Leave alpha data uncompressed, if compression does not yield in smaller data

Author: brianpopow

src/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs

@@ -264,12 +264,15 @@ public void Encode<TPixel>(Image<TPixel> image, Stream stream)
         /// <typeparam name="TPixel">The type of the pixel.</typeparam>
         /// <param name="image">The <see cref="Image{TPixel}"/> to encode from.</param>
         /// <param name="alphaData">The destination buffer to write the encoded alpha data to.</param>
-        /// <returns>The size of the data in bytes.</returns>
+        /// <returns>The size of the compressed data in bytes.
+        /// If the size of the data is the same as the pixel count, the compression would not yield in smaller data and is left uncompressed.
+        /// </returns>
         public int EncodeAlphaImageData<TPixel>(Image<TPixel> image, IMemoryOwner<byte> alphaData)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             int width = image.Width;
             int height = image.Height;
+            int pixelCount = width * height;
 
             // Convert image pixels to bgra array.
             this.ConvertPixelsToBgra(image, width, height);
@@ -278,6 +281,12 @@ public int EncodeAlphaImageData<TPixel>(Image<TPixel> image, IMemoryOwner<byte>
             this.EncodeStream(image);
             this.bitWriter.Finish();
             int size = this.bitWriter.NumBytes();
+            if (size >= pixelCount)
+            {
+                // Compressing would not yield in smaller data -> leave the data uncompressed.
+                return pixelCount;
+            }
+
             this.bitWriter.WriteToBuffer(alphaData.GetSpan());
             return size;
         }

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

@@ -302,6 +302,7 @@ public void Encode<TPixel>(Image<TPixel> image, Stream stream)
         {
             int width = image.Width;
             int height = image.Height;
+            int pixelCount = width * height;
             Span<byte> y = this.Y.GetSpan();
             Span<byte> u = this.U.GetSpan();
             Span<byte> v = this.V.GetSpan();
@@ -329,10 +330,16 @@ public void Encode<TPixel>(Image<TPixel> image, Stream stream)
 
             // Extract and encode alpha channel data, if present.
             int alphaDataSize = 0;
+            bool alphaCompressionSucceeded = false;
             if (hasAlpha)
             {
                 // TODO: This can potentially run in an separate task.
                 this.AlphaData = AlphaEncoder.EncodeAlpha(image, this.configuration, this.memoryAllocator, this.alphaCompression, out alphaDataSize);
+                if (alphaDataSize < pixelCount)
+                {
+                    // Only use compressed data, if the compressed data is actually smaller then the uncompressed data.
+                    alphaCompressionSucceeded = true;
+                }
             }
 
             // Stats-collection loop.
@@ -376,7 +383,7 @@ public void Encode<TPixel>(Image<TPixel> image, Stream stream)
                 (uint)height,
                 hasAlpha,
                 hasAlpha ? this.AlphaData.GetSpan().Slice(0, alphaDataSize) : Span<byte>.Empty,
-                this.alphaCompression);
+                this.alphaCompression && alphaCompressionSucceeded);
         }
 
         /// <inheritdoc/>