Fix alpha companding.

src/ImageSharp/ColorSpaces/Companding/SRgbCompanding.cs

@@ -22,8 +22,8 @@ namespace SixLabors.ImageSharp.ColorSpaces.Companding
     /// </remarks>
     public static class SRgbCompanding
     {
-        private const int Length = Scale + 2;
-        private const int Scale = (1 << 14) - 1;
+        private const int Length = Scale + 2; // 256kb @ 16bit precision.
+        private const int Scale = (1 << 16) - 1;
 
         private static readonly Lazy<float[]> LazyCompressTable = new Lazy<float[]>(() =>
         {
@@ -129,10 +129,10 @@ public static unsafe void Compress(Span<Vector4> vectors)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static void Expand(ref Vector4 vector)
         {
+            // Alpha is already a linear representation of opacity so we do not want to convert it.
             vector.X = Expand(vector.X);
             vector.Y = Expand(vector.Y);
             vector.Z = Expand(vector.Z);
-            vector.W = Expand(vector.W);
         }
 
         /// <summary>
@@ -142,10 +142,10 @@ public static void Expand(ref Vector4 vector)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static void Compress(ref Vector4 vector)
         {
+            // Alpha is already a linear representation of opacity so we do not want to convert it.
             vector.X = Compress(vector.X);
             vector.Y = Compress(vector.Y);
             vector.Z = Compress(vector.Z);
-            vector.W = Compress(vector.W);
         }
 
         /// <summary>
@@ -192,7 +192,9 @@ private static unsafe void CompandAvx2(Span<Vector4> vectors, float[] table)
                     Vector256<float> low = Avx2.GatherVector256(tablePointer, truncated, sizeof(float));
                     Vector256<float> high = Avx2.GatherVector256(tablePointer, Avx2.Add(truncated, offset), sizeof(float));
 
-                    vectorsBase = Numerics.Lerp(low, high, Avx.Subtract(multiplied, truncatedF));
+                    // Alpha is already a linear representation of opacity so we do not want to convert it.
+                    Vector256<float> companded = Numerics.Lerp(low, high, Avx.Subtract(multiplied, truncatedF));
+                    vectorsBase = Avx.Blend(companded, vectorsBase, Numerics.BlendAlphaControl);
                     vectorsBase = ref Unsafe.Add(ref vectorsBase, 1);
                 }
             }
@@ -216,17 +218,15 @@ private static unsafe void CompandScalar(Span<Vector4> vectors, float[] table)
                     float f0 = multiplied.X;
                     float f1 = multiplied.Y;
                     float f2 = multiplied.Z;
-                    float f3 = multiplied.W;
 
                     uint i0 = (uint)f0;
                     uint i1 = (uint)f1;
                     uint i2 = (uint)f2;
-                    uint i3 = (uint)f3;
 
+                    // Alpha is already a linear representation of opacity so we do not want to convert it.
                     vectorsBase.X = Numerics.Lerp(tablePointer[i0], tablePointer[i0 + 1], f0 - (int)i0);
                     vectorsBase.Y = Numerics.Lerp(tablePointer[i1], tablePointer[i1 + 1], f1 - (int)i1);
                     vectorsBase.Z = Numerics.Lerp(tablePointer[i2], tablePointer[i2 + 1], f2 - (int)i2);
-                    vectorsBase.W = Numerics.Lerp(tablePointer[i3], tablePointer[i3 + 1], f3 - (int)i3);
 
                     vectorsBase = ref Unsafe.Add(ref vectorsBase, 1);
                 }

src/ImageSharp/Common/Helpers/Numerics.cs

@@ -19,7 +19,7 @@ namespace SixLabors.ImageSharp
     internal static class Numerics
     {
 #if SUPPORTS_RUNTIME_INTRINSICS
-        private const int BlendAlphaControl = 0b_10_00_10_00;
+        public const int BlendAlphaControl = 0b_10_00_10_00;
         private const int ShuffleAlphaControl = 0b_11_11_11_11;
 #endif
 

src/ImageSharp/PixelFormats/Utils/Vector4Converters.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Six Labors.
+// Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
 using System;
@@ -45,4 +45,4 @@ internal static void ApplyBackwardConversionModifiers(Span<Vector4> vectors, Pix
             }
         }
     }
-}
+}

tests/ImageSharp.Tests/PixelFormats/PixelOperations/PixelOperationsTests.cs

@@ -160,23 +160,17 @@ public void FromScaledVector4(int count)
                 (s, d) =>
                     {
                         Span<TPixel> destPixels = d.GetSpan();
-                        this.Operations.FromVector4Destructive(this.Configuration, (Span<Vector4>)s, destPixels, PixelConversionModifiers.Scale);
+                        this.Operations.FromVector4Destructive(this.Configuration, s, destPixels, PixelConversionModifiers.Scale);
                     });
         }
 
