Re: [Lcms-user] RGB-to-XYZ-to-RGB weirdness

[Martí M. <mar...@li...>]

Hi,

XYZ gamut is huge, so using tetrahedral interpolation from XYZ to RGB is 
likely to result in errors. I would use cmsFLAGS_NOOPTIMIZE in xyzToRGB 
color transform to force all the math.

Another option is to use float or double values.

Regards

Marti


On 10/20/2017 9:50 PM, Derek B. Noonburg wrote:
> I'm working on some color management code, using LCMS2, and I'm running
> into some behavior that I don't understand.
>
> If I convert sRGB to XYZ and then back to sRGB, I get significantly
> different RGB values at the output.
>
> If I do sRGB -> Lab -> sRGB, I get exactly the same values back.
>
> The really weird part is: if I convert XYZ to Lab and then to sRGB (sRGB
> -> XYZ -> Lab -> sRGB), I get the expected RGB values again.  So the
> sRGB-to-XYZ conversion is fine, but the XYZ-to-sRGB conversion is wrong.
> Or, more likely, I'm doing something wrong...  but I'm not sure what.
>
> Can someone explain what's going on here?
>
> I'm running LCMS 2.8 on Linux.  Here's my test program:
>
> #include <stdio.h>
> #include <lcms2.h>
>
> int main(int argc, char *argv[]) {
>    cmsHPROFILE rgbProfile = cmsCreate_sRGBProfile();
>    cmsHPROFILE labProfile = cmsCreateLab2Profile(NULL);
>    cmsHPROFILE xyzProfile = cmsCreateXYZProfile();
>    cmsHTRANSFORM rgbToLab = cmsCreateTransform(rgbProfile, TYPE_RGB_8,
> 					      labProfile, TYPE_Lab_16,
> 					      INTENT_ABSOLUTE_COLORIMETRIC,
> 					      0);
>    cmsHTRANSFORM labToRGB = cmsCreateTransform(labProfile, TYPE_Lab_16,
> 					      rgbProfile, TYPE_RGB_8,
> 					      INTENT_ABSOLUTE_COLORIMETRIC,
> 					      0);
>    cmsHTRANSFORM rgbToXYZ = cmsCreateTransform(rgbProfile, TYPE_RGB_8,
> 					      xyzProfile, TYPE_XYZ_16,
> 					      INTENT_ABSOLUTE_COLORIMETRIC,
> 					      0);
>    cmsHTRANSFORM xyzToRGB = cmsCreateTransform(xyzProfile, TYPE_XYZ_16,
> 					      rgbProfile, TYPE_RGB_8,
> 					      INTENT_ABSOLUTE_COLORIMETRIC,
> 					      0);
>
>    unsigned char rgbIn[3] = {0x6a, 0x3c, 0x31};
>
>    // rgb1 matches rgbIn
>    unsigned short lab1[3];
>    unsigned char rgb1[3];
>    cmsDoTransform(rgbToLab, rgbIn, lab1, 1);
>    cmsDoTransform(labToRGB, lab1, rgb1, 1);
>    printf("RGB %02x %02x %02x -> Lab %04x %04x %04x -> RGB %02x %02x %02x\n",
> 	 rgbIn[0], rgbIn[1], rgbIn[2],
> 	 lab1[0], lab1[1], lab1[2],
> 	 rgb1[0], rgb1[1], rgb1[2]);
>
>    // rgb2 does not match rgbIn
>    unsigned short xyz2[3];
>    unsigned char rgb2[3];
>    cmsDoTransform(rgbToXYZ, rgbIn, xyz2, 1);
>    cmsDoTransform(xyzToRGB, xyz2, rgb2, 1);
>    printf("RGB %02x %02x %02x -> Lab %04x %04x %04x -> RGB %02x %02x %02x\n",
> 	 rgbIn[0], rgbIn[1], rgbIn[2],
> 	 xyz2[0], xyz2[1], xyz2[2],
> 	 rgb2[0], rgb2[1], rgb2[2]);
>
>    // rgb3 matches rgbIn
>    cmsCIEXYZ xyzF;
>    cmsCIELab labF;
>    unsigned short lab3[3];
>    unsigned char rgb3[3];
>    cmsXYZEncoded2Float(&xyzF, xyz2);
>    cmsXYZ2Lab(NULL, &labF, &xyzF);
>    cmsFloat2LabEncoded(lab3, &labF);
>    cmsDoTransform(labToRGB, lab3, rgb3, 1);
>    printf("RGB %02x %02x %02x -> XYZ %04x %04x %04x -> Lab %04x %04x %04x -> RGB %02x %02x %02x\n",
> 	 rgbIn[0], rgbIn[1], rgbIn[2],
> 	 xyz2[0], xyz2[1], xyz2[2],
> 	 lab3[0], lab3[1], lab3[2],
> 	 rgb3[0], rgb3[1], rgb3[2]);
> }
>
> - Derek
>
>
> ------------------------------------------------------------------------------
> Check out the vibrant tech community on one of the world's most
> engaging tech sites, Slashdot.org! http://sdm.link/slashdot
> _______________________________________________
> Lcms-user mailing list
> Lcm...@li...
> https://lists.sourceforge.net/lists/listinfo/lcms-user
>