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

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

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