";
 
             // Filtered test images from http://www.schaik.com/pngsuite/pngsuite_fil_png.html
             public const string Filter0 = "Png/filter0.png";

tests/Images/Input/Png/gray-2-tRNS.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:c36d10c8498bbe148bc44d41e0243137e47b07402dfb8e58978d1a9b5029cabc
+size 265

tests/Images/Input/Png/gray-4-tRNS.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:217248c079a0ed7896803a2bf5d9719b6060ef898314a5f6ab898c6c4349f341
+size 267

tests/Images/Input/Png/gray-8-tRNS.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:6e14dce9350fc40f7ee542d1dc788fbf6c1c0178515ea8b76ffa0b18bcf54e8a
+size 267