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

[Derek B. N. <de...@gl...>]

Thanks, that makes sense.

- Derek


On 2017 Oct 23, Martí Maria wrote:
> 
> 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
>>
>>
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