Ensure correct disposal method is carried across

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -8,6 +8,7 @@ namespace ImageSharp.Formats
     using System;
     using System.Buffers;
     using System.IO;
+    using System.Runtime.CompilerServices;
     using System.Text;
 
     using ImageSharp.PixelFormats;
@@ -332,6 +333,7 @@ namespace ImageSharp.Formats
         /// </summary>
         /// <param name="imageDescriptor">The <see cref="GifImageDescriptor"/>.</param>
         /// <param name="indices">The pixel array to write to.</param>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private void ReadFrameIndices(GifImageDescriptor imageDescriptor, byte[] indices)
         {
             int dataSize = this.currentStream.ReadByte();
@@ -366,7 +368,7 @@ namespace ImageSharp.Formats
                 // This initializes the image to become fully transparent because the alpha channel is zero.
                 this.image = Image.Create<TPixel>(imageWidth, imageHeight, this.metaData, this.configuration);
 
-                this.SetFrameDelay(this.metaData);
+                this.SetFrameMetaData(this.metaData);
 
                 image = this.image;
             }
@@ -380,7 +382,7 @@ namespace ImageSharp.Formats
 
                 currentFrame = this.previousFrame.Clone();
 
-                this.SetFrameDelay(currentFrame.MetaData);
+                this.SetFrameMetaData(currentFrame.MetaData);
 
                 image = currentFrame;
 
@@ -506,14 +508,20 @@ namespace ImageSharp.Formats
         }
 
         /// <summary>
-        /// Sets the frame delay in the metadata.
+        /// Sets the frames metadata.
         /// </summary>
         /// <param name="metaData">The meta data.</param>
-        private void SetFrameDelay(IMetaData metaData)
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private void SetFrameMetaData(IMetaData metaData)
         {
             if (this.graphicsControlExtension != null && this.graphicsControlExtension.DelayTime > 0)
             {
-                metaData.FrameDelay = this.graphicsControlExtension.DelayTime;
+                if (this.graphicsControlExtension.DelayTime > 0)
+                {
+                    metaData.FrameDelay = this.graphicsControlExtension.DelayTime;
+                }
+
+                metaData.DisposalMethod = this.graphicsControlExtension.DisposalMethod;
             }
         }
     }

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -191,7 +191,7 @@ namespace ImageSharp.Formats
             {
                 Width = (short)image.Width,
                 Height = (short)image.Height,
-                GlobalColorTableFlag = false, // Always false for now.
+                GlobalColorTableFlag = false, // TODO: Always false for now.
                 GlobalColorTableSize = this.bitDepth - 1,
                 BackgroundColorIndex = (byte)tranparencyIndex
             };
@@ -312,16 +312,10 @@ namespace ImageSharp.Formats
         private void WriteGraphicalControlExtension<TPixel>(ImageBase<TPixel> image, IMetaData metaData, EndianBinaryWriter writer, int transparencyIndex)
             where TPixel : struct, IPixel<TPixel>
         {
-            // TODO: Check transparency logic.
-            bool hasTransparent = transparencyIndex < 255;
-            DisposalMethod disposalMethod = hasTransparent
-                ? DisposalMethod.RestoreToBackground
-                : DisposalMethod.Unspecified;
-
             GifGraphicsControlExtension extension = new GifGraphicsControlExtension()
             {
-                DisposalMethod = disposalMethod,
-                TransparencyFlag = hasTransparent,
+                DisposalMethod = metaData.DisposalMethod,
+                TransparencyFlag = transparencyIndex < 255,
                 TransparencyIndex = transparencyIndex,
                 DelayTime = metaData.FrameDelay
             };

src/ImageSharp/MetaData/IMetaData.cs

@@ -5,6 +5,8 @@
 
 namespace ImageSharp
 {
+    using ImageSharp.Formats;
+
     /// <summary>
     /// Encapsulates the metadata of an image frame.
     /// </summary>
@@ -12,10 +14,17 @@ namespace ImageSharp
     {
         /// <summary>
         /// Gets or sets the frame delay for animated images.
-        /// If not 0, this field specifies the number of hundredths (1/100) of a second to
+        /// If not 0, when utilized in Gif animation, this field specifies the number of hundredths (1/100) of a second to
         /// wait before continuing with the processing of the Data Stream.
         /// The clock starts ticking immediately after the graphic is rendered.
         /// </summary>
         int FrameDelay { get; set; }
+
+        /// <summary>
+        /// Gets or sets the disposal method for animated images.
+        /// Primarily used in Gif animation, this field indicates the way in which the graphic is to
+        /// be treated after being displayed.
+        /// </summary>
+        DisposalMethod DisposalMethod { get; set; }
     }
 }

src/ImageSharp/MetaData/ImageFrameMetaData.cs

@@ -5,6 +5,8 @@
 
 namespace ImageSharp
 {
+    using ImageSharp.Formats;
+
     /// <summary>
     /// Encapsulates the metadata of an image frame.
     /// </summary>
@@ -29,14 +31,13 @@ namespace ImageSharp
             DebugGuard.NotNull(other, nameof(other));
 
             this.FrameDelay = other.FrameDelay;
+            this.DisposalMethod = other.DisposalMethod;
         }
 
-        /// <summary>
-        /// Gets or sets the frame delay for animated images.
-        /// If not 0, this field specifies the number of hundredths (1/100) of a second to
-        /// wait before continuing with the processing of the Data Stream.
-        /// The clock starts ticking immediately after the graphic is rendered.
-        /// </summary>
+        /// <inheritdoc/>
         public int FrameDelay { get; set; }
+
+        /// <inheritdoc/>
+        public DisposalMethod DisposalMethod { get; set; }
     }
 }

src/ImageSharp/MetaData/ImageMetaData.cs

@@ -7,6 +7,7 @@ namespace ImageSharp
 {
     using System;
     using System.Collections.Generic;
+    using ImageSharp.Formats;
 
     /// <summary>
     /// Encapsulates the metadata of an image.
@@ -52,6 +53,7 @@ namespace ImageSharp
             this.VerticalResolution = other.VerticalResolution;
             this.Quality = other.Quality;
             this.FrameDelay = other.FrameDelay;
+            this.DisposalMethod = other.DisposalMethod;
             this.RepeatCount = other.RepeatCount;
 
             foreach (ImageProperty property in other.Properties)
@@ -83,10 +85,10 @@ namespace ImageSharp
 
             set
             {
-              if (value > 0)
-              {
-                  this.horizontalResolution = value;
-              }
+                if (value > 0)
+                {
+                    this.horizontalResolution = value;
+                }
             }
         }
 
@@ -116,14 +118,12 @@ namespace ImageSharp
         /// </summary>
         public ExifProfile ExifProfile { get; set; }
 
-        /// <summary>
-        /// Gets or sets the frame delay for animated images.
-        /// If not 0, this field specifies the number of hundredths (1/100) of a second to
-        /// wait before continuing with the processing of the Data Stream.
-        /// The clock starts ticking immediately after the graphic is rendered.
-        /// </summary>
+        /// <inheritdoc/>
         public int FrameDelay { get; set; }
 
+        /// <inheritdoc/>
+        public DisposalMethod DisposalMethod { get; set; }
+
         /// <summary>
         /// Gets the list of properties for storing meta information about this image.
         /// </summary>