[Lcms-user] RGB to XYZ conversion

[Adrian Knagg-B. <aje...@gm...>]

Hello,

I'm trying to convert data from a source RGB colorspace to HSLuv (that
color space is a bit niche but I specifically want to use it for a
saturation stretching function). I did an initial test using the C
implementation from hsluv.org and sRGB source data and it works well,
but I want to be able to cope with any RGB source profile so my plan
was to use LittleCMS to convert the source data to XYZ and then use
the hsluv.org functions to convert XYZ to HSLuv.

However I'm getting unexpected results from the initial conversion to
XYZ and I wonder if I'm doing something wrong. Here's what I'm trying,
first I make a transform:
```
    cmsHPROFILE xyz = cmsCreateXYZProfile();
    cmsUInt32Number formatrgb = TYPE_RGB_FLT_PLANAR;
    cmsUInt32Number formatxyz = TYPE_XYZ_FLT_PLANAR;
    cmsHTRANSFORM workingtoxyz = cmsCreateTransform(gfit.icc_profile,
formatrgb, xyz, formatxyz, com.pref.icc.processing_intent, 0);
     cmsCloseProfile(xyz);
```
gfit.icc_profile is a cmsHPROFILE and contains a sRGB profile, and
com.pref.processing_intent is set to INTENT_PERCEPTUAL: those work
fine throughout the rest of the code.
But when I apply it with cmsDoTransform() the effect on an image looks
completely wrong compared with my initial test using hsluv's rgb2xyz()
function. I tested the RGB to XYZ conversion using the following:
```
    float test_rgb[] = {0.75f, 0.5f, 0.3f};
    float test_xyz[];
    printf("RGB test: %f %f %f\n", test_rgb[0], test_rgb[1], test_rgb[2]);
    hsluv_rgbtoxyz(test_rgb[0], test_rgb[1], test_rgb[2],
&test_xyz[0], &test_xyz[1], &test_xyz[2]);
    printf("HSLuv rgb2xyz: %f %f %f\n", test_xyz[0], test_xyz[1], test_xyz[2]);
     cmsDoTransform(workingtoxyz, (void*) &test_rgb, (void*) &test_xyz, 1);
    printf("LCMS rgb2xyz: %f %f %f\n", test_xyz[0], test_xyz[1], test_xyz[2]);
```
And the results are completely different:

HSLuv rgb2xyz: 0.305239 0.269471 0.105229
LCMS rgb2xyz: 0.160394 0.137083 0.040251

I assume I'm doing something wrong but I can't see what - grateful for
any help you can offer.

Thanks,

Adrian.