organizing DCT code

ImageSharp.sln.DotSettings

@@ -342,6 +342,7 @@
 	<s:Boolean x:Key="/Default/CodeStyle/CSharpUsing/QualifiedUsingAtNestedScope/@EntryValue">True</s:Boolean>
 	<s:String x:Key="/Default/CodeStyle/Naming/CSharpNaming/Abbreviations/=AC/@EntryIndexedValue">AC</s:String>
 	<s:String x:Key="/Default/CodeStyle/Naming/CSharpNaming/Abbreviations/=DC/@EntryIndexedValue">DC</s:String>
+	<s:String x:Key="/Default/CodeStyle/Naming/CSharpNaming/Abbreviations/=DCT/@EntryIndexedValue">DCT</s:String>
 	<s:String x:Key="/Default/CodeStyle/Naming/CSharpNaming/Abbreviations/=EOF/@EntryIndexedValue">EOF</s:String>
 	<s:String x:Key="/Default/CodeStyle/Naming/CSharpNaming/Abbreviations/=FDCT/@EntryIndexedValue">FDCT</s:String>
 	<s:String x:Key="/Default/CodeStyle/Naming/CSharpNaming/Abbreviations/=IDCT/@EntryIndexedValue">IDCT</s:String>

src/ImageSharp/Formats/Jpeg/Common/Block8x8.cs

@@ -44,6 +44,12 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Common
             }
         }
 
+        public short this[int y, int x]
+        {
+            get => this[(y * 8) + x];
+            set => this[(y * 8) + x] = value;
+        }
+
         public static bool operator ==(Block8x8 left, Block8x8 right)
         {
             return left.Equals(right);
@@ -83,9 +89,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Common
             short* fp = blockPtr->data;
             fp[idx] = value;
         }
-
-        public short GetValueAt(int x, int y) => this[(y * 8) + x];
-
+        
         public Block8x8F AsFloatBlock()
         {
             // TODO: Optimize this
@@ -112,6 +116,22 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Common
             Unsafe.CopyBlock(ref destRef, ref selfRef, Size * sizeof(short));
         }
 
+        public void CopyTo(Span<int> destination)
+        {
+            for (int i = 0; i < Size; i++)
+            {
+                destination[i] = this[i];
+            }
+        }
+
+        public void LoadFrom(Span<int> source)
+        {
+            for (int i = 0; i < Size; i++)
+            {
+                this[i] = (short)source[i];
+            }
+        }
+
         [Conditional("DEBUG")]
         private static void GuardBlockIndex(int idx)
         {

src/ImageSharp/Formats/Jpeg/Common/Block8x8F.cs

@@ -83,6 +83,12 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Common
             }
         }
 
+        public float this[int y, int x]
+        {
+            get => this[(y * 8) + x];
+            set => this[(y * 8) + x] = value;
+        }
+
         /// <summary>
         /// Pointer-based "Indexer" (getter part)
         /// </summary>
@@ -113,17 +119,17 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Common
             fp[idx] = value;
         }
 
-        public Block8x8 AsInt16Block()
-        {
-            // TODO: Optimize this
-            var result = default(Block8x8);
-            for (int i = 0; i < Size; i++)
-            {
-                result[i] = (short)this[i];
-            }
+        //public Block8x8 AsInt16Block()
+        //{
+        //    // TODO: Optimize this
+        //    var result = default(Block8x8);
+        //    for (int i = 0; i < Size; i++)
+        //    {
+        //        result[i] = (short)this[i];
+        //    }
 
-            return result;
-        }
+        //    return result;
+        //}
 
         /// <summary>
         /// Fill the block with defaults (zeroes)
@@ -413,6 +419,30 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Common
             a.V7R = DivideRound(a.V7R, b.V7R);
         }
 
