Support for Oklab/Oklch?

[sommerluk]

Oklab is an alternative to Cielab. It works with the same logic, but claims to be more perceptually uniform than Cielab. It uses a D65 whitepoint. There are conversions defined from and to XYZ-D65 (a matrix + a cubic root respective power of three + another matrix).

Since its inclusion in the CSS Color Module Level 4 [W3C Candidate Recommendation Draft] it’s gaining wider support.

Would it be possible to support Oklab (and its derived cylindrical variant, Oklch) in LittleCMS out-of-the-box? So that pipelines can directly convert between Oklab/Oklch and an arbitrary ICC profile?

[sommerluk]

Here is the definition of Oklab: https://bottosson.github.io/posts/oklab/

[mm2]

That seems a nice addition, and would be as easy as adding a color space profile bult-in. What I need is the set equations to convert CIE Lab* to this space and vice-versa. Also it would need some sort of chromatic adaptation to convert from D65 to D50, which is where ICC colorimetry is based.

[sommerluk]

That’s great to hear!

Currently, I’m doing this conversion in my own code. Probably this is not directly useful for you, because it’s hard-code and depends on C++ and Qt. But those are the steps:

Conversion from an arbitrary ICC profile to Oklch

Arbitrary ICC profile → XYZ-D50

This is done with LittleCMS as usual. Let’s call the result V₁.

XYZ-D50 → XYZ-D65

Let’s call the result V₂. The chromatic adaption is done with the Bradford transformation, as proposed here. Formula: V₂ = M⁻¹ × V₁.

XYZ-D65 → Oklab

The transformation is done as described here in the Oklab definition. [Note that l, m and s might be negative. Therefore, in my own code, I do not use std::pow(num, 1.0 / 3) which would not work for a negative num. Instead, I use std::cbrt(num) which is defined also for a negative num.]

Oklab → Oklch

This can be done via void cmsLab2LCh(cmsCIELCh*LCh, const cmsCIELab* Lab)

Conversion from Oklch to an arbitrary ICC profile

Oklch → Oklab

This can be done via void cmsLCh2Lab(cmsCIELab* Lab, const cmsCIELCh* LCh)

Oklab → XYZ-D65

The transformation is done as described here in the Oklab definition. Let’s call the result V₂.

XYZ-D65 → XYZ-D50

Let’s call the result V₁. The chromatic adaption is done with the Bradford transformation, as proposed here. Formula: V₁ = M × V₂.

XYZ-D50 → Arbitrary ICC profile

This is done with LittleCMS as usual.

[mm2]

@sommerluk In f4e9f9122a001b71219d9b2d2a6ec5b8c784d785 you can see a preliminary implementation of OkLab space. This built-in profile cannot be saved as an ICC file, so its use is programmatically only.

It is a colorspace profile, so It can deal with OkLab-> whatever and whatever->OkLab. Note that it uses D50 as PCS white point.

The stages are:

[Conversion D60 to D65] ->[Conversion to LMS cone space] -> [Non-linearity] -> [Final matrix to OkLab] The input direction pipeline is like this one but reversed.

I still have to figure out how to deal with white point in unadapted absolute colorimetric, probably a D65 should be used as media white. It should do its way to 2.16, which is still months ahead.

Thanks again for the idea.

[sommerluk]

That's great! I've downloaded it and played around, and it works fine.

Maybe it makes sense to add

#define TYPE_OKLAB_DBL          (FLOAT_SH(1)|COLORSPACE_SH(PT_MCH3)|CHANNELS_SH(3)|BYTES_SH(0))

to the public header?

Thanks for implementing this as fast! That's a super-conveniant feature!

[mm2]

Full support is now added by 6cabbcec8c229bf09dfa13b2b68eb0ab9f1030c8

[sommerluk]

That's great. Thanks a lot!

[killyorz]

int main()
{
    cmsSetLogErrorHandlerTHR(nullptr, log);
    cmsContext ctx = cmsCreateContext(nullptr, nullptr);

    cmsCIExyY D65xyY;
    cmsWhitePointFromTemp( &D65xyY, 6504);
    cmsHPROFILE oklabProfile = cmsCreateLab4Profile(&D65xyY);
    setupMetadata(ctx, oklabProfile);

    // Strict transformation between LAB and XYZ
    cmsSetDeviceClass(oklabProfile, cmsSigColorSpaceClass);
    cmsSetColorSpace(oklabProfile, cmsSigMCH3Data);
    cmsSetPCS(oklabProfile, cmsSigXYZData);
    cmsSetHeaderRenderingIntent(oklabProfile, INTENT_RELATIVE_COLORIMETRIC);

    const double M_D65_D50[] =
    {
       1.047886, 0.022919, -0.050216,
       0.029582, 0.990484, -0.017079,
      -0.009252, 0.015073,  0.751678
    };

    const double M_D50_D65[] =
    {
         0.955512609517083, -0.023073214184645,  0.063308961782107,
        -0.028324949364887,  1.009942432477107,  0.021054814890112,
         0.012328875695483, -0.020535835374141,  1.330713916450354
    };

