Reduce memory used during LZW decoding

src/ImageSharp/Formats/Gif/GifDecoderCore.cs

@@ -252,7 +252,7 @@ namespace ImageSharp.Formats
             GifImageDescriptor imageDescriptor = this.ReadImageDescriptor();
 
             byte[] localColorTable = null;
-
+            byte[] indices = null;
             try
             {
                 // Determine the color table for this frame. If there is a local one, use it otherwise use the global color table.
@@ -264,7 +264,9 @@ namespace ImageSharp.Formats
                     this.currentStream.Read(localColorTable, 0, length);
                 }
 
-                byte[] indices = this.ReadFrameIndices(imageDescriptor);
+                indices = ArrayPool<byte>.Shared.Rent(imageDescriptor.Width * imageDescriptor.Height);
+
+                this.ReadFrameIndices(imageDescriptor, indices);
                 this.ReadFrameColors(indices, localColorTable ?? this.globalColorTable, length, imageDescriptor);
 
                 // Skip any remaining blocks
@@ -276,6 +278,8 @@ namespace ImageSharp.Formats
                 {
                     ArrayPool<byte>.Shared.Return(localColorTable);
                 }
+
+                ArrayPool<byte>.Shared.Return(indices);
             }
         }
 
@@ -283,13 +287,13 @@ namespace ImageSharp.Formats
         /// Reads the frame indices marking the color to use for each pixel.
         /// </summary>
         /// <param name="imageDescriptor">The <see cref="GifImageDescriptor"/>.</param>
-        /// <returns>The <see cref="T:byte[]"/></returns>
+        /// <param name="indices">The pixel array to write to.</param>
-        private byte[] ReadFrameIndices(GifImageDescriptor imageDescriptor)
+        private void ReadFrameIndices(GifImageDescriptor imageDescriptor, byte[] indices)
         {
             int dataSize = this.currentStream.ReadByte();
             using (LzwDecoder lzwDecoder = new LzwDecoder(this.currentStream))
             {
-                return lzwDecoder.DecodePixels(imageDescriptor.Width, imageDescriptor.Height, dataSize);
+                lzwDecoder.DecodePixels(imageDescriptor.Width, imageDescriptor.Height, dataSize, indices);
             }
         }
 

src/ImageSharp/Formats/Gif/LzwDecoder.cs

@@ -2,6 +2,7 @@
 // Copyright (c) James Jackson-South and contributors.
 // Licensed under the Apache License, Version 2.0.
 // </copyright>
+
 namespace ImageSharp.Formats
 {
     using System;
@@ -83,13 +84,13 @@ namespace ImageSharp.Formats
         /// <param name="width">The width of the pixel index array.</param>
         /// <param name="height">The height of the pixel index array.</param>
         /// <param name="dataSize">Size of the data.</param>
-        /// <returns>The decoded and uncompressed array.</returns>
+        /// <param name="pixels">The pixel array to decode to.</param>
-        public byte[] DecodePixels(int width, int height, int dataSize)
+        public void DecodePixels(int width, int height, int dataSize, byte[] pixels)
         {
             Guard.MustBeLessThan(dataSize, int.MaxValue, nameof(dataSize));
 
-            // The resulting index table.
+            // The resulting index table length.
-            byte[] pixels = new byte[width * height];
+            int length = width * height;
 
             // Calculate the clear code. The value of the clear code is 2 ^ dataSize
             int clearCode = 1 << dataSize;
@@ -124,7 +125,7 @@ namespace ImageSharp.Formats
             }
 
             byte[] buffer = new byte[255];
-            while (xyz < pixels.Length)
+            while (xyz < length)
             {
                 if (top == 0)
                 {
@@ -221,8 +222,6 @@ namespace ImageSharp.Formats
                 // Clear missing pixels
                 pixels[xyz++] = (byte)this.pixelStack[top];
             }
-
-            return pixels;
         }
 
         /// <inheritdoc />