Fix png to gif transparency

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -69,7 +69,7 @@ namespace ImageSharp.Formats
             this.Quantizer = this.options.Quantizer ?? new OctreeQuantizer<TPixel>();
 
             // Do not use IDisposable pattern here as we want to preserve the stream.
-            EndianBinaryWriter writer = new EndianBinaryWriter(Endianness.LittleEndian, stream);
+            var writer = new EndianBinaryWriter(Endianness.LittleEndian, stream);
 
             // Ensure that quality can be set but has a fallback.
             int quality = this.options.Quality > 0 ? this.options.Quality : image.MetaData.Quality;
@@ -82,7 +82,7 @@ namespace ImageSharp.Formats
             this.hasFrames = image.Frames.Any();
 
             // Dithering when animating gifs is a bad idea as we introduce pixel tearing across frames.
-            IQuantizer<TPixel> ditheredQuantizer = (IQuantizer<TPixel>)this.Quantizer;
+            var ditheredQuantizer = (IQuantizer<TPixel>)this.Quantizer;
             ditheredQuantizer.Dither = !this.hasFrames;
 
             QuantizedImage<TPixel> quantized = ditheredQuantizer.Quantize(image, quality);
@@ -96,7 +96,7 @@ namespace ImageSharp.Formats
             this.WriteLogicalScreenDescriptor(image, writer, index);
 
             // Write the first frame.
-            this.WriteGraphicalControlExtension(image, writer, index);
+            this.WriteGraphicalControlExtension(image.MetaData, writer, index);
             this.WriteComments(image, writer);
             this.WriteImageDescriptor(image, writer);
             this.WriteColorTable(quantized, writer);
@@ -113,7 +113,7 @@ namespace ImageSharp.Formats
                     ImageFrame<TPixel> frame = image.Frames[i];
                     QuantizedImage<TPixel> quantizedFrame = ditheredQuantizer.Quantize(frame, quality);
 
-                    this.WriteGraphicalControlExtension(frame, writer, this.GetTransparentIndex(quantizedFrame));
+                    this.WriteGraphicalControlExtension(frame.MetaData, writer, this.GetTransparentIndex(quantizedFrame));
                     this.WriteImageDescriptor(frame, writer);
                     this.WriteColorTable(quantizedFrame, writer);
                     this.WriteImageData(quantizedFrame, writer);
@@ -187,7 +187,7 @@ namespace ImageSharp.Formats
         private void WriteLogicalScreenDescriptor<TPixel>(Image<TPixel> image, EndianBinaryWriter writer, int tranparencyIndex)
             where TPixel : struct, IPixel<TPixel>
         {
-            GifLogicalScreenDescriptor descriptor = new GifLogicalScreenDescriptor
+            var descriptor = new GifLogicalScreenDescriptor
             {
                 Width = (short)image.Width,
                 Height = (short)image.Height,
@@ -199,7 +199,7 @@ namespace ImageSharp.Formats
             writer.Write((ushort)descriptor.Width);
             writer.Write((ushort)descriptor.Height);
 
