Move disposing the alpha data to the AlphaEncoder

src/ImageSharp/Formats/Webp/AlphaEncoder.cs

@@ -13,8 +13,10 @@ namespace SixLabors.ImageSharp.Formats.Webp
     /// <summary>
     /// Methods for encoding the alpha data of a VP8 image.
     /// </summary>
-    internal static class AlphaEncoder
+    internal class AlphaEncoder : IDisposable
     {
+        private IMemoryOwner<byte> alphaData;
+
         /// <summary>
         /// Encodes the alpha channel data.
         /// Data is either compressed as lossless webp image or uncompressed.
@@ -26,12 +28,12 @@ namespace SixLabors.ImageSharp.Formats.Webp
         /// <param name="compress">Indicates, if the data should be compressed with the lossless webp compression.</param>
         /// <param name="size">The size in bytes of the alpha data.</param>
         /// <returns>The encoded alpha data.</returns>
-        public static IMemoryOwner<byte> EncodeAlpha<TPixel>(Image<TPixel> image, Configuration configuration, MemoryAllocator memoryAllocator, bool compress, out int size)
+        public IMemoryOwner<byte> EncodeAlpha<TPixel>(Image<TPixel> image, Configuration configuration, MemoryAllocator memoryAllocator, bool compress, out int size)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             int width = image.Width;
             int height = image.Height;
-            IMemoryOwner<byte> alphaData = ExtractAlphaChannel(image, configuration, memoryAllocator);
+            this.alphaData = ExtractAlphaChannel(image, configuration, memoryAllocator);
 
             if (compress)
             {
@@ -51,15 +53,15 @@ namespace SixLabors.ImageSharp.Formats.Webp
                 // The transparency information will be stored in the green channel of the ARGB quadruplet.
                 // The green channel is allowed extra transformation steps in the specification -- unlike the other channels,
                 // that can improve compression.
-                using Image<Rgba32> alphaAsImage = DispatchAlphaToGreen(image, alphaData.GetSpan());
+                using Image<Rgba32> alphaAsImage = DispatchAlphaToGreen(image, this.alphaData.GetSpan());
 
-                size = lossLessEncoder.EncodeAlphaImageData(alphaAsImage, alphaData);
+                size = lossLessEncoder.EncodeAlphaImageData(alphaAsImage, this.alphaData);
 
-                return alphaData;
+                return this.alphaData;
             }
 
             size = width * height;
-            return alphaData;
+            return this.alphaData;
         }
 
         /// <summary>
@@ -124,5 +126,8 @@ namespace SixLabors.ImageSharp.Formats.Webp
 
             return alphaDataBuffer;
         }
+
+        /// <inheritdoc/>
+        public void Dispose() => this.alphaData?.Dispose();
     }
 }

src/ImageSharp/Formats/Webp/Lossy/Vp8Encoder.cs

@@ -241,11 +241,6 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
 
         public int DqUvDc { get; private set; }
 
-        /// <summary>
-        /// Gets or sets the alpha data.
-        /// </summary>
-        private IMemoryOwner<byte> AlphaData { get; set; }
-
         /// <summary>
         /// Gets the luma component.
         /// </summary>
@@ -331,10 +326,13 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             // Extract and encode alpha channel data, if present.
             int alphaDataSize = 0;
             bool alphaCompressionSucceeded = false;
+            using var alphaEncoder = new AlphaEncoder();
+            Span<byte> alphaData = Span<byte>.Empty;
             if (hasAlpha)
             {
                 // TODO: This can potentially run in an separate task.
-                this.AlphaData = AlphaEncoder.EncodeAlpha(image, this.configuration, this.memoryAllocator, this.alphaCompression, out alphaDataSize);
+                IMemoryOwner<byte> encodedAlphaData = alphaEncoder.EncodeAlpha(image, this.configuration, this.memoryAllocator, this.alphaCompression, out alphaDataSize);
+                alphaData = encodedAlphaData.GetSpan();
                 if (alphaDataSize < pixelCount)
                 {
                     // Only use compressed data, if the compressed data is actually smaller then the uncompressed data.
@@ -382,7 +380,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 (uint)width,
                 (uint)height,
                 hasAlpha,
-                hasAlpha ? this.AlphaData.GetSpan().Slice(0, alphaDataSize) : Span<byte>.Empty,
+                alphaData,
                 this.alphaCompression && alphaCompressionSucceeded);
         }
 
@@ -392,7 +390,6 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             this.Y.Dispose();
             this.U.Dispose();
             this.V.Dispose();
-            this.AlphaData?.Dispose();
         }
 
         /// <summary>