[Lcms-user] What does cmsFLAGS_NOOPTIMIZE actually do?

[Boudewijn R. <bo...@va...>]

Hi!

I've had a long discussion with Elle Stone who did a huge comparison of the results image applications have when converting between 16 bits linear rgb and srgb. It turned out that Krita gave different results from tifficc, so I dug in a bit and found that the reason was that I didn't use the cmsFLAGS_NOOPTIMIZE flag when creating transforms, while tifficc does. Code like this:


    quint16 src[4];
    src[0] = 257;
    src[1] = 257;
    src[2] = 257;
    src[3] = 65535;

    quint16 dst[4];

    cmsHPROFILE sRgbProfile = cmsCreate_sRGBProfile();
    QByteArray rawData = linearRgb->profile()->rawData(); // from krita... it's a linear scRGB profile, as attached.
    cmsHPROFILE linearRgbProfile = cmsOpenProfileFromMem((void*)rawData.constData(), rawData.size());

    cmsHTRANSFORM tf = cmsCreateTransform(linearRgbProfile,
                                          TYPE_BGRA_16,
                                          sRgbProfile,
                                          TYPE_BGRA_16,
                                          INTENT_RELATIVE_COLORIMETRIC,
                                          cmsFLAGS_NOOPTIMIZE | cmsFLAGS_BLACKPOINTCOMPENSATION);

    cmsDoTransform(tf, (quint8*)&src, (quint8*)&dst, 1);

    qDebug() << dst[0] << dst[1] << dst[2];


returns 3266,3266,3266, while 


    cmsHTRANSFORM tf = cmsCreateTransform(linearRgbProfile,
                                          TYPE_BGRA_16,
                                          sRgbProfile,
                                          TYPE_BGRA_16,
                                          INTENT_RELATIVE_COLORIMETRIC,
                                          cmsFLAGS_BLACKPOINTCOMPENSATION);

returns 1595,1595,1595

Which surprises me, since I would have thought optimization wouldn't have any influence on the actual results obtained. Is this a bug, or should I always use cmsFLAGS_NOOPTIMIZE to get accurate results?

-- 
Boudewijn Rempt
http://www.valdyas.org, http://www.krita.org, http://www.boudewijnrempt.nl