Re: [Lcms-user] RGB to XYZ conversion

[<mar...@li...>]

Hello,

I have written a small program to check this and found it works fine to me.
At first glance I see a couple of things on your code, but didn't analyze in
detail. float_xyz[] do not compile, you need to set the size, which at least
should be 3. Another thing is the formatters TYPE_RGB_FLT_PLANAR and
TYPE_XYZ_FLT_PLANAR that are not in lcms2.h, therefore you have defined them
elsewhere.  Check if correct. 

Anyway, this is the program I did that gives the right values.

#include "lcms2.h"

#define TYPE_RGB_FLT_PLANAR
(FLOAT_SH(1)|COLORSPACE_SH(PT_RGB)|CHANNELS_SH(3)|BYTES_SH(4)|PLANAR_SH(1))
#define TYPE_XYZ_FLT_PLANAR
(FLOAT_SH(1)|COLORSPACE_SH(PT_XYZ)|CHANNELS_SH(3)|BYTES_SH(4)|PLANAR_SH(1))

int main()
{
    float rgb[] = { 0.75f, 0.5f, 0.3f };
    float xyz[3];

    cmsHPROFILE srgb_icc = cmsCreate_sRGBProfile();
    cmsHPROFILE xyz_icc = cmsCreateXYZProfile();

    cmsHTRANSFORM workingtoxyz = cmsCreateTransform(srgb_icc,
TYPE_RGB_FLT_PLANAR, xyz_icc, TYPE_XYZ_FLT_PLANAR, INTENT_PERCEPTUAL, 0);

    cmsCloseProfile(xyz_icc);
    cmsCloseProfile(srgb_icc);

    cmsDoTransform(workingtoxyz, rgb, xyz, 1);

    printf("LCMS rgb2xyz: %f %f %f\n", xyz[0], xyz[1], xyz[2]);
    cmsDeleteTransform(workingtoxyz);

   return 0;
}

Output:
LCMS rgb2xyz: 0.320747 0.274139 0.080336

You can verify the values using this calculator
http://www.brucelindbloom.com/index.html?ColorCalculator.html

Hope that helps

Marti Maria
The LittleCMS Project
https://www.littlecms.com



> -----Original Message-----
> From: Adrian Knagg-Baugh <aje...@gm...>
> Sent: Thursday, June 15, 2023 8:54 PM
> To: lcm...@li...
> Subject: [Lcms-user] RGB to XYZ conversion
> 
> 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.
> 
> 
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