Hello,
I'm trying to use lcms2 to convert sRGB values from a linear double
precision representation into a gamma-corrected 8-bit representation
(using the sRGB-specific gamma correction curve). For in-gamut values,
that is a basic operation that doesn't really require a CMS.
I'm more interested in how out-of-gamut values are treated (e.g. when
one of the double precision values is greater than one).
Can lcms be configured to handle this using one of the the four intent
modes, for instance to desaturate the color instead of just clamping
each component separately?
I've played around a bit (source attached) but was unable to produce
such behaviour. In all cases, lcms2 just clamped the individual
components without showing signs of doing some kind of more
sophisticated projection onto the gamut. This can be a problem because
clamping the components will lead to a hue shift, which may be
unintended. Is it just a problem of my setup, or am I expecting
something that lcms2 does not currently do?
I've never used this library before, so any advice would be much
appreciated!
Thank you very much,
Wenzel
=====================
#include <lcms2.h>
#include <stdio.h>
#include <stdint.h>
/* Create a linear sRGB profile (based on cmsCreate_sRGBProfile)*/
cmsHPROFILE create_sRGBProfileLinear() {
cmsCIExyY D65;
cmsCIExyYTRIPLE Rec709Primaries = {
{0.6400, 0.3300, 1.0},
{0.3000, 0.6000, 1.0},
{0.1500, 0.0600, 1.0}
};
cmsWhitePointFromTemp(&D65, 6504);
cmsToneCurve* gamma = cmsBuildGamma(NULL, 1.0);
cmsToneCurve* gamma3[] = { gamma, gamma, gamma };
return cmsCreateRGBProfile(&D65, &Rec709Primaries, gamma3);
}
int main(void) {
cmsHPROFILE profile_sRGB = cmsCreate_sRGBProfile();
cmsHPROFILE profile_sRGB_linear = create_sRGBProfileLinear();
cmsHTRANSFORM transform = cmsCreateTransform(
profile_sRGB, TYPE_RGB_DBL, profile_sRGB, TYPE_RGB_8,
INTENT_RELATIVE_COLORIMETRIC, 0);
{
/* Inside of gamut: prints "188 188 188" -- that looks good. */
double a[] = {0.5, 0.5, 0.5};
uint8_t out[3];
cmsDoTransform(transform, a, out, 1);
printf("%i %i %i\n", (int) out[0], (int) out[1], (int) out[2]);
}
{
/* Outside of gamut: prints "188 188 255" ?? -- I would have
expected that the other channels would be scaled down due
to gamut mapping. */
double a[] = {0.5, 0.5, 1000};
uint8_t out[3];
cmsDoTransform(transform, a, out, 1);
printf("%i %i %i\n", (int) out[0], (int) out[1], (int) out[2]);
}
cmsDeleteTransform(transform);
cmsCloseProfile(profile_sRGB);
cmsCloseProfile(profile_sRGB_linear);
}
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