This should write transparency

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -656,8 +656,8 @@ namespace SixLabors.ImageSharp.Formats.Png
                         scanlineSpan,
                         rowSpan,
                         pngMetaData.HasTrans,
-                        pngMetaData.Luminance16Trans,
+                        pngMetaData.Luminance16Trans.GetValueOrDefault(),
-                        pngMetaData.LuminanceTrans);
+                        pngMetaData.LuminanceTrans.GetValueOrDefault());
 
                     break;
 
@@ -690,8 +690,8 @@ namespace SixLabors.ImageSharp.Formats.Png
                         this.bytesPerPixel,
                         this.bytesPerSample,
                         pngMetaData.HasTrans,
-                        pngMetaData.Rgb48Trans,
+                        pngMetaData.Rgb48Trans.GetValueOrDefault(),
-                        pngMetaData.Rgb24Trans);
+                        pngMetaData.Rgb24Trans.GetValueOrDefault());
 
                     break;
 
@@ -740,8 +740,8 @@ namespace SixLabors.ImageSharp.Formats.Png
                         pixelOffset,
                         increment,
                         pngMetaData.HasTrans,
-                        pngMetaData.Luminance16Trans,
+                        pngMetaData.Luminance16Trans.GetValueOrDefault(),
-                        pngMetaData.LuminanceTrans);
+                        pngMetaData.LuminanceTrans.GetValueOrDefault());
 
                     break;
 
@@ -779,8 +779,8 @@ namespace SixLabors.ImageSharp.Formats.Png
                         this.bytesPerPixel,
                         this.bytesPerSample,
                         pngMetaData.HasTrans,
-                        pngMetaData.Rgb48Trans,
+                        pngMetaData.Rgb48Trans.GetValueOrDefault(),
-                        pngMetaData.Rgb24Trans);
+                        pngMetaData.Rgb24Trans.GetValueOrDefault());
 
                     break;
 

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -292,7 +292,7 @@ namespace SixLabors.ImageSharp.Formats.Png
 
             if (pngMetaData.HasTrans)
             {
-                //Write transparency header
+                this.WriteTransparencyMarkers(stream, pngMetaData);
             }
 
             this.WritePhysicalChunk(stream, metaData);
@@ -305,6 +305,50 @@ namespace SixLabors.ImageSharp.Formats.Png
             quantized?.Dispose();
         }
 
+        /// <summary>
+        /// Writes the transparency markers to the stream
+        /// </summary>
+        /// <param name="stream">The <see cref="Stream"/> containing image data.</param>
+        /// <param name="pngMetaData">The image meta data.</param>
+        private void WriteTransparencyMarkers(Stream stream, PngMetaData pngMetaData)
+        {
+            if (pngMetaData.ColorType == PngColorType.Rgb)
+            {
+                if (pngMetaData.Rgb48Trans != null)
+                {
+                    var r = BitConverter.GetBytes(pngMetaData.Rgb48Trans.Value.R);
+                    var g = BitConverter.GetBytes(pngMetaData.Rgb48Trans.Value.R);
+                    var B = BitConverter.GetBytes(pngMetaData.Rgb48Trans.Value.B);
+
+                    var alphaArray = r.Concat(g).Concat(B).ToArray();
+
+                    this.WriteChunk(stream, PngChunkType.PaletteAlpha, alphaArray, 0, alphaArray.Length);
+                }
+                else if (pngMetaData.Rgb24Trans != null)
+                {
+                    var alphaArray = new byte[6];
+                    alphaArray[0] = pngMetaData.Rgb24Trans.Value.R;
+                    alphaArray[2] = pngMetaData.Rgb24Trans.Value.G;
+                    alphaArray[4] = pngMetaData.Rgb24Trans.Value.B;
+                    this.WriteChunk(stream, PngChunkType.PaletteAlpha, alphaArray, 0, alphaArray.Length);
+                }
+            }
+            else if (pngMetaData.ColorType == PngColorType.Grayscale)
+            {
+                if (pngMetaData.Luminance16Trans != null)
+                {
+                    var alphaArray = BitConverter.GetBytes(pngMetaData.Luminance16Trans.Value);
+
+                    this.WriteChunk(stream, PngChunkType.PaletteAlpha, alphaArray, 0, alphaArray.Length);
+                }
+                else if (pngMetaData.LuminanceTrans != null)
+                {
+                    var alphaArray = new byte[2];
+                    alphaArray[0] = pngMetaData.LuminanceTrans.Value;
+                }
+            }
+        }
+
         /// <inheritdoc />
         public void Dispose()
         {
@@ -848,7 +892,10 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// Writes the chunk end to the stream.
         /// </summary>
         /// <param name="stream">The <see cref="Stream"/> containing image data.</param>
-        private void WriteEndChunk(Stream stream) => this.WriteChunk(stream, PngChunkType.End, null);
+        private void WriteEndChunk(Stream stream)
+        {
+            this.WriteChunk(stream, PngChunkType.End, null);
+        }
 
         /// <summary>
         /// Writes a chunk to the stream.
@@ -856,7 +903,10 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <param name="stream">The <see cref="Stream"/> to write to.</param>
         /// <param name="type">The type of chunk to write.</param>
         /// <param name="data">The <see cref="T:byte[]"/> containing data.</param>
-        private void WriteChunk(Stream stream, PngChunkType type, byte[] data) => this.WriteChunk(stream, type, data, 0, data?.Length ?? 0);
+        private void WriteChunk(Stream stream, PngChunkType type, byte[] data)
+        {
+            this.WriteChunk(stream, type, data, 0, data?.Length ?? 0);
+        }
 
         /// <summary>
         /// Writes a chunk of a specified length to the stream at the given offset.

src/ImageSharp/Formats/Png/PngMetaData.cs

@@ -47,22 +47,22 @@ namespace SixLabors.ImageSharp.Formats.Png
         /// <summary>
         /// Gets or sets the Rgb 24 transparent color. This represents any color in an 8 bit Rgb24 encoded png that should be transparent
         /// </summary>
-        public Rgb24 Rgb24Trans { get; set; }
+        public Rgb24? Rgb24Trans { get; set; }
 
         /// <summary>
         /// Gets or sets the Rgb 48 transparent color. This represents any color in a 16 bit Rgb24 encoded png that should be transparent
         /// </summary>
-        public Rgb48 Rgb48Trans { get; set; }
+        public Rgb48? Rgb48Trans { 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 byte LuminanceTrans { get; set; }
+        public byte? LuminanceTrans { 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
         /// </summary>
-        public ushort Luminance16Trans { get; set; }
+        public ushort? Luminance16Trans { get; set; }
 
         /// <summary>
         /// Gets or sets a value indicating whether the image has transparency chunk and markers were decoded