+        public void RoundInto(ref Block8x8 dest)
+        {
+            for (int i = 0; i < Size; i++)
+            {
+                float val = this[i];
+                if (val < 0)
+                {
+                    val -= 0.5f;
+                }
+                else
+                {
+                    val += 0.5f;
+                }
+                dest[i] = (short)val;
+            }
+        }
+
+        public Block8x8 RoundAsInt16Block()
+        {
+            var result = default(Block8x8);
+            this.RoundInto(ref result);
+            return result;
+        }
+
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private static Vector4 DivideRound(Vector4 dividend, Vector4 divisor)
         {

tests/ImageSharp.Tests/Formats/Jpg/Block8x8FTests.cs

@@ -291,10 +291,10 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
         public unsafe void UnzigDivRound(int seed)
         {
             Block8x8F block = new Block8x8F();
-            block.LoadFrom(Create8x8RandomFloatData(-2000, 2000, seed));
+            block.LoadFrom(Create8x8RoundedRandomFloatData(-2000, 2000, seed));
 
             Block8x8F qt = new Block8x8F();
-            qt.LoadFrom(Create8x8RandomFloatData(-2000, 2000, seed));
+            qt.LoadFrom(Create8x8RoundedRandomFloatData(-2000, 2000, seed));
 
             UnzigData unzig = UnzigData.Create();
 
@@ -314,19 +314,40 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             }
         }
 
+        //[Fact]
+        //public void AsInt16Block()
+        //{
+        //    float[] data = Create8x8FloatData();
+
+        //    var source = default(Block8x8F);
+        //    source.LoadFrom(data);
+
+        //    Block8x8 dest = source.AsInt16Block();
+
+        //    for (int i = 0; i < Block8x8F.Size; i++)
+        //    {
+        //        Assert.Equal((short)data[i], dest[i]);
+        //    }
+        //}
+
         [Fact]
-        public void AsInt16Block()
+        public void RoundInto()
         {
-            float[] data = Create8x8FloatData();
+            float[] data = Create8x8RandomFloatData(-1000, 1000);
 
             var source = default(Block8x8F);
             source.LoadFrom(data);
+            var dest = default(Block8x8);
 
-            Block8x8 dest = source.AsInt16Block();
+            source.RoundInto(ref dest);
 
-            for (int i = 0; i < Block8x8F.Size; i++)
+            for (int i = 0; i < Block8x8.Size; i++)
             {
-                Assert.Equal((short)data[i], dest[i]);
+                float expectedFloat = data[i];
+                short expectedShort = (short) Math.Round(expectedFloat);
+                short actualShort = dest[i];
+
+                Assert.Equal(expectedShort, actualShort);
             }
         }
     }

tests/ImageSharp.Tests/Formats/Jpg/Block8x8Tests.cs

@@ -115,12 +115,12 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
         }
 
         [Fact]
-        public void GetValueAt()
+        public void IndexerYX()
         {
             var block = default(Block8x8);
             block[8 * 3 + 5] = 42;
 
-            short value = block.GetValueAt(5, 3);
+            short value = block[3, 5];
 
             Assert.Equal(42, value);
         }

tests/ImageSharp.Tests/Formats/Jpg/DCTTests.cs

@@ -76,7 +76,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             [InlineData(3)]
             public void TransformIDCT(int seed)
             {
-                Span<float> sourceArray = JpegUtilityTestFixture.Create8x8RandomFloatData(-200, 200, seed);
+                Span<float> sourceArray = JpegUtilityTestFixture.Create8x8RoundedRandomFloatData(-200, 200, seed);
                 float[] expectedDestArray = new float[64];
                 float[] tempArray = new float[64];
 
@@ -107,7 +107,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             [InlineData(2)]
             public void FDCT8x4_LeftPart(int seed)
             {
-                Span<float> src = JpegUtilityTestFixture.Create8x8RandomFloatData(-200, 200, seed);
+                Span<float> src = JpegUtilityTestFixture.Create8x8RoundedRandomFloatData(-200, 200, seed);
                 Block8x8F srcBlock = new Block8x8F();
                 srcBlock.LoadFrom(src);
 
