Add PngTransparentColorBehavior enum

src/ImageSharp/Formats/Png/IPngEncoderOptions.cs

@@ -70,8 +70,9 @@ namespace SixLabors.ImageSharp.Formats.Png
         PngChunkFilter? ChunkFilter { get; }
 
         /// <summary>
-        /// Gets a value indicating whether fully transparent pixels should be converted to black pixels.
+        /// Gets a value indicating whether fully transparent pixels that may contain R, G, B values which are not 0,
+        /// should be converted to transparent black, which can yield in better compression in some cases.
         /// </summary>
-        bool MakeTransparentBlack { get; }
+        PngTransparentColorBehavior TransparentColorBehavior { get; }
     }
 }

src/ImageSharp/Formats/Png/PngEncoder.cs

@@ -47,7 +47,7 @@ namespace SixLabors.ImageSharp.Formats.Png
         public bool IgnoreMetadata { get; set; }
 
         /// <inheritdoc/>
-        public bool MakeTransparentBlack { get; set; }
+        public PngTransparentColorBehavior TransparentColorBehavior { get; set; }
 
         /// <summary>
         /// Encodes the image to the specified stream from the <see cref="Image{TPixel}"/>.

src/ImageSharp/Formats/Png/PngEncoderCore.cs

@@ -145,10 +145,11 @@ namespace SixLabors.ImageSharp.Formats.Png
             PngMetadata pngMetadata = metadata.GetFormatMetadata(PngFormat.Instance);
             PngEncoderOptionsHelpers.AdjustOptions<TPixel>(this.options, pngMetadata, out this.use16Bit, out this.bytesPerPixel);
             Image<TPixel> clonedImage = null;
-            if (this.options.MakeTransparentBlack)
+            bool clearTransparency = this.options.TransparentColorBehavior == PngTransparentColorBehavior.Clear;
+            if (clearTransparency)
             {
                 clonedImage = image.Clone();
-                MakeTransparentPixelsBlack(clonedImage);
+                ClearTransparentPixels(clonedImage);
             }
 
             IndexedImageFrame<TPixel> quantized = this.CreateQuantizedImage(image, clonedImage);
@@ -162,7 +163,7 @@ namespace SixLabors.ImageSharp.Formats.Png
             this.WritePhysicalChunk(stream, metadata);
             this.WriteExifChunk(stream, metadata);
             this.WriteTextChunks(stream, pngMetadata);
-            this.WriteDataChunks(this.options.MakeTransparentBlack ? clonedImage : image, quantized, stream);
+            this.WriteDataChunks(clearTransparency ? clonedImage : image, quantized, stream);
             this.WriteEndChunk(stream);
 
             stream.Flush();
@@ -190,11 +191,11 @@ namespace SixLabors.ImageSharp.Formats.Png
         }
 
         /// <summary>
-        /// Makes transparent pixels black.
+        /// Convert transparent pixels, to transparent black pixels, which can yield to better compression in some cases.
         /// </summary>
         /// <typeparam name="TPixel">The type of the pixel.</typeparam>
         /// <param name="image">The cloned image where the transparent pixels will be changed.</param>
-        private static void MakeTransparentPixelsBlack<TPixel>(Image<TPixel> image)
+        private static void ClearTransparentPixels<TPixel>(Image<TPixel> image)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             Rgba32 rgba32 = default;
@@ -207,7 +208,7 @@ namespace SixLabors.ImageSharp.Formats.Png
 
                     if (rgba32.A == 0)
                     {
-                        span[x].FromRgba32(Color.Black);
+                        span[x].FromRgba32(Color.Transparent);
                     }
                 }
             }
@@ -224,7 +225,7 @@ namespace SixLabors.ImageSharp.Formats.Png
             where TPixel : unmanaged, IPixel<TPixel>
         {
             IndexedImageFrame<TPixel> quantized;
-            if (this.options.MakeTransparentBlack)
+            if (this.options.TransparentColorBehavior == PngTransparentColorBehavior.Clear)
             {
                 quantized = PngEncoderOptionsHelpers.CreateQuantizedFrame(this.options, clonedImage);
                 this.bitDepth = PngEncoderOptionsHelpers.CalculateBitDepth(this.options, quantized);

src/ImageSharp/Formats/Png/PngEncoderOptions.cs

@@ -31,7 +31,7 @@ namespace SixLabors.ImageSharp.Formats.Png
             this.InterlaceMethod = source.InterlaceMethod;
             this.ChunkFilter = source.ChunkFilter;
             this.IgnoreMetadata = source.IgnoreMetadata;
-            this.MakeTransparentBlack = source.MakeTransparentBlack;
+            this.TransparentColorBehavior = source.TransparentColorBehavior;
         }
 
         /// <inheritdoc/>
@@ -68,6 +68,6 @@ namespace SixLabors.ImageSharp.Formats.Png
         public bool IgnoreMetadata { get; set; }
 
         /// <inheritdoc/>
-        public bool MakeTransparentBlack { get; set; }
+        public PngTransparentColorBehavior TransparentColorBehavior { get; set; }
     }
 }

src/ImageSharp/Formats/Png/PngTransparentColorBehavior.cs

@@ -0,0 +1,22 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the GNU Affero General Public License, Version 3.
+
+namespace SixLabors.ImageSharp.Formats.Png
+{
+    /// <summary>
+    /// Enum indicating how the transparency should be handled on encoding.
+    /// </summary>
+    public enum PngTransparentColorBehavior
+    {
+        /// <summary>
+        /// Converts fully transparent pixels that may contain R, G, B values which are not 0,
+        /// to transparent black, which can yield in better compression in some cases.
+        /// </summary>
+        Clear,
+
+        /// <summary>
+        /// The transparency will be kept as is.
+        /// </summary>
+        Preserve
+    }
+}

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

@@ -392,17 +392,15 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
 
         [Theory]
         [InlineData(PngColorType.Palette)]
-        [InlineData(PngColorType.Rgb)]
         [InlineData(PngColorType.RgbWithAlpha)]
-        [InlineData(PngColorType.Grayscale)]
         [InlineData(PngColorType.GrayscaleWithAlpha)]
-        public void Encode_WithMakeTransparentBlackOption_Works(PngColorType colorType)
+        public void Encode_WithPngTransparentColorBehaviorClear_Works(PngColorType colorType)
         {
             // arrange
             var image = new Image<Rgba32>(50, 50);
             var encoder = new PngEncoder()
             {
-                MakeTransparentBlack = true,
+                TransparentColorBehavior = PngTransparentColorBehavior.Clear,
                 ColorType = colorType
             };
             Rgba32 rgba32 = Color.Blue;
@@ -442,7 +440,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
 
                 if (y > 25)
                 {
-                    expectedColor = Color.Black;
+                    expectedColor = Color.Transparent;
                 }
 
                 for (int x = 0; x < actual.Width; x++)