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
scRGB.icm
Description: application/vnd.iccprofile
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