@@ -129,7 +129,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             [InlineData(2)]
             public void FDCT8x4_RightPart(int seed)
             {
-                Span<float> src = JpegUtilityTestFixture.Create8x8RandomFloatData(-200, 200, seed);
+                Span<float> src = JpegUtilityTestFixture.Create8x8RoundedRandomFloatData(-200, 200, seed);
                 Block8x8F srcBlock = new Block8x8F();
                 srcBlock.LoadFrom(src);
 
@@ -151,7 +151,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
             [InlineData(2)]
             public void TransformFDCT(int seed)
             {
-                Span<float> src = JpegUtilityTestFixture.Create8x8RandomFloatData(-200, 200, seed);
+                Span<float> src = JpegUtilityTestFixture.Create8x8RoundedRandomFloatData(-200, 200, seed);
                 Block8x8F srcBlock = new Block8x8F();
                 srcBlock.LoadFrom(src);
 

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

@@ -0,0 +1,113 @@
+// ReSharper disable InconsistentNaming
+namespace SixLabors.ImageSharp.Tests.Formats.Jpg
+{
+    using System;
+
+    using SixLabors.ImageSharp.Tests.Formats.Jpg.Utils;
+
+    using Xunit;
+
+    public partial class ReferenceImplementationsTests
+    {
+        public class FastFloatingPointDCT
+        {
+            [Theory]
+            [InlineData(42, 0)]
+            [InlineData(1, 0)]
+            [InlineData(2, 0)]
+            public void ForwardThenInverse(int seed, int startAt)
+            {
+                int[] data = JpegUtilityTestFixture.Create8x8RandomIntData(-200, 200, seed);
+                float[] src = data.ConvertAllToFloat();
+                float[] dest = new float[64];
+                float[] temp = new float[64];
+
+                ReferenceImplementations.FastFloatingPointDCT.fDCT2D_llm(src, dest, temp, true);
+                ReferenceImplementations.FastFloatingPointDCT.iDCT2D_llm(dest, src, temp);
+
+                for (int i = startAt; i < 64; i++)
+                {
+                    float expected = data[i];
+                    float actual = (float)src[i];
+
+                    Assert.Equal(expected, actual, new ApproximateFloatComparer(2f));
+                }
+            }
+
+            [Theory]
+            [InlineData(42)]
+            [InlineData(1)]
+            [InlineData(2)]
+            public void IDCT_IsEquivalentTo_StandardIntegerImplementation(int seed)
+            {
+                int[] intData = JpegUtilityTestFixture.Create8x8RandomIntData(-200, 200, 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);
+
+                for (int i = 0; i < 64; i++)
+                {
+                    float expected = intData[i];
+                    float actual = dest[i];
+
+                    Assert.Equal(expected, actual, new ApproximateFloatComparer(1f));
+                }
+            }
+
+            [Theory]
+            [InlineData(42)]
+            [InlineData(1)]
+            [InlineData(2)]
+            public void IDCT_IsEquivalentTo_AccurateImplementation(int seed)
+            {
+                int[] intData = JpegUtilityTestFixture.Create8x8RandomIntData(-200, 200, seed);
+                float[] floatSrc = intData.ConvertAllToFloat();
+
+                ReferenceImplementations.AccurateDCT.TransformIDCTInplace(intData);
+
+                float[] dest = new float[64];
+                float[] temp = new float[64];
+
+                ReferenceImplementations.FastFloatingPointDCT.iDCT2D_llm(floatSrc, dest, temp);
+
+                for (int i = 0; i < 64; i++)
+                {
+                    float expected = intData[i];
+                    float actual = dest[i];
+
+                    Assert.Equal(expected, actual, new ApproximateFloatComparer(1f));
+                }
+            }
+            
+            [Theory]
+            [InlineData(42)]
+            [InlineData(1)]
+            [InlineData(2)]
+            public void FDCT_IsEquivalentTo_StandardIntegerImplementation(int seed)
+            {
+                int[] intData = JpegUtilityTestFixture.Create8x8RandomIntData(-200, 200, seed);
+                float[] floatSrc = intData.ConvertAllToFloat();
+
+                ReferenceImplementations.StandardIntegerDCT.TransformFDCTInplace(intData);
+
+                float[] dest = new float[64];
+                float[] temp = new float[64];
+
+                ReferenceImplementations.FastFloatingPointDCT.fDCT2D_llm(floatSrc, dest, temp, offsetSourceByNeg128: true);
+
+                for (int i = 0; i < 64; i++)
+                {
+                    float expected = intData[i];
+                    float actual = dest[i];
+
+                    Assert.Equal(expected, actual, new ApproximateFloatComparer(1f));
+                }
+            }
+        }
+    }
+}