         [Theory]
         [MemberData(nameof(ArraySizesData))]
         public void FromCompandedScaledVector4(int count)
         {
-            void SourceAction(ref Vector4 v)
-            {
-                SRgbCompanding.Expand(ref v);
-            }
+            void SourceAction(ref Vector4 v) => SRgbCompanding.Expand(ref v);
 
-            void ExpectedAction(ref Vector4 v)
-            {
-                SRgbCompanding.Compress(ref v);
-            }
+            void ExpectedAction(ref Vector4 v) => SRgbCompanding.Compress(ref v);
 
             Vector4[] source = CreateVector4TestData(count, (ref Vector4 v) => SourceAction(ref v));
             TPixel[] expected = CreateScaledExpectedPixelData(source, (ref Vector4 v) => ExpectedAction(ref v));
@@ -219,7 +213,8 @@ void ExpectedAction(ref Vector4 v)
                 expected,
                 (s, d) =>
                 {
-                    PixelConversionModifiers modifiers = this.HasUnassociatedAlpha ? PixelConversionModifiers.Premultiply
+                    PixelConversionModifiers modifiers = this.HasUnassociatedAlpha
+                        ? PixelConversionModifiers.Premultiply
                         : PixelConversionModifiers.None;
 
                     this.Operations.FromVector4Destructive(this.Configuration, s, d.GetSpan(), modifiers);
@@ -254,7 +249,8 @@ void ExpectedAction(ref Vector4 v)
                 expected,
                 (s, d) =>
                 {
-                    PixelConversionModifiers modifiers = this.HasUnassociatedAlpha ? PixelConversionModifiers.Premultiply
+                    PixelConversionModifiers modifiers = this.HasUnassociatedAlpha
+                        ? PixelConversionModifiers.Premultiply
                         : PixelConversionModifiers.None;
 
                     this.Operations.FromVector4Destructive(
@@ -297,7 +293,8 @@ void ExpectedAction(ref Vector4 v)
                 expected,
                 (s, d) =>
                 {
-                    PixelConversionModifiers modifiers = this.HasUnassociatedAlpha ? PixelConversionModifiers.Premultiply
+                    PixelConversionModifiers modifiers = this.HasUnassociatedAlpha
+                        ? PixelConversionModifiers.Premultiply
                         : PixelConversionModifiers.None;
 
                     this.Operations.FromVector4Destructive(
@@ -343,7 +340,7 @@ public void Generic_To<TDestPixel>(TestPixel<TDestPixel> dummy)
 
             PixelConverterTests.ReferenceImplementations.To<TPixel, TDestPixel>(this.Configuration, source, expected);
 
-            TestOperation(source, expected, (s, d) => this.Operations.To(this.Configuration, (ReadOnlySpan<TPixel>)s, d.GetSpan()));
+            TestOperation(source, expected, (s, d) => this.Operations.To(this.Configuration, s, d.GetSpan()));
         }
 
         [Theory]
@@ -356,11 +353,11 @@ public void ToScaledVector4(int count)
             TestOperation(
                 source,
                 expected,
-                (s, d) =>
-                    {
-                        Span<Vector4> destVectors = d.GetSpan();
-                        this.Operations.ToVector4(this.Configuration, (ReadOnlySpan<TPixel>)s, destVectors, PixelConversionModifiers.Scale);
-                    });
+                (s, d) => this.Operations.ToVector4(
+                    this.Configuration,
+                    s,
+                    d.GetSpan(),
+                    PixelConversionModifiers.Scale));
         }
 
         [Theory]
@@ -369,13 +366,9 @@ public void ToCompandedScaledVector4(int count)
         {
             void SourceAction(ref Vector4 v)
             {
-                SRgbCompanding.Compress(ref v);
             }
 
-            void ExpectedAction(ref Vector4 v)
-            {
-                SRgbCompanding.Expand(ref v);
-            }
+            void ExpectedAction(ref Vector4 v) => SRgbCompanding.Expand(ref v);
 
             TPixel[] source = CreateScaledPixelTestData(count, (ref Vector4 v) => SourceAction(ref v));
             Vector4[] expected = CreateExpectedScaledVector4Data(source, (ref Vector4 v) => ExpectedAction(ref v));
@@ -396,13 +389,9 @@ public void ToPremultipliedVector4(int count)
         {
             void SourceAction(ref Vector4 v)
             {
-                Numerics.UnPremultiply(ref v);
             }
 
