How can I get Look up table or conversion matrix in cmsHTRANSFORM

[lazysquid]

Hello. I'm trying to apply CMS to my renderer which is implemented using OpenGL.

Specifically I supposed generated image in GPU buffer(rendered image) is in sRGB space and I'm trying to convert that image into my monitor's RGB space inside of my fragment shader.

So I made simple transform which looks like this.

hTransform = cmsCreateTransform(profile_sRGB, TYPE_BGR_8, profile_leftMonitor, TYPE_BGR_8, INTENT_PERCEPTUAL, 0);

To accomplish CMS inside of GPU I should send matrix or LUT as form of uniform variable. So that my fragment shader can access to information about color transform .

I have basic knowldege about CMS so I thought cmsHTRANSFORM has some sort of conversion matrix or look up table to compute source to target RGB conversion.

So first thing that I tried to find was 3x3(or 4x4) conversion matrix or look up table
which convert RGB in color space A to RGB in color space B in cmsHTRANSFORM.
But I couldn't find anything like that in API documents.
Maybe I don't know the keyword related to it.
Does LCMS support LUT or 3x3 conversion matrix?

If not, is there any way to achieve CMS in my renderer?

[mm2]

Hi, what you intend to do is easy. Once you got the RGB8 to RGB8 transform, you can create a LUT table by sampling it at regular intervals. Something like:

for (r=0; r < 256; r += 16)
for (g=0; g < 256; g+=16)
for (b=0; b < 256; b+=16)
{
put r, g, b into rgb[] table
cmsDoTransform(rgb, rgb_out, 1);
keep rgb_out as LUT node
}

Then upload the LUT to the GPU and perform trilinear interpolation.

Those folks from media player classic are using this way:
https://voxelium.wordpress.com/2010/09/20/icc-color-management-in-media-player-classic-home-cinema/

Regards