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

@@ -0,0 +1,73 @@
+namespace SixLabors.ImageSharp.Tests.Formats.Jpg
+{
+    using System;
+
+    using SixLabors.ImageSharp.Formats.Jpeg.Common;
+    using SixLabors.ImageSharp.Tests.Formats.Jpg.Utils;
+
+    using Xunit;
+    using Xunit.Abstractions;
+
+    public partial class ReferenceImplementationsTests
+    {
+        
+        public class StandardIntegerDCT
+        {
+            public StandardIntegerDCT(ITestOutputHelper output)
+            {
+                this.Output = output;
+            }
+
+            private ITestOutputHelper Output { get; }
+
+            [Theory]
+            [InlineData(42)]
+            [InlineData(1)]
+            [InlineData(2)]
+            public void IDCT_IsEquivalentTo_AccurateImplementation(int seed)
+            {
+                int[] data = Create8x8RandomIntData(-1000, 1000, seed);
+
+                Block8x8 source = default(Block8x8);
+                source.LoadFrom(data);
+
+                Block8x8 expected = ReferenceImplementations.AccurateDCT.TransformIDCT(ref source);
+                Block8x8 actual = ReferenceImplementations.StandardIntegerDCT.TransformIDCT(ref source);
+
+                long diff = Block8x8.TotalDifference(ref expected, ref actual);
+                this.Output.WriteLine(expected.ToString());
+                this.Output.WriteLine(actual.ToString());
+                this.Output.WriteLine("DIFFERENCE: "+diff);
+            }
+
+
+            [Theory]
+            [InlineData(42, 0)]
+            [InlineData(1, 0)]
+            [InlineData(2, 0)]
+            public void ForwardThenInverse(int seed, int startAt)
+            {
+                Span<int> original = JpegUtilityTestFixture.Create8x8RandomIntData(-200, 200, seed);
+
+                Span<int> block = original.AddScalarToAllValues(128);
+
+                ReferenceImplementations.StandardIntegerDCT.TransformFDCTInplace(block);
+
+                for (int i = 0; i < 64; i++)
+                {
+                    block[i] /= 8;
+                }
+
+                ReferenceImplementations.StandardIntegerDCT.TransformIDCTInplace(block);
+
+                for (int i = startAt; i < 64; i++)
+                {
+                    float expected = original[i];
+                    float actual = (float)block[i];
+
+                    Assert.Equal(expected, actual, new ApproximateFloatComparer(3f));
+                }
+            }
+        }
+    }
+}

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

@@ -3,124 +3,18 @@
 
 using SixLabors.ImageSharp.Formats.Jpeg.GolangPort.Utils;
 
-using Xunit;
 using Xunit.Abstractions;
 
 // ReSharper disable InconsistentNaming
 namespace SixLabors.ImageSharp.Tests.Formats.Jpg
 {
-    using System;
-
     using SixLabors.ImageSharp.Tests.Formats.Jpg.Utils;
 
