Moved encode method choice to the JpegEncoderCore

src/ImageSharp/Formats/Jpeg/Components/Encoder/HuffmanScanEncoder.cs

@@ -85,7 +85,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
         /// <param name="luminanceQuantTable">Luminance quantization table provided by the callee</param>
         /// <param name="chrominanceQuantTable">Chrominance quantization table provided by the callee</param>
         /// <param name="cancellationToken">The token to monitor for cancellation.</param>
-        private void Encode444<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuantTable, ref Block8x8F chrominanceQuantTable, CancellationToken cancellationToken)
+        public void Encode444<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuantTable, ref Block8x8F chrominanceQuantTable, CancellationToken cancellationToken)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             var unzig = ZigZag.CreateUnzigTable();
@@ -129,6 +129,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
                         ref unzig);
                 }
             }
+
+            // Pad the last byte with 1's.
+            this.Emit(0x7f, 7);
+            this.target.Write(this.emitBuffer, 0, this.emitLen);
         }
 
         /// <summary>
@@ -140,7 +144,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
         /// <param name="luminanceQuantTable">Luminance quantization table provided by the callee</param>
         /// <param name="chrominanceQuantTable">Chrominance quantization table provided by the callee</param>
         /// <param name="cancellationToken">The token to monitor for cancellation.</param>
-        private void Encode420<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuantTable, ref Block8x8F chrominanceQuantTable, CancellationToken cancellationToken)
+        public void Encode420<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuantTable, ref Block8x8F chrominanceQuantTable, CancellationToken cancellationToken)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             // TODO: Need a JpegScanEncoder<TPixel> class or struct that encapsulates the scan-encoding implementation. (Similar to JpegScanDecoder.)
@@ -199,6 +203,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
                         ref unzig);
                 }
             }
+
+            // Pad the last byte with 1's.
+            this.Emit(0x7f, 7);
+            this.target.Write(this.emitBuffer, 0, this.emitLen);
         }
 
 
@@ -209,7 +217,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
         /// <param name="pixels">The pixel accessor providing access to the image pixels.</param>
         /// <param name="luminanceQuantTable">Luminance quantization table provided by the callee</param>
         /// <param name="cancellationToken">The token to monitor for cancellation.</param>
-        private void EncodeGrayscale<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuantTable, CancellationToken cancellationToken)
+        public void EncodeGrayscale<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuantTable, CancellationToken cancellationToken)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             var unzig = ZigZag.CreateUnzigTable();
@@ -239,33 +247,6 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
                         ref unzig);
                 }
             }
-        }
-
-        public void WriteStartOfScan<TPixel>(
-            Image<TPixel> image,
-            JpegColorType? colorType,
-            JpegSubsample? subsample,
-            ref Block8x8F luminanceQuantTable,
-            ref Block8x8F chrominanceTable,
-            CancellationToken cancellationToken)
-            where TPixel : unmanaged, IPixel<TPixel>
-        {
-            if (colorType == JpegColorType.Luminance)
-            {
-                this.EncodeGrayscale(image, ref luminanceQuantTable, cancellationToken);
-            }
-            else
-            {
-                switch (subsample)
-                {
-                    case JpegSubsample.Ratio444:
-                        this.Encode444(image, ref luminanceQuantTable, ref chrominanceTable, cancellationToken);
-                        break;
-                    case JpegSubsample.Ratio420:
-                        this.Encode420(image, ref luminanceQuantTable, ref chrominanceTable, cancellationToken);
-                        break;
-                }
-            }
 
             // Pad the last byte with 1's.
             this.Emit(0x7f, 7);

src/ImageSharp/Formats/Jpeg/JpegEncoderCore.cs

@@ -86,9 +86,9 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         private readonly int? quality;
 
         /// <summary>
-        /// Gets or sets the subsampling method to use.
+        /// Component count.
         /// </summary>
-        private readonly JpegColorType? colorType;
+        private readonly int componentCount;
 
         /// <summary>
         /// The output stream. All attempted writes after the first error become no-ops.
@@ -103,7 +103,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         {
             this.quality = options.Quality;
             this.subsample = options.Subsample;
-            this.colorType = options.ColorType;
+            this.componentCount = (options.ColorType == JpegColorType.Luminance) ? 1 : 3;
         }
 
         /// <summary>
@@ -129,9 +129,6 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             this.outputStream = stream;
             ImageMetadata metadata = image.Metadata;
 
-            // Compute number of components based on color type in options.
-            int componentCount = (this.colorType == JpegColorType.Luminance) ? 1 : 3;
-
             // System.Drawing produces identical output for jpegs with a quality parameter of 0 and 1.
             int qlty = Numerics.Clamp(this.quality ?? metadata.GetJpegMetadata().Quality, 1, 100);
             this.subsample ??= qlty >= 91 ? JpegSubsample.Ratio444 : JpegSubsample.Ratio420;
@@ -153,7 +150,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             Block8x8F luminanceQuantTable = default;
             Block8x8F chrominanceQuantTable = default;
             InitQuantizationTable(0, scale, ref luminanceQuantTable);
-            if (componentCount > 1)
+            if (this.componentCount > 1)
             {
                 InitQuantizationTable(1, scale, ref chrominanceQuantTable);
             }
@@ -177,13 +174,23 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
             this.WriteStartOfScan(image, componentCount, cancellationToken);
 
             // Write the scan compressed data.
-            new HuffmanScanEncoder(stream).WriteStartOfScan(
+            var scanEncoder = new HuffmanScanEncoder(stream);
-                image,
+            if (this.componentCount == 1)
-                this.colorType,
+            {
-                this.subsample,
+                scanEncoder.EncodeGrayscale(image, ref luminanceQuantTable, cancellationToken);
-                ref luminanceQuantTable,
+            }
-                ref chrominanceQuantTable,
+            else
-                cancellationToken);
+            {
+                switch (subsample)
+                {
+                    case JpegSubsample.Ratio444:
+                        scanEncoder.Encode444(image, ref luminanceQuantTable, ref chrominanceQuantTable, cancellationToken);
+                        break;
+                    case JpegSubsample.Ratio420:
+                        scanEncoder.Encode420(image, ref luminanceQuantTable, ref chrominanceQuantTable, cancellationToken);
+                        break;
+                }
+            }
 
             // Write the End Of Image marker.
             this.buffer[0] = JpegConstants.Markers.XFF;