better IDCT constants, but it did not help

tests/ImageSharp.Tests/Formats/Jpg/ReferenceImplementationsTests.FastFloatingPointDCT.cs

@@ -34,26 +34,13 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
 
                 this.CompareBlocks(data.ConvertAllToFloat(), src, 2f);
             }
-
-            //[Theory(Skip = "Sandboxing only! (Incorrect reference implementation)")]
-            //[InlineData(42)]
-            //[InlineData(1)]
-            //[InlineData(2)]
-            //public void IDCT_IsEquivalentTo_StandardIntegerImplementation(int seed)
-            //{
-            //    int[] intData = JpegUtilityTestFixture.Create8x8RandomIntData(-1000, 1000, seed);
-            //    Span<float> floatSrc = intData.ConvertAllToFloat();
-
-            //    ReferenceImplementations.StandardIntegerDCT.TransformIDCTInplace(intData);
-
-            //    float[] dest = new float[64];
-            //    float[] temp = new float[64];
-
-            //    ReferenceImplementations.FastFloatingPointDCT.iDCT2D_llm(floatSrc, dest, temp);
-
-            //    this.CompareBlocks(intData.ConvertAllToFloat(), dest, 1f);
-            //}
-
+            
+            // [Fact]
+            public void CalcConstants()
+            {
+                ReferenceImplementations.FastFloatingPointDCT.PrintConstants(this.Output);
+            }
+            
             [Theory]
             [InlineData(42, 1000)]
             [InlineData(1, 1000)]
@@ -76,25 +63,6 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
                 this.CompareBlocks(intData.ConvertAllToFloat(), dest, 1f);
             }
             
-            //[Theory]
-            //[InlineData(42)]
-            //[InlineData(1)]
-            //[InlineData(2)]
-            //public void FDCT_IsEquivalentTo_StandardIntegerImplementation(int seed)
-            //{
-            //    int[] intData = JpegUtilityTestFixture.Create8x8RandomIntData(-1000, 1000, seed);
-            //    float[] floatSrc = intData.ConvertAllToFloat();
-
-            //    ReferenceImplementations.StandardIntegerDCT.Subtract128_TransformFDCT_Upscale8_Inplace(intData);
-
-            //    float[] dest = new float[64];
-            //    float[] temp = new float[64];
-
-            //    ReferenceImplementations.FastFloatingPointDCT.fDCT2D_llm(floatSrc, dest, temp, subtract128FromSource: true);
-
-            //    this.CompareBlocks(intData.ConvertAllToFloat(), dest, 2f);
-            //}
-
             [Theory]
             [InlineData(42)]
             [InlineData(1)]

tests/ImageSharp.Tests/Formats/Jpg/Utils/ReferenceImplementations.FastFloatingPointDCT.cs

@@ -8,8 +8,18 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg.Utils
     using SixLabors.ImageSharp.Formats.Jpeg.Common;
     using SixLabors.ImageSharp.Formats.Jpeg.GolangPort.Utils;
 
+    using Xunit.Abstractions;
+
     internal static partial class ReferenceImplementations
     {
+        /// <summary>
+        /// Contains a non-optimized port of:
+        /// https://github.com/norishigefukushima/dct_simd/blob/master/dct/dct8x8_simd.cpp
+        /// 
+        /// The main purpose of this code is testing and documentation, it is intented to be similar to it's original counterpart.
+        /// DO NOT clean it!
+        /// DO NOT StyleCop it!
+        /// </summary>
         internal static class FastFloatingPointDCT
         {
             public static Block8x8F TransformIDCT(ref Block8x8F source)
@@ -38,6 +48,23 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg.Utils
                 return result;
             }
 
+            private static double cos(double x) => Math.Cos(x);
+
+            private const double M_PI = Math.PI;
+
+            private static readonly double M_SQRT2 = Math.Sqrt(2);
+
+            public static float[] PrintConstants(ITestOutputHelper output)
+            {
+                float[] r = new float[8];
+                for (int i = 0; i < 8; i++)
+                {
+                    r[i] = (float)(cos((double)i / 16.0 * M_PI) * M_SQRT2);
+                    output?.WriteLine($"float r{i} = {r[i]:R}f;");
+                }
+                return r;
+            }
+
             /// <summary>
             /// https://github.com/norishigefukushima/dct_simd/blob/master/dct/dct8x8_simd.cpp#L200
             /// </summary>
@@ -48,14 +75,15 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg.Utils
                 float a0, a1, a2, a3, b0, b1, b2, b3;
                 float z0, z1, z2, z3, z4;
 
-                //float r0 = 1.414214f;
-                float r1 = 1.387040f;
+                // see: PrintConstants()
+                float r0 = 1.41421354f;
+                float r1 = 1.3870399f;
                 float r2 = 1.306563f;
-                float r3 = 1.175876f;
-                //float r4 = 1.000000f;
-                float r5 = 0.785695f;
-                float r6 = 0.541196f;
-                float r7 = 0.275899f;
+                float r3 = 1.17587554f;
+                float r4 = 1f;
+                float r5 = 0.785694957f;
+                float r6 = 0.5411961f;
+                float r7 = 0.27589938f;
 
                 z0 = y[1] + y[7];
                 z1 = y[3] + y[5];