-            void ExpectedAction(ref Vector4 v)
-            {
-                Numerics.Premultiply(ref v);
-            }
+            void ExpectedAction(ref Vector4 v) => Numerics.Premultiply(ref v);
 
             TPixel[] source = CreatePixelTestData(count, (ref Vector4 v) => SourceAction(ref v));
             Vector4[] expected = CreateExpectedVector4Data(source, (ref Vector4 v) => ExpectedAction(ref v));
@@ -419,13 +408,9 @@ public void ToPremultipliedScaledVector4(int count)
         {
             void SourceAction(ref Vector4 v)
             {
-                Numerics.UnPremultiply(ref v);
             }
 
-            void ExpectedAction(ref Vector4 v)
-            {
-                Numerics.Premultiply(ref v);
-            }
+            void ExpectedAction(ref Vector4 v) => Numerics.Premultiply(ref v);
 
             TPixel[] source = CreateScaledPixelTestData(count, (ref Vector4 v) => SourceAction(ref v));
             Vector4[] expected = CreateExpectedScaledVector4Data(source, (ref Vector4 v) => ExpectedAction(ref v));
@@ -446,8 +431,6 @@ public void ToCompandedPremultipliedScaledVector4(int count)
         {
             void SourceAction(ref Vector4 v)
             {
-                Numerics.UnPremultiply(ref v);
-                SRgbCompanding.Compress(ref v);
             }
 
             void ExpectedAction(ref Vector4 v)
@@ -1006,15 +989,9 @@ public void ToRgba64Bytes(int count)
         [Theory]
         [MemberData(nameof(ArraySizesData))]
         public void PackFromRgbPlanes(int count)
-        {
-            SimdUtilsTests.TestPackFromRgbPlanes<TPixel>(
+            => SimdUtilsTests.TestPackFromRgbPlanes<TPixel>(
                 count,
-                (
-                    r,
-                    g,
-                    b,
-                    actual) => PixelOperations<TPixel>.Instance.PackFromRgbPlanes(this.Configuration, r, g, b, actual));
-        }
+                (r, g, b, actual) => PixelOperations<TPixel>.Instance.PackFromRgbPlanes(this.Configuration, r, g, b, actual));
 
         public delegate void RefAction<T1>(ref T1 arg1);
 
@@ -1071,7 +1048,7 @@ internal static Vector4[] CreateVector4TestData(int length, RefAction<Vector4> v
 
             for (int i = 0; i < result.Length; i++)
             {
-                Vector4 v = GetVector(rnd);
+                Vector4 v = GetScaledVector(rnd);
                 vectorModifier?.Invoke(ref v);
 
                 result[i] = v;
@@ -1088,7 +1065,7 @@ internal static TPixel[] CreatePixelTestData(int length, RefAction<Vector4> vect
 
             for (int i = 0; i < result.Length; i++)
             {
-                Vector4 v = GetVector(rnd);
+                Vector4 v = GetScaledVector(rnd);
 
                 vectorModifier?.Invoke(ref v);
 
@@ -1106,7 +1083,7 @@ internal static TPixel[] CreateScaledPixelTestData(int length, RefAction<Vector4
 
             for (int i = 0; i < result.Length; i++)
             {
-                Vector4 v = GetVector(rnd);
+                Vector4 v = GetScaledVector(rnd);
 
                 vectorModifier?.Invoke(ref v);
 
@@ -1129,10 +1106,8 @@ internal static byte[] CreateByteTestData(int length, int seed = 42)
             return result;
         }
 
-        internal static Vector4 GetVector(Random rnd)
-        {
-            return new Vector4((float)rnd.NextDouble(), (float)rnd.NextDouble(), (float)rnd.NextDouble(), (float)rnd.NextDouble());
-        }
+        internal static Vector4 GetScaledVector(Random rnd)
+            => new Vector4((float)rnd.NextDouble(), (float)rnd.NextDouble(), (float)rnd.NextDouble(), (float)rnd.NextDouble());
 
         [StructLayout(LayoutKind.Sequential)]
         internal unsafe struct OctetBytes
@@ -1177,14 +1152,22 @@ public void Verify()
                 {
                     Span<Vector4> expected = MemoryMarshal.Cast<TDest, Vector4>(this.ExpectedDestBuffer.AsSpan());
                     Span<Vector4> actual = MemoryMarshal.Cast<TDest, Vector4>(this.ActualDestBuffer.GetSpan());
+                    var comparer = new ApproximateFloatComparer(0.0001F);
 