-    public class ReferenceImplementationsTests : JpegUtilityTestFixture
+    public partial class ReferenceImplementationsTests : JpegUtilityTestFixture
     {
         public ReferenceImplementationsTests(ITestOutputHelper output)
             : base(output)
         {
         }
-
-
-        [Theory]
-        [InlineData(42)]
-        [InlineData(1)]
-        [InlineData(2)]
-        public void Idct_FloatingPointReferenceImplementation_IsEquivalentToIntegerImplementation(int seed)
-        {
-            int[] intData = Create8x8RandomIntData(-200, 200, seed);
-            Span<float> floatSrc = intData.ConvertAllToFloat();
-
-            ReferenceImplementations.IntegerDCT.TransformIDCTInplace(intData);
-
-            float[] dest = new float[64];
-            float[] temp = new float[64];
-
-            ReferenceImplementations.FastFloatingPointDCT.iDCT2D_llm(floatSrc, dest, temp);
-
-            for (int i = 0; i < 64; i++)
-            {
-                float expected = intData[i];
-                float actual = dest[i];
-
-                Assert.Equal(expected, actual, new ApproximateFloatComparer(1f));
-            }
-        }
-
-        [Theory]
-        [InlineData(42, 0)]
-        [InlineData(1, 0)]
-        [InlineData(2, 0)]
-        public void IntegerDCT_ForwardThenInverse(int seed, int startAt)
-        {
-            Span<int> original = Create8x8RandomIntData(-200, 200, seed);
-
-            Span<int> block = original.AddScalarToAllValues(128);
-
-            ReferenceImplementations.IntegerDCT.TransformFDCTInplace(block);
-
-            for (int i = 0; i < 64; i++)
-            {
-                block[i] /= 8;
-            }
-
-            ReferenceImplementations.IntegerDCT.TransformIDCTInplace(block);
-
-            for (int i = startAt; i < 64; i++)
-            {
-                float expected = original[i];
-                float actual = (float)block[i];
-
-                Assert.Equal(expected, actual, new ApproximateFloatComparer(3f));
-            }
-
-        }
-
-        [Theory]
-        [InlineData(42, 0)]
-        [InlineData(1, 0)]
-        [InlineData(2, 0)]
-        public void FloatingPointDCT_ReferenceImplementation_ForwardThenInverse(int seed, int startAt)
-        {
-            int[] data = Create8x8RandomIntData(-200, 200, seed);
-            float[] src = data.ConvertAllToFloat();
-            float[] dest = new float[64];
-            float[] temp = new float[64];
-
-            ReferenceImplementations.FastFloatingPointDCT.fDCT2D_llm(src, dest, temp, true);
-            ReferenceImplementations.FastFloatingPointDCT.iDCT2D_llm(dest, src, temp);
-
-            for (int i = startAt; i < 64; i++)
-            {
-                float expected = data[i];
-                float actual = (float)src[i];
-
-                Assert.Equal(expected, actual, new ApproximateFloatComparer(2f));
-            }
-        }
-
-        [Theory]
-        [InlineData(42)]
-        [InlineData(1)]
-        [InlineData(2)]
-        public void Fdct_FloatingPointReferenceImplementation_IsEquivalentToIntegerImplementation(int seed)
-        {
-            int[] intData = Create8x8RandomIntData(-200, 200, seed);
-            float[] floatSrc = intData.ConvertAllToFloat();
-
-            ReferenceImplementations.IntegerDCT.TransformFDCTInplace(intData);
-
-            float[] dest = new float[64];
-            float[] temp = new float[64];
-
-            ReferenceImplementations.FastFloatingPointDCT.fDCT2D_llm(floatSrc, dest, temp, offsetSourceByNeg128: true);
-
-            for (int i = 0; i < 64; i++)
-            {
-                float expected = intData[i];
-                float actual = dest[i];
-
-                Assert.Equal(expected, actual, new ApproximateFloatComparer(1f));
-            }
-        }
     }
 }

tests/ImageSharp.Tests/Formats/Jpg/Utils/JpegUtilityTestFixture.cs

@@ -76,8 +76,26 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg.Utils
             return result;
         }
 
