Quant table adjustment method

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

@@ -139,12 +139,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
         public void Encode444<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuantTable, ref Block8x8F chrominanceQuantTable, CancellationToken cancellationToken)
             where TPixel : unmanaged, IPixel<TPixel>
         {
-            // Calculate reciprocal quantization tables for FDCT method
+            FastFloatingPointDCT.AdjustToFDCT(ref luminanceQuantTable);
-            for (int i = 0; i < Block8x8F.Size; i++)
+            FastFloatingPointDCT.AdjustToFDCT(ref chrominanceQuantTable);
-            {
-                luminanceQuantTable[i] = FastFloatingPointDCT.DctReciprocalAdjustmentCoefficients[i] / luminanceQuantTable[i];
-                chrominanceQuantTable[i] = FastFloatingPointDCT.DctReciprocalAdjustmentCoefficients[i] / chrominanceQuantTable[i];
-            }
 
             this.huffmanTables = HuffmanLut.TheHuffmanLut;
 
@@ -206,12 +202,8 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
         public void Encode420<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuantTable, ref Block8x8F chrominanceQuantTable, CancellationToken cancellationToken)
             where TPixel : unmanaged, IPixel<TPixel>
         {
-            // Calculate reciprocal quantization tables for FDCT method
+            FastFloatingPointDCT.AdjustToFDCT(ref luminanceQuantTable);
-            for (int i = 0; i < Block8x8F.Size; i++)
+            FastFloatingPointDCT.AdjustToFDCT(ref chrominanceQuantTable);
-            {
-                luminanceQuantTable[i] = FastFloatingPointDCT.DctReciprocalAdjustmentCoefficients[i] / luminanceQuantTable[i];
-                chrominanceQuantTable[i] = FastFloatingPointDCT.DctReciprocalAdjustmentCoefficients[i] / chrominanceQuantTable[i];
-            }
 
             this.huffmanTables = HuffmanLut.TheHuffmanLut;
 
@@ -279,11 +271,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
         public void EncodeGrayscale<TPixel>(Image<TPixel> pixels, ref Block8x8F luminanceQuantTable, CancellationToken cancellationToken)
             where TPixel : unmanaged, IPixel<TPixel>
         {
-            // Calculate reciprocal quantization tables for FDCT method
+            FastFloatingPointDCT.AdjustToFDCT(ref luminanceQuantTable);
-            for (int i = 0; i < Block8x8F.Size; i++)
-            {
-                luminanceQuantTable[i] = FastFloatingPointDCT.DctReciprocalAdjustmentCoefficients[i] / luminanceQuantTable[i];
-            }
 
             this.huffmanTables = HuffmanLut.TheHuffmanLut;
 
@@ -325,16 +313,12 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
         /// </summary>
         /// <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="quantTable">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, CancellationToken cancellationToken)
+        public void EncodeRgb<TPixel>(Image<TPixel> pixels, ref Block8x8F quantTable, CancellationToken cancellationToken)
             where TPixel : unmanaged, IPixel<TPixel>
         {
-            // Calculate reciprocal quantization tables for FDCT method
+            FastFloatingPointDCT.AdjustToFDCT(ref quantTable);
-            for (int i = 0; i < Block8x8F.Size; i++)
-            {
-                luminanceQuantTable[i] = FastFloatingPointDCT.DctReciprocalAdjustmentCoefficients[i] / luminanceQuantTable[i];
-            }
 
             this.huffmanTables = HuffmanLut.TheHuffmanLut;
 
@@ -360,19 +344,19 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components.Encoder
                         QuantIndex.Luminance,
                         prevDCR,
                         ref pixelConverter.R,
-                        ref luminanceQuantTable);
+                        ref quantTable);
 
                     prevDCG = this.WriteBlock(
                         QuantIndex.Luminance,
                         prevDCG,
                         ref pixelConverter.G,
-                        ref luminanceQuantTable);
+                        ref quantTable);
 
                     prevDCB = this.WriteBlock(
                         QuantIndex.Luminance,
                         prevDCB,
                         ref pixelConverter.B,
-                        ref luminanceQuantTable);
+                        ref quantTable);
 
                     if (this.IsStreamFlushNeeded)
                     {

src/ImageSharp/Formats/Jpeg/Components/FastFloatingPointDCT.cs

@@ -62,10 +62,10 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
         /// <code>
         /// scalefactor[k] = cos(k*PI/16) * sqrt(2)    for k=1..7
         /// </code>
-        /// Values are also scaled by 8 so DCT code won't do unnecessary division.
+        /// Values are also scaled by 8 so DCT code won't do extra division/multiplication.
         /// </para>
         /// </remarks>
-        public static readonly float[] DctReciprocalAdjustmentCoefficients = new float[]
+        private static readonly float[] DctReciprocalAdjustmentCoefficients = new float[]
         {
             0.125f, 0.09011998f, 0.09567086f, 0.10630376f, 0.125f, 0.15909483f, 0.23096988f, 0.45306373f,
             0.09011998f, 0.064972885f, 0.068974845f, 0.07664074f, 0.09011998f, 0.11470097f, 0.16652f, 0.32664075f,
@@ -77,6 +77,21 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Components
             0.45306373f, 0.32664075f, 0.34675997f, 0.38529903f, 0.45306373f, 0.5766407f, 0.8371526f, 1.642134f,
         };
 
+        /// <summary>
+        /// Adjusts given quantization table to be complient with FDCT implementation.
+        /// </summary>
+        /// <remarks>
+        /// See <see cref="DctReciprocalAdjustmentCoefficients"/> docs for explanation.
+        /// </remarks>
+        /// <param name="quantizationtable">Quantization table to adjust.</param>
+        public static void AdjustToFDCT(ref Block8x8F quantizationtable)
+        {
+            for (int i = 0; i < Block8x8F.Size; i++)
+            {
+                quantizationtable[i] = DctReciprocalAdjustmentCoefficients[i] / quantizationtable[i];
+            }
+        }
+
         /// <summary>
         /// Apply 2D floating point FDCT inplace.
         /// </summary>