Merge pull request #2987 from SixLabors/icc-pcs

Enforce ICC D50 PCS illuminant when converting colours between profiles

Author: JimBobSquarePants

src/ImageSharp/ColorProfiles/ColorProfileConverterExtensionsIcc.cs

@@ -60,8 +60,8 @@ internal static TTo ConvertUsingIccProfile<TFrom, TTo>(this ColorProfileConverte
         ColorProfileConverter pcsConverter = new(new ColorConversionOptions
         {
             MemoryAllocator = converter.Options.MemoryAllocator,
-            SourceWhitePoint = new CieXyz(converter.Options.SourceIccProfile.Header.PcsIlluminant),
-            TargetWhitePoint = new CieXyz(converter.Options.TargetIccProfile.Header.PcsIlluminant),
+            SourceWhitePoint = KnownIlluminants.D50Icc,
+            TargetWhitePoint = KnownIlluminants.D50Icc
         });
 
         // Normalize the source, then convert to the PCS space.
@@ -104,8 +104,8 @@ internal static void ConvertUsingIccProfile<TFrom, TTo>(this ColorProfileConvert
         ColorProfileConverter pcsConverter = new(new ColorConversionOptions
         {
             MemoryAllocator = converter.Options.MemoryAllocator,
-            SourceWhitePoint = new CieXyz(converter.Options.SourceIccProfile.Header.PcsIlluminant),
-            TargetWhitePoint = new CieXyz(converter.Options.TargetIccProfile.Header.PcsIlluminant),
+            SourceWhitePoint = KnownIlluminants.D50Icc,
+            TargetWhitePoint = KnownIlluminants.D50Icc
         });
 
         using IMemoryOwner<Vector4> pcsBuffer = converter.Options.MemoryAllocator.Allocate<Vector4>(source.Length);

src/ImageSharp/ColorProfiles/KnownIlluminants.cs

@@ -9,6 +9,7 @@ namespace SixLabors.ImageSharp.ColorProfiles;
 /// </summary>
 /// <remarks>
 /// Coefficients taken from: http://www.brucelindbloom.com/index.html?Eqn_ChromAdapt.html
+/// and https://color.org/specification/ICC.1-2022-05.pdf
 /// <br />
 /// Descriptions taken from: http://en.wikipedia.org/wiki/Standard_illuminant
 /// </remarks>
@@ -30,10 +31,15 @@ public static class KnownIlluminants
     public static CieXyz C { get; } = new(0.98074F, 1F, 1.18232F);
 
     /// <summary>
-    /// Gets the Horizon Light. ICC profile PCS illuminant.
+    /// Gets the Horizon Light.
     /// </summary>
     public static CieXyz D50 { get; } = new(0.96422F, 1F, 0.82521F);
 
+    /// <summary>
+    /// Gets the D50 illuminant used in the ICC profile specification.
+    /// </summary>
+    public static CieXyz D50Icc { get; } = new(0.9642F, 1F, 0.8249F);
+
     /// <summary>
     /// Gets the Mid-morning / Mid-afternoon Daylight illuminant.
     /// </summary>

tests/ImageSharp.Tests/ColorProfiles/Icc/ColorProfileConverterTests.Icc.cs

@@ -42,7 +42,7 @@ public class ColorProfileConverterTests(ITestOutputHelper testOutputHelper)
     [InlineData(TestIccProfiles.RommRgb, TestIccProfiles.StandardRgbV4)] // CMYK -> LAB -> CMYK (different bit depth v2 LUTs, 16-bit vs 8-bit)
     [InlineData(TestIccProfiles.Fogra39, TestIccProfiles.StandardRgbV2, 0.0005)] // CMYK -> LAB -> XYZ -> RGB (different LUT tags, A2B vs TRC) --- tolerance slightly higher due to difference in inverse curve implementation
     [InlineData(TestIccProfiles.StandardRgbV2, TestIccProfiles.Fogra39)] // RGB -> XYZ -> LAB -> CMYK (different LUT tags, TRC vs A2B)
-    public void CanConvertIccProfiles(string sourceProfile, string targetProfile, double tolerance = 0.00005)
+    public void CanConvertIccProfiles(string sourceProfile, string targetProfile, double tolerance = 0.000005)
     {
         List<Vector4> actual = Inputs.ConvertAll(input => GetActualTargetValues(input, sourceProfile, targetProfile));
         AssertConversion(sourceProfile, targetProfile, actual, tolerance, testOutputHelper);
@@ -63,7 +63,7 @@ public void CanConvertIccProfiles(string sourceProfile, string targetProfile, do
     [InlineData(TestIccProfiles.Fogra39, TestIccProfiles.StandardRgbV2, 0.0005)] // CMYK -> LAB -> XYZ -> RGB (different LUT tags, A2B vs TRC) --- tolerance slightly higher due to difference in inverse curve implementation
     [InlineData(TestIccProfiles.StandardRgbV2, TestIccProfiles.Fogra39)] // RGB -> XYZ -> LAB -> CMYK (different LUT tags, TRC vs A2B)
     [InlineData(TestIccProfiles.Issue129, TestIccProfiles.StandardRgbV4)] // CMYK -> LAB -> -> XYZ -> RGB
-    public void CanBulkConvertIccProfiles(string sourceProfile, string targetProfile, double tolerance = 0.00005)
+    public void CanBulkConvertIccProfiles(string sourceProfile, string targetProfile, double tolerance = 0.000005)
     {
         List<Vector4> actual = GetBulkActualTargetValues(Inputs, sourceProfile, targetProfile);
         AssertConversion(sourceProfile, targetProfile, actual, tolerance, testOutputHelper);