Re: [Lcms-user] How does a transformation work

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

Ok, 

The procedure to create a gray profile is simple. 
First measure the transfer function of your gray device to XYZ. That could be a gamma curve or anything.
Then measure the white point of your gray space. The adaptation to D50 is already done by lcms.
Then create a tone curve with your transfer function. 
Finally, by using white point and curve, create a profile.

Please note you cannot map contone values for input to output since the output profile is unknown. So, you cannot map 0 to 40 and 255 to 240. You can only characterize which XYZ you get with gray = 0 and gray = 255.

Example:

    cmsToneCurve* tone;
    cmsUInt16Number curve[256];
    int i;
    cmsHPROFILE hProfile;

    for (i = 0; i < 256; i++)
    {        
        curve[i] = (cmsUInt16Number) round(65535.0 * pow(i / 255.0, 2.2));
    }

    tone = cmsBuildTabulatedToneCurve16(0, 256, curve);

    hProfile = cmsCreateGrayProfile(cmsD50_xyY(), tone);
    cmsFreeToneCurve(tone);

    cmsSaveProfileToFile(hProfile, "gray_test.icc");
    cmsCloseProfile(hProfile);

Regards

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


> -----Original Message-----
> From: flap <fb...@oh...>
> Sent: Friday, December 29, 2023 2:21 PM
> To: lcm...@li...
> Subject: Re: [Lcms-user] How does a transformation work
> 
> Am Freitag, 29. Dezember 2023, 14:01:40 CET schrieb
> mar...@li...:
> > Hi,
> >
> > From your explanation, it seems to me that the transformation you want
> > has
> nothing to do with color management. You
> > know nothing about the L* of your grayscale and need only some
> > scaling. I
> would rather use simple math in this case:
> >
> > Gray_out = (Gray_in * 40) /51 + 40;
> >
> > You could just use a for() loop. This is going to be faster that using
> > any
> color management routines.
> 
> Hmm. It just was a test to understand how it (may) works.
> The profile I create is used in the muPDF library to create a greyscale out from
> a PDF document. I would expect, any kind of PDF could be rasterized into a
> greyscale raster this way.
> 
> Cheers,
> Jürgen
> 
> 
> 
> 
> 
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