    cmsStage* D65toD50 = cmsStageAllocMatrix(ctx, 3, 3, M_D65_D50, NULL);
    cmsStage* D50toD65 = cmsStageAllocMatrix(ctx, 3, 3, M_D50_D65, NULL);

    const double M_D65_LMS[] =
    {
        0.819022437996703, 0.3619062600528904, -0.1288737815209879,
        0.03298365393238847, 0.9292868615863434, 0.03614466635064236,
        0.04817718935962421, 0.2642395317527308, 0.6335478284694309
    };

    const double M_LMS_D65[] =
    {
        1.226879875845924, -0.5578149944602171, 0.2813910456659647,
       -0.04057574521480083, 1.112286803280317, -0.07171105806551635,
       -0.07637293667466008, -0.42149333240224324, 1.5869240198367818
    };

    cmsStage* D65toLMS = cmsStageAllocMatrix(ctx, 3, 3, M_D65_LMS, NULL);
    cmsStage* LMStoD65 = cmsStageAllocMatrix(ctx, 3, 3, M_LMS_D65, NULL);

    const double RootParameters[] = {1.0 / 3.0, 1, 0, 0};
    const double CubeParameters[] = {3.0, 1, 0, 0};

    cmsToneCurve* Root = cmsBuildParametricToneCurve(ctx, 6, RootParameters);
    cmsToneCurve* Cube = cmsBuildParametricToneCurve(ctx, 6, CubeParameters);

    cmsToneCurve* Roots[3] = { Root, Root, Root };
    cmsToneCurve* Cubes[3] = { Cube, Cube, Cube };

    cmsStage* NonLinearityFw = cmsStageAllocToneCurves(ctx, 3, Roots);
    cmsStage* NonLinearityRv = cmsStageAllocToneCurves(ctx, 3, Cubes);

    const double M_LMSprime_OkLab[] =
    {
        0.21045426830931396, 0.7936177747023053, -0.0040720430116192585,
        1.9779985324311686, -2.42859224204858, 0.450593709617411,
        0.025904042465547734, 0.7827717124575297, -0.8086757549230774
    };

    const double M_OkLab_LMSprime[] =
    {
        1.0, 0.3963377773761749, 0.21580375730991364,
        1.0, -0.10556134581565857, -0.0638541728258133,
        1.0, -0.08948417752981186, -1.2914855480194092
    };

    cmsStage* LMSprime_OkLab = cmsStageAllocMatrix(ctx, 3, 3, M_LMSprime_OkLab, NULL);
    cmsStage* OkLab_LMSprime = cmsStageAllocMatrix(ctx, 3, 3, M_OkLab_LMSprime, NULL);

    //LAB -> XYZD50
    cmsPipeline* AToB = cmsPipelineAlloc(ctx, 3, 3);
    cmsPipelineInsertStage(AToB, cmsAT_END, cmsStageDup(OkLab_LMSprime));      // Matrix = LAB -> LMS
    cmsPipelineInsertStage(AToB, cmsAT_END, cmsStageDup(NonLinearityRv));
    cmsPipelineInsertStage(AToB, cmsAT_END, cmsStageDup(LMStoD65));            // Matrix = LMS -> XYZ
    cmsPipelineInsertStage(AToB, cmsAT_END, cmsStageDup(D65toD50));            // Matrix = D65 -> D50
    cmsWriteTag(oklabProfile, cmsSigDToB0Tag, AToB);

    //XYZD50 -> LAB
    cmsPipeline* BToA = cmsPipelineAlloc(ctx, 3, 3);
    cmsPipelineInsertStage(BToA, cmsAT_END, cmsStageDup(D50toD65));            // Matrix = D50 -> D65
    cmsPipelineInsertStage(BToA, cmsAT_END, cmsStageDup(D65toLMS));            // Matrix = XYZ -> LMS
    cmsPipelineInsertStage(BToA, cmsAT_END, cmsStageDup(NonLinearityFw));
    cmsPipelineInsertStage(BToA, cmsAT_END, cmsStageDup(LMSprime_OkLab));      // Matrix = LMS -> LAB
    cmsWriteTag(oklabProfile, cmsSigBToD0Tag, BToA);

    cmsPipelineFree(AToB);
    cmsPipelineFree(BToA);

    cmsFreeToneCurve(Root);
    cmsFreeToneCurve(Cube);

    if (!cmsMD5computeID(oklabProfile)) {
        std::cerr << "Failed MD5 computation" << std::endl;
        return -1;
    }

    const std::string profileName{"oklab.icc"};

    if (!cmsSaveProfileToFile(oklabProfile, profileName.c_str())) {
        std::cerr << "CANNOT WRITE PROFILE" << std::endl;
        return -2;
    }
}

Hi, @mm2, I have created an ICC profile for oklab in this situation, but I found that when a or b is negative, when using cmsCreateTransform to convert oklab to an integer colour space, such as rgb8 or rgb16, the result of the conversion seems to be incorrect (a and b seem to be clipped to 0 before the conversion) . But when converting to floating point such as rgbF16 or rgbF32 the result is correct, could the error be caused by Optimisation? Thanks a lot!

