Make sure the alpha flag in VP8X and VP8L are the same

src/ImageSharp/Formats/Webp/BitWriter/BitWriterBase.cs

@@ -63,15 +63,6 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
         /// </summary>
         public abstract void Finish();
 
-        /// <summary>
-        /// Writes the encoded image to the stream.
-        /// </summary>
-        /// <param name="stream">The stream to write to.</param>
-        /// <param name="exifProfile">The exif profile.</param>
-        /// <param name="width">The width of the image.</param>
-        /// <param name="height">The height of the image.</param>
-        public abstract void WriteEncodedImageToStream(Stream stream, ExifProfile exifProfile, uint width, uint height);
-
         protected void ResizeBuffer(int maxBytes, int sizeRequired)
         {
             int newSize = (3 * maxBytes) >> 1;
@@ -142,7 +133,8 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
         /// <param name="exifProfile">A exif profile or null, if it does not exist.</param>
         /// <param name="width">The width of the image.</param>
         /// <param name="height">The height of the image.</param>
-        protected void WriteVp8XHeader(Stream stream, ExifProfile exifProfile, uint width, uint height)
+        /// <param name="hasAlpha">Flag indicating, if a alpha channel is present.</param>
+        protected void WriteVp8XHeader(Stream stream, ExifProfile exifProfile, uint width, uint height, bool hasAlpha)
         {
             if (width > MaxDimension || height > MaxDimension)
             {
@@ -162,6 +154,12 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
                 flags |= 8;
             }
 
+            if (hasAlpha)
+            {
+                // Set alpha bit.
+                flags |= 16;
+            }
+
             Span<byte> buf = this.scratchBuffer.AsSpan(0, 4);
             stream.Write(WebpConstants.Vp8XMagicBytes);
             BinaryPrimitives.WriteUInt32LittleEndian(buf, WebpConstants.Vp8XChunkSize);

src/ImageSharp/Formats/Webp/BitWriter/Vp8BitWriter.cs

@@ -399,8 +399,15 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             }
         }
 
-        /// <inheritdoc/>
-        public override void WriteEncodedImageToStream(Stream stream, ExifProfile exifProfile, uint width, uint height)
+        /// <summary>
+        /// Writes the encoded image to the stream.
+        /// </summary>
+        /// <param name="stream">The stream to write to.</param>
+        /// <param name="exifProfile">The exif profile.</param>
+        /// <param name="width">The width of the image.</param>
+        /// <param name="height">The height of the image.</param>
+        /// <param name="hasAlpha">Flag indicating, if a alpha channel is present.</param>
+        public void WriteEncodedImageToStream(Stream stream, ExifProfile exifProfile, uint width, uint height, bool hasAlpha)
         {
             bool isVp8X = false;
             byte[] exifBytes = null;
@@ -433,7 +440,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             riffSize += WebpConstants.TagSize + WebpConstants.ChunkHeaderSize + vp8Size;
 
             // Emit headers and partition #0
-            this.WriteWebpHeaders(stream, size0, vp8Size, riffSize, isVp8X, width, height, exifProfile);
+            this.WriteWebpHeaders(stream, size0, vp8Size, riffSize, isVp8X, width, height, exifProfile, hasAlpha);
             bitWriterPartZero.WriteToStream(stream);
 
             // Write the encoded image to the stream.
@@ -616,14 +623,14 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             while (it.Next());
         }
 
-        private void WriteWebpHeaders(Stream stream, uint size0, uint vp8Size, uint riffSize, bool isVp8X, uint width, uint height, ExifProfile exifProfile)
+        private void WriteWebpHeaders(Stream stream, uint size0, uint vp8Size, uint riffSize, bool isVp8X, uint width, uint height, ExifProfile exifProfile, bool hasAlpha)
         {
             this.WriteRiffHeader(stream, riffSize);
 
             // Write VP8X, header if necessary.
             if (isVp8X)
             {
-                this.WriteVp8XHeader(stream, exifProfile, width, height);
+                this.WriteVp8XHeader(stream, exifProfile, width, height, hasAlpha);
             }
 
             this.WriteVp8Header(stream, vp8Size);

src/ImageSharp/Formats/Webp/BitWriter/Vp8LBitWriter.cs

@@ -127,8 +127,15 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             this.used = 0;
         }
 
-        /// <inheritdoc/>
-        public override void WriteEncodedImageToStream(Stream stream, ExifProfile exifProfile, uint width, uint height)
+        /// <summary>
+        /// Writes the encoded image to the stream.
+        /// </summary>
+        /// <param name="stream">The stream to write to.</param>
+        /// <param name="exifProfile">The exif profile.</param>
+        /// <param name="width">The width of the image.</param>
+        /// <param name="height">The height of the image.</param>
+        /// <param name="hasAlpha">Flag indicating, if a alpha channel is present.</param>
+        public void WriteEncodedImageToStream(Stream stream, ExifProfile exifProfile, uint width, uint height, bool hasAlpha)
         {
             bool isVp8X = false;
             byte[] exifBytes = null;
@@ -153,7 +160,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.BitWriter
             // Write VP8X, header if necessary.
             if (isVp8X)
             {
-                this.WriteVp8XHeader(stream, exifProfile, width, height);
+                this.WriteVp8XHeader(stream, exifProfile, width, height, hasAlpha);
             }
 
             // Write magic bytes indicating its a lossless webp.

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

@@ -234,7 +234,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossless
             this.EncodeStream(image);
 
             // Write bytes from the bitwriter buffer to the stream.
-            this.bitWriter.WriteEncodedImageToStream(stream, image.Metadata.ExifProfile, (uint)width, (uint)height);
+            this.bitWriter.WriteEncodedImageToStream(stream, image.Metadata.ExifProfile, (uint)width, (uint)height, hasAlpha);
         }
 
         /// <summary>

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

@@ -317,6 +317,8 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
             this.bitWriter = new Vp8BitWriter(expectedSize, this);
 
             // TODO: EncodeAlpha();
+            bool hasAlpha = false;
+
             // Stats-collection loop.
             this.StatLoop(width, height, yStride, uvStride);
             it.Init();
@@ -348,7 +350,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
 
             // Write bytes from the bitwriter buffer to the stream.
             image.Metadata.SyncProfiles();
-            this.bitWriter.WriteEncodedImageToStream(stream, image.Metadata.ExifProfile, (uint)width, (uint)height);
+            this.bitWriter.WriteEncodedImageToStream(stream, image.Metadata.ExifProfile, (uint)width, (uint)height, hasAlpha);
         }
 
         /// <inheritdoc/>