Hi, At first I have to clarify a very common misconception: there is no such thing like a "linear" sRGB. sRGB is a color space very precisely defined.
See http://www.color.org/chardata/rgb/srgb.pdf http://www.color.org/chardata/rgb/srgb.xalter If you define a colorspace that has transfer functions of gamma 1.0, primaries Rec709 and D65 as white point you cannot call it "sRGB", just because this is not sRGB but something close but not sRGB. At that point, lcms2, as most of the modern CMM, uses what ICC describe as "smart profiles, dumb CMM". In this approach the profiles instructs the CMM how to do the gamut remapping and the CMM blindly obeys what the profile says. Then, looking at your code, you instruct the CMM to do exactly what you get: to apply a tone curves and to remain inside a set of primaries. So, it comes at no surprise you get no gamut mapping, it is just that you didn't tell the CMM how to do it. Let me show an example of what cold be done with a proper profile with embedded gamut mapping. Instead of using the simple cmsCreateRGBProfile () built-in, please download and try a real profile. You got some good sRGB ones from here: http://www.color.org/srgbprofiles.xalter#v4pref Please try the preference sRGB. With ICC trans you can check the profile without any coding. BTW, obviously you cannot use the same sRGB as the input, that would make impossible to encode out of gamut colors, and doing (0.5, 0.5, 1000) clips the value on the input - it doesn't even reach the profile. transicc -i*Lab -o sRGB_v4_ICC_preference.icc LittleCMS ColorSpace conversion calculator - 4.2 [LittleCMS 2.04] Enter values, 'q' to quit L*? 50 a*? -128 b*? 23 R=0.0000 G=198.5075 B=124.5013 I'm converting an extremely saturated green to a in-gamut sRGB value by using gamut remapping. If I try the same by using just plain conversion: transicc -i*Lab -o *sRGB LittleCMS ColorSpace conversion calculator - 4.2 [LittleCMS 2.04] Enter values, 'q' to quit L*? 50 a*? -125 b*? 3 R=-900.4487 G=154.3234 B=109.4446 Quite different green! lcms2 unbounded mode gives you the RGB without clipping, please note the R=-900!! Please compare (0,154, 109) with (0, 199, 125). It would be nice if you get a chance to see the original Lab=(50, -128, 23) but I'm afraid no device can reproduce it because its extreme saturation. Hope that helps Regards Marti Maria The LittleCMS Project http://www.littlecms.com El 07/09/2012 2:15, Wenzel Jakob escribió: > 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); > } > > ------------------------------------------------------------------------------ > Live Security Virtual Conference > Exclusive live event will cover all the ways today's security and > threat landscape has changed and how IT managers can respond. Discussions > will include endpoint security, mobile security and the latest in malware > threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ > _______________________________________________ > Lcms-user mailing list > Lcms-user@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/lcms-user > ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