-            PackedField field = default(PackedField);
+            var field = default(PackedField);
             field.SetBit(0, descriptor.GlobalColorTableFlag);  // 1   : Global color table flag = 1 || 0 (GCT used/ not used)
             field.SetBits(1, 3, descriptor.GlobalColorTableSize); // 2-4 : color resolution
             field.SetBit(4, false); // 5   : GCT sort flag = 0
@@ -251,7 +251,7 @@ namespace ImageSharp.Formats
         private void WriteComments<TPixel>(Image<TPixel> image, EndianBinaryWriter writer)
             where TPixel : struct, IPixel<TPixel>
         {
-            if (this.options.IgnoreMetadata == true)
+            if (this.options.IgnoreMetadata)
             {
                 return;
             }
@@ -278,44 +278,15 @@ namespace ImageSharp.Formats
         /// <summary>
         /// Writes the graphics control extension to the stream.
         /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
-        /// <param name="image">The <see cref="Image{TPixel}"/> to encode.</param>
-        /// <param name="writer">The stream to write to.</param>
-        /// <param name="transparencyIndex">The index of the color in the color palette to make transparent.</param>
-        private void WriteGraphicalControlExtension<TPixel>(Image<TPixel> image, EndianBinaryWriter writer, int transparencyIndex)
-            where TPixel : struct, IPixel<TPixel>
-        {
-            this.WriteGraphicalControlExtension(image, image.MetaData, writer, transparencyIndex);
-        }
-
-        /// <summary>
-        /// Writes the graphics control extension to the stream.
-        /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
-        /// <param name="imageFrame">The <see cref="ImageFrame{TPixel}"/> to encode.</param>
-        /// <param name="writer">The stream to write to.</param>
-        /// <param name="transparencyIndex">The index of the color in the color palette to make transparent.</param>
-        private void WriteGraphicalControlExtension<TPixel>(ImageFrame<TPixel> imageFrame, EndianBinaryWriter writer, int transparencyIndex)
-            where TPixel : struct, IPixel<TPixel>
-        {
-            this.WriteGraphicalControlExtension(imageFrame, imageFrame.MetaData, writer, transparencyIndex);
-        }
-
-        /// <summary>
-        /// Writes the graphics control extension to the stream.
-        /// </summary>
-        /// <typeparam name="TPixel">The pixel format.</typeparam>
-        /// <param name="image">The <see cref="ImageBase{TPixel}"/> to encode.</param>
         /// <param name="metaData">The metadata of the image or frame.</param>
         /// <param name="writer">The stream to write to.</param>
         /// <param name="transparencyIndex">The index of the color in the color palette to make transparent.</param>
-        private void WriteGraphicalControlExtension<TPixel>(ImageBase<TPixel> image, IMetaData metaData, EndianBinaryWriter writer, int transparencyIndex)
+        private void WriteGraphicalControlExtension(IMetaData metaData, EndianBinaryWriter writer, int transparencyIndex)
-            where TPixel : struct, IPixel<TPixel>
         {
-            GifGraphicsControlExtension extension = new GifGraphicsControlExtension()
+            var extension = new GifGraphicsControlExtension
             {
                 DisposalMethod = metaData.DisposalMethod,
-                TransparencyFlag = transparencyIndex < 255,
+                TransparencyFlag = true, // TODO: The spec here is unclear. Can we get away with this?
                 TransparencyIndex = transparencyIndex,
                 DelayTime = metaData.FrameDelay
             };
@@ -326,7 +297,7 @@ namespace ImageSharp.Formats
             this.buffer[2] = 4;
             writer.Write(this.buffer, 0, 3);
 
-            PackedField field = default(PackedField);
+            var field = default(PackedField);
             field.SetBits(3, 3, (int)extension.DisposalMethod); // 1-3 : Reserved, 4-6 : Disposal
 
             // TODO: Allow this as an option.
@@ -356,7 +327,7 @@ namespace ImageSharp.Formats
             writer.Write((ushort)image.Width);
             writer.Write((ushort)image.Height);
 
-            PackedField field = default(PackedField);
+            var field = default(PackedField);
             field.SetBit(0, true); // 1: Local color table flag = 1 (LCT used)
             field.SetBit(1, false); // 2: Interlace flag 0
             field.SetBit(2, false); // 3: Sort flag 0
@@ -409,7 +380,7 @@ namespace ImageSharp.Formats
         private void WriteImageData<TPixel>(QuantizedImage<TPixel> image, EndianBinaryWriter writer)
             where TPixel : struct, IPixel<TPixel>
         {
-            using (LzwEncoder encoder = new LzwEncoder(image.Pixels, (byte)this.bitDepth))
+            using (var encoder = new LzwEncoder(image.Pixels, (byte)this.bitDepth))
             {
                 encoder.Encode(writer.BaseStream);
             }