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Enforce ICC D50 PCS illuminant when converting colours between profiles #2987

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Merged
JimBobSquarePants merged 2 commits into from
Oct 13, 2025
+13 −7
Merged

Enforce ICC D50 PCS illuminant when converting colours between profiles #2987

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efc4cf5
Enforce ICC D50 PCS illuminant
waacton Sep 18, 2025
819d0fc
Merge branch 'main' into icc-pcs
JimBobSquarePants Oct 12, 2025
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8 changes: 4 additions & 4 deletions src/ImageSharp/ColorProfiles/ColorProfileConverterExtensionsIcc.cs
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Expand Up @@ -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,
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@JimBobSquarePants JimBobSquarePants Sep 29, 2025

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Should we not be testing for a version number < 5 and assigning the value based on the version?

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@waacton waacton Oct 3, 2025

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Potentially, though whether the ICC code in ImageSharp can support v5+ correctly at all is another question. I wouldn't be surprised if v5-level features were only added to the reference implementation after it was ported here (and now that the reference has been refactored, could be harder to match up). Adapting between PCS illuminants seems a sensible step but it might not meet v5 spec, given how wordy section 10.2.22 spectralViewingConditionsType is for the svcn tag.

And their description on https://www.color.org/icclabs.xalter tries to emphasise how different v5 is:

this new system is not a simple "dot" release to the V4 spec and that a V5 profile is very different than a V2 or V4

in many industries, v4 (and even v2) meets existing color management needs and in these industries there will be no drive to move to adopt the v5 specification

I've still not got around to implementing full support for v4 profiles in Unicolour so I've largely buried my head in the sand for v5 🙈. I'm hedging my bets that because v4 took painstaking effort to meet, v5 surely can't be a smoother experience.

In summary I think it's a choice between:

  • Explicitly no support for v5 until (or if) it becomes more widely adopted
  • Best attempt to support v5 while acknowledging it might deviate from spec

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@JimBobSquarePants JimBobSquarePants Oct 13, 2025

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Thanks for confirming. Yeah, definitely best to deal with V4 for now and V5 if and when it's ever needed.

TargetWhitePoint = KnownIlluminants.D50Icc
});

// Normalize the source, then convert to the PCS space.
Expand Down Expand Up @@ -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);
Expand Down
8 changes: 7 additions & 1 deletion src/ImageSharp/ColorProfiles/KnownIlluminants.cs
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Expand Up @@ -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>
Expand All @@ -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>
Expand Down
4 changes: 2 additions & 2 deletions tests/ImageSharp.Tests/ColorProfiles/Icc/ColorProfileConverterTests.Icc.cs
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Expand Up @@ -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);
Expand All @@ -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);
Expand Down
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