[mm2]

You are including only BtoD /DToB tags which according the spec are to be used as a complement specialized on floats. You should use AtoB/BToA tags and if you want more precision on floats then also add DToB/BToD . If you could use unstable, 2.16 has an OkLab implementation on cmsCreate_OkLabProfile(). You can look at the code too.

[killyorz]

Okay, thank you so much!

You are including only BtoD /DToB tags which according the spec are to be used as a complement specialized on floats. You should use AtoB/BToA tags and if you want more precision on floats then also add DToB/BToD . If you could use unstable, 2.16 has an OkLab implementation on cmsCreate_OkLabProfile(). You can look at the code too.

[killyorz]

You are including only BtoD /DToB tags which according the spec are to be used as a complement specialized on floats. You should use AtoB/BToA tags and if you want more precision on floats then also add DToB/BToD . If you could use unstable, 2.16 has an OkLab implementation on cmsCreate_OkLabProfile(). You can look at the code too.

Well, I tried to create the oklab icc profile in krita using cmsCreate_OkLabProfile(), but since krita uses cmsSaveProfileToMem, it doesn't seem to be able to do it successfully, which is really sad.

[mm2]

This particular profile cannot be saved. The ICC file format does not support the combination of stages in the pipeline. It only works as a built-in.

[killyorz]

This particular profile cannot be saved. The ICC file format does not support the combination of stages in the pipeline. It only works as a built-in.

Okay, I got it. Thank you.

[killyorz]

Hi, @mm2, I created the oklab profile using cmsCreate_OkLabProfile() but it doesn't seem to convert to rgb16 correctly, here's the code I'm testing with:

#define TYPE_LABA_F32 (FLOAT_SH(1)|COLORSPACE_SH(PT_MCH3)|EXTRA_SH(1)|CHANNELS_SH(3)|BYTES_SH(4))

    cmsUInt16Number rgb[3];
    cmsFloat32Number lab[3];

    cmsHPROFILE labProfile = cmsCreate_OkLabProfile(NULL);
    cmsHPROFILE rgbProfile = cmsCreate_sRGBProfile();

    cmsHTRANSFORM hBack = cmsCreateTransform(labProfile, TYPE_LABA_F32, rgbProfile, TYPE_RGB_16, INTENT_RELATIVE_COLORIMETRIC, 0);
    cmsHTRANSFORM hForth = cmsCreateTransform(rgbProfile, TYPE_RGB_16, labProfile, TYPE_LABA_F32, INTENT_RELATIVE_COLORIMETRIC, 0);

    cmsCloseProfile(labProfile);
    cmsCloseProfile(rgbProfile);

    rgb[0] = 0;
    rgb[1] = 0;
    rgb[2] = 65535;

    cmsDoTransform(hForth, rgb, &lab, 1);
    cmsDoTransform(hBack, lab, &rgb, 1);

    cmsDeleteTransform(hBack);
    cmsDeleteTransform(hForth);

    std::cout<<rgb[0]<<' '<<rgb[1]<<' '<<rgb[2]<< std::endl;

    //Target results: 0 0 65535
    //Actual results: 22025 22020 22012

[mm2]

Thanks for reporting. This seems not related to OkLab but on transforms going float->integer. If you use floats on rgb the roundtrip works well, so it is not the profile. I will file a bug regarding this issue, not limited to OkLab.

See below the code that works fine

`

define TYPE_LABA_F32 (FLOAT_SH(1)|COLORSPACE_SH(PT_MCH3)|EXTRA_SH(1)|CHANNELS_SH(3)|BYTES_SH(4))

cmsFloat32Number rgb[3];
cmsFloat32Number lab[4];

cmsHPROFILE labProfile = cmsCreate_OkLabProfile(NULL);
cmsHPROFILE rgbProfile = cmsCreate_sRGBProfile();

cmsHTRANSFORM hBack = cmsCreateTransform(labProfile, TYPE_LABA_F32, rgbProfile, TYPE_RGB_FLT, INTENT_RELATIVE_COLORIMETRIC, 0);
cmsHTRANSFORM hForth = cmsCreateTransform(rgbProfile, TYPE_RGB_FLT, labProfile, TYPE_LABA_F32, INTENT_RELATIVE_COLORIMETRIC, 0);

cmsCloseProfile(labProfile);
cmsCloseProfile(rgbProfile);

rgb[0] = 0;
rgb[1] = 0;
rgb[2] = 1.0f;

cmsDoTransform(hForth, rgb, &lab, 1);
cmsDoTransform(hBack, lab, &rgb, 1);

cmsDeleteTransform(hBack);
cmsDeleteTransform(hForth);

`

[killyorz]

Okay, that's great. Thanks！

[mm2]

It is now fixed by 4c0c66e7109a903c470159ecd3d753e8a4d56c79, but I have to check it more carefully.

[killyorz]

It is now fixed by 4c0c66e, but I have to check it more carefully.

Now it works fine in krita! Really thank you for your help!