-        internal static float[] Create8x8RandomFloatData(int minValue, int maxValue, int seed = 42)
-            => SpanExtensions.ConvertAllToFloat(Create8x8RandomIntData(minValue, maxValue, seed));
+        internal static float[] Create8x8RoundedRandomFloatData(int minValue, int maxValue, int seed = 42)
+            => Create8x8RandomIntData(minValue, maxValue, seed).ConvertAllToFloat();
+
+        public static float[] Create8x8RandomFloatData(float minValue, float maxValue, int seed = 42)
+        {
+            Random rnd = new Random(seed);
+            float[] result = new float[64];
+            for (int i = 0; i < 8; i++)
+            {
+                for (int j = 0; j < 8; j++)
+                {
+                    double val = rnd.NextDouble();
+                    val *= maxValue - minValue;
+                    val += minValue;
+                    
+                    result[i * 8 + j] = (float)val;
+                }
+            }
+            return result;
+        }
 
         internal void Print8x8Data<T>(T[] data) => this.Print8x8Data(new Span<T>(data));
 

tests/ImageSharp.Tests/Formats/Jpg/Utils/LibJpegTools.ComponentData.cs

@@ -121,7 +121,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg.Utils
             internal float GetBlockValue(Block8x8 block, int x, int y)
             {
                 float d = (this.MaxVal - this.MinVal);
-                float val = block.GetValueAt(x, y);
+                float val = block[y, x];
                 val -= this.MinVal;
                 val /= d;
                 return val;

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

@@ -0,0 +1,121 @@
+namespace SixLabors.ImageSharp.Tests.Formats.Jpg.Utils
+{
+    using System;
+
+    using SixLabors.ImageSharp.Formats.Jpeg.Common;
+
+    internal static partial class ReferenceImplementations
+    {
+        /// <summary>
+        /// Reference implementations based on:
+        /// https://github.com/keithw/mympeg2enc/blob/master/idct.c#L222
+        /// Claiming:
+        /// /* reference idct taken from "ieeetest.c"
+        ///  * Written by Tom Lane (tgl@cs.cmu.edu).
+        ///  * Released to public domain 11/22/93.
+        ///  */  
+        /// </summary>
+        internal static class AccurateDCT
+        {
+            private static double[,] CosLut = InitCosLut();
+
+
+            public static Block8x8 TransformIDCT(ref Block8x8 block)
+            {
+                Block8x8F temp = block.AsFloatBlock();
+                Block8x8F res0 = TransformIDCT(ref temp);
+                return res0.RoundAsInt16Block();
+            }
+
+            public static void TransformIDCTInplace(Span<int> span)
+            {
+                var temp = new Block8x8();
+                temp.LoadFrom(span);
+                Block8x8 result = TransformIDCT(ref temp);
+                result.CopyTo(span);
+            }
+
+            public static Block8x8 TransformFDCT(ref Block8x8 block)
+            {
+                Block8x8F temp = block.AsFloatBlock();
+                Block8x8F res0 = TransformFDCT(ref temp);
+                return res0.RoundAsInt16Block();
+            }
+
+            public static void TransformFDCTInplace(Span<int> span)
+            {
+                var temp = new Block8x8();
+                temp.LoadFrom(span);
+                Block8x8 result = TransformFDCT(ref temp);
+                result.CopyTo(span);
+            }
+
+
+
+            private static double[,] InitCosLut()
+            {
+                double[,] coslu = new double[8,8];
+                int a, b;
+                double tmp;
+
+                for (a = 0; a < 8; a++)
+                for (b = 0; b < 8; b++)
+                {
+                    tmp = Math.Cos((double)((a + a + 1) * b) * (3.14159265358979323846 / 16.0));
+                    if (b == 0)
+                    {
+                        tmp /= Math.Sqrt(2.0);
+                    }
+                    coslu[a, b] = tmp * 0.5;
+                }
+                return coslu;
+            }
+
+            public static Block8x8F TransformIDCT(ref Block8x8F block)
+            {
+                int x, y, u, v;
+                double tmp, tmp2;
+                Block8x8F res = default(Block8x8F);
+
+                for (y=0; y<8; y++) {
+                    for (x=0; x<8; x++) {
+                        tmp = 0.0;
+                        for (v=0; v<8; v++) {
+                            tmp2 = 0.0;
+                            for (u=0; u<8; u++) {
+                                tmp2 += (double) block[v * 8 + u] * CosLut[x, u];
+                            }
+                            tmp += CosLut[y, v] * tmp2;
+                        }
+                        res[y, x] = (float)tmp;
+                    }
+                }
+                return res;
+            }
+
+
+            public static Block8x8F TransformFDCT(ref Block8x8F block)
+            {
+                int x, y, u, v;
+                double tmp, tmp2;
+                Block8x8F res = default(Block8x8F);
+
+                for (v=0; v<8; v++) {   
+                    for (u=0; u<8; u++) {   
+                        tmp = 0.0;   
+                        for (y=0; y<8; y++) {   
+                            tmp2 = 0.0;   
+                            for (x=0; x<8; x++) {   
+                                tmp2 += (double) block[y, x] * CosLut[x, u];
+                            }
+                            tmp += CosLut[y, v] * tmp2;   
+                        }
+                        res[v, u] = (float)tmp;   
+                    }   
+                }
+
+                return res;
+            }   
+        }
+    }
+}

