Use luminance quant table for all channel with RGB

Author: brianpopow

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

@@ -237,17 +237,16 @@ public void EncodeGrayscale<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanc
         /// <typeparam name="TPixel">The pixel format.</typeparam>
         /// <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="chrominanceQuantTable">Chrominance quantization table provided by the callee.</param>
         /// <param name="cancellationToken">The token to monitor for cancellation.</param>
-        public void EncodeRgb<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuantTable, ref Block8x8F chrominanceQuantTable, CancellationToken cancellationToken)
+        public void EncodeRgb<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuantTable, CancellationToken cancellationToken)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             this.huffmanTables = HuffmanLut.TheHuffmanLut;
 
             var unzig = ZigZag.CreateUnzigTable();
 
             // ReSharper disable once InconsistentNaming
-            int prevDCY = 0, prevDCCb = 0, prevDCCr = 0;
+            int prevDCR = 0, prevDCG = 0, prevDCB = 0;
 
             ImageFrame<TPixel> frame = pixels.Frames.RootFrame;
             Buffer2D<TPixel> pixelBuffer = frame.PixelBuffer;
@@ -264,25 +263,25 @@ public void EncodeRgb<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuant
                 {
                     pixelConverter.Convert(x, y, ref currentRows);
 
-                    prevDCY = this.WriteBlock(
+                    prevDCR = this.WriteBlock(
                         QuantIndex.Luminance,
-                        prevDCY,
+                        prevDCR,
                         ref pixelConverter.R,
                         ref luminanceQuantTable,
                         ref unzig);
 
-                    prevDCCb = this.WriteBlock(
-                        QuantIndex.Chrominance,
-                        prevDCCb,
+                    prevDCG = this.WriteBlock(
+                        QuantIndex.Luminance,
+                        prevDCG,
                         ref pixelConverter.G,
-                        ref chrominanceQuantTable,
+                        ref luminanceQuantTable,
                         ref unzig);
 
-                    prevDCCr = this.WriteBlock(
-                        QuantIndex.Chrominance,
-                        prevDCCr,
+                    prevDCB = this.WriteBlock(
+                        QuantIndex.Luminance,
+                        prevDCB,
                         ref pixelConverter.B,
-                        ref chrominanceQuantTable,
+                        ref luminanceQuantTable,
                         ref unzig);
                 }
             }

src/ImageSharp/Formats/Jpeg/JpegEncoderCore.cs

@@ -160,7 +160,7 @@ public void Encode<TPixel>(Image<TPixel> image, Stream stream, CancellationToken
                         scanEncoder.Encode420(image, ref luminanceQuantTable, ref chrominanceQuantTable, cancellationToken);
                         break;
                     case JpegColorType.Rgb:
-                        scanEncoder.EncodeRgb(image, ref luminanceQuantTable, ref chrominanceQuantTable, cancellationToken);
+                        scanEncoder.EncodeRgb(image, ref luminanceQuantTable, cancellationToken);
                         break;
                 }
             }
@@ -620,6 +620,17 @@ private void WriteStartOfFrame(int width, int height, int componentCount, ReadOn
                             0x11,
                             0x11
                         };
+
+                        if (this.colorType == JpegColorType.Rgb)
+                        {
+                            chroma = stackalloc byte[]
+                            {
+                                0x00,
+                                0x00,
+                                0x00
+                            };
+                        }
+
                         break;
                     case JpegColorType.YCbCrRatio420:
                         subsamples = stackalloc byte[]
@@ -669,6 +680,17 @@ private void WriteStartOfScan(int componentCount, ReadOnlySpan<byte> componentId
                 0x11
             };
 
+            // Use the same DC/AC tables for all channels for RGB.
+            if (this.colorType == JpegColorType.Rgb)
+            {
+                huffmanId = stackalloc byte[]
+                {
+                    0x00,
+                    0x00,
+                    0x00
+                };
+            }
+
             // Write the SOS (Start Of Scan) marker "\xff\xda" followed by 12 bytes:
             // - the marker length "\x00\x0c",
             // - the number of components "\x03",