-                    var comparer = new ApproximateFloatComparer(0.001f);
                     for (int i = 0; i < count; i++)
                     {
-                        // ReSharper disable PossibleNullReferenceException
                         Assert.Equal(expected[i], actual[i], comparer);
+                    }
+                }
+                else if (typeof(IPixel).IsAssignableFrom(typeof(TDest)))
+                {
+                    Span<TDest> expected = this.ExpectedDestBuffer.AsSpan();
+                    Span<TDest> actual = this.ActualDestBuffer.GetSpan();
+                    var comparer = new ApproximateFloatComparer(0.0001F);
 
-                        // ReSharper restore PossibleNullReferenceException
+                    for (int i = 0; i < count; i++)
+                    {
+                        Assert.Equal((IPixel)expected[i], (IPixel)actual[i], comparer);
                     }
                 }
                 else

tests/ImageSharp.Tests/TestUtilities/ApproximateFloatComparer.cs

@@ -3,6 +3,7 @@
 
 using System.Collections.Generic;
 using System.Numerics;
+using SixLabors.ImageSharp.PixelFormats;
 
 namespace SixLabors.ImageSharp.Tests
 {
@@ -12,6 +13,7 @@ namespace SixLabors.ImageSharp.Tests
     internal readonly struct ApproximateFloatComparer :
         IEqualityComparer<float>,
         IEqualityComparer<Vector2>,
+        IEqualityComparer<IPixel>,
         IEqualityComparer<Vector4>,
         IEqualityComparer<ColorMatrix>
     {
@@ -32,30 +34,42 @@ public bool Equals(float x, float y)
         }
 
         /// <inheritdoc/>
-        public int GetHashCode(float obj) => obj.GetHashCode();
+        public int GetHashCode(float obj)
+            => obj.GetHashCode();
 
         /// <inheritdoc/>
-        public bool Equals(Vector2 x, Vector2 y) => this.Equals(x.X, y.X) && this.Equals(x.Y, y.Y);
+        public bool Equals(Vector2 x, Vector2 y)
+            => this.Equals(x.X, y.X) && this.Equals(x.Y, y.Y);
 
         /// <inheritdoc/>
-        public int GetHashCode(Vector2 obj) => obj.GetHashCode();
+        public int GetHashCode(Vector2 obj)
+            => obj.GetHashCode();
 
         /// <inheritdoc/>
-        public bool Equals(Vector4 x, Vector4 y) => this.Equals(x.X, y.X) && this.Equals(x.Y, y.Y) && this.Equals(x.Z, y.Z) && this.Equals(x.W, y.W);
+        public bool Equals(IPixel x, IPixel y)
+            => this.Equals(x.ToScaledVector4(), y.ToScaledVector4());
+
+        public int GetHashCode(IPixel obj)
+            => obj.ToScaledVector4().GetHashCode();
+
+        /// <inheritdoc/>
+        public bool Equals(Vector4 x, Vector4 y)
+            => this.Equals(x.X, y.X)
+            && this.Equals(x.Y, y.Y)
+            && this.Equals(x.Z, y.Z)
+            && this.Equals(x.W, y.W);
 
         /// <inheritdoc/>
-        public int GetHashCode(Vector4 obj) => obj.GetHashCode();
+        public int GetHashCode(Vector4 obj)
+            => obj.GetHashCode();
 
         /// <inheritdoc/>
         public bool Equals(ColorMatrix x, ColorMatrix y)
-        {
-            return
-            this.Equals(x.M11, y.M11) && this.Equals(x.M12, y.M12) && this.Equals(x.M13, y.M13) && this.Equals(x.M14, y.M14)
+            => this.Equals(x.M11, y.M11) && this.Equals(x.M12, y.M12) && this.Equals(x.M13, y.M13) && this.Equals(x.M14, y.M14)
             && this.Equals(x.M21, y.M21) && this.Equals(x.M22, y.M22) && this.Equals(x.M23, y.M23) && this.Equals(x.M24, y.M24)
             && this.Equals(x.M31, y.M31) && this.Equals(x.M32, y.M32) && this.Equals(x.M33, y.M33) && this.Equals(x.M34, y.M34)
             && this.Equals(x.M41, y.M41) && this.Equals(x.M42, y.M42) && this.Equals(x.M43, y.M43) && this.Equals(x.M44, y.M44)
             && this.Equals(x.M51, y.M51) && this.Equals(x.M52, y.M52) && this.Equals(x.M53, y.M53) && this.Equals(x.M54, y.M54);
-        }
 
         /// <inheritdoc/>
         public int GetHashCode(ColorMatrix obj) => obj.GetHashCode();