tests/ImageSharp.Tests/Formats/Jpg/Utils/ReferenceImplementations.IntegerDCT.cs → tests/ImageSharp.Tests/Formats/Jpg/Utils/ReferenceImplementations.StandardIntegerDCT.cs

@@ -2,12 +2,40 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg.Utils
 {
     using System;
 
+    using SixLabors.ImageSharp.Formats.Jpeg.Common;
+
     internal static partial class ReferenceImplementations
     {
         /// <summary>
-        /// The "original" libjpeg/golang based DCT implementation is used as reference implementation for tests.
+        /// Contains the "original" golang based DCT/IDCT implementations as reference implementations.
+        /// 1. ===== Forward DCT =====
+        /// **** The original golang source claims:
+        /// It is based on the code in jfdctint.c from the Independent JPEG Group,
+        /// found at http://www.ijg.org/files/jpegsrc.v8c.tar.gz.
+        /// 
+        /// **** Could be found here as well:
+        /// https://github.com/mozilla/mozjpeg/blob/master/jfdctint.c 
+        /// 
+        /// 2. ===== Inverse DCT =====
+        /// 
+        /// The golang source claims:
+        /// http://standards.iso.org/ittf/PubliclyAvailableStandards/ISO_IEC_13818-4_2004_Conformance_Testing/Video/verifier/mpeg2decode_960109.tar.gz
+        /// The referenced MPEG2 code claims:
+        /// /**********************************************************/
+        /// /* inverse two dimensional DCT, Chen-Wang algorithm       */
+        /// /* (cf. IEEE ASSP-32, pp. 803-816, Aug. 1984)             */
+        /// /* 32-bit integer arithmetic (8 bit coefficients)         */
+        /// /* 11 mults, 29 adds per DCT                              */
+        /// /*                                      sE, 18.8.91       */
+        /// /**********************************************************/
+        /// /* coefficients extended to 12 bit for IEEE1180-1990      */
+        /// /* compliance                           sE,  2.1.94       */
+        /// /**********************************************************/
+        /// 
+        /// **** The code looks pretty similar to the standard libjpeg IDCT, but without quantization:
+        /// https://github.com/mozilla/mozjpeg/blob/master/jidctint.c
         /// </summary>
-        public static class IntegerDCT
+        public static class StandardIntegerDCT
         {
             private const int fix_0_298631336 = 2446;
             private const int fix_0_390180644 = 3196;
@@ -37,6 +65,26 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg.Utils
             /// </summary>
             private const int CenterJSample = 128;
 
+            public static Block8x8 TransformFDCT(ref Block8x8 block)
+            {
+                int[] temp = new int[Block8x8.Size];
+                block.CopyTo(temp);
+                TransformFDCTInplace(temp);
+                var result = default(Block8x8);
+                result.LoadFrom(temp);
+                return result;
+            }
+
+            public static Block8x8 TransformIDCT(ref Block8x8 block)
+            {
+                int[] temp = new int[Block8x8.Size];
+                block.CopyTo(temp);
+                TransformIDCTInplace(temp);
+                var result = default(Block8x8);
+                result.LoadFrom(temp);
+                return result;
+            }
+            
             /// <summary>
             /// Performs a forward DCT on an 8x8 block of coefficients, including a level shift.
             /// Leave results scaled up by an overall factor of 8.