Re: [Lcms-user] Trying to make a UI for colorspaces, have some questions.

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

Hi,

> Okay, so you are suggesting I try to take a set of colors, generated spaced
> out over the different channels(so if I were to make 10 samples per
> channel, RGB would give me 10³ colors and CMYK would give me 10⁴ colors...
> perhaps a bit too many samples). Then transform those colors to XYZ(using
> relative colorimetric mapping as indicated by your suggestion of the AtoB1
> tag), then to xyY, find the outliers, draw a polygon around them, and then
> use that to give feedback on the gamut?

Yes, could work. Just use 5^3 or 5^4 points, would be be enough for  
this purpose.


> That still doesn't help me because when I use this code, which is the only
> version of the code that compiles and doesn't crash(it crashes when I try
> to read the PCS as an array)...
> if (cmsIsTag(d->profile, cmsSigColorantTableTag)) {
>            d->namedColorList = ((cmsNAMEDCOLORLIST *)cmsReadTag
> (d->profile, cmsSigColorantTableTag));
>            for (cmsUInt16Number
> i=0;i<cmsNamedColorCount(d->namedColorList);i++) {
>                char name;
>                char prefix;
>                char suffix;
>                cmsUInt16Number pcs;
>                cmsUInt16Number col;
>                cmsNamedColorInfo(d->namedColorList, i, &name, &prefix,
> &suffix, &pcs, &col);
>                qDebug()<<d->name<<i<<","<< name<<","<< prefix<<","<<
> suffix;
>                if (pcs){qDebug()<<pcs;} else {qDebug()<<"no pcs
> retrieved";}
>            }
>        }
> }



Try this, with the sizes the manual says:


>> Name: Pointer to a 256-char array to get the name, NULL to ignore.
>> Prefix: Pointer to a 33-char array to get the prefix, NULL to ignore
>> Suffix: Pointer to a 33-char array to get the suffix, NULL to ignore.
>> PCS: Pointer to a 3-cmsUInt16Number to get the encoded PCS, NULL to ignore
>> Colorant: Pointer to a 16-cmsUInt16Number to get the encoded Colorant,
>> NULL to ignore


  for (cmsUInt16Number i=0;i<cmsNamedColorCount(d->namedColorList);i++) {
    char name[256];
    char prefix[33];
    char suffix[33];
    cmsUInt16Number pcs[3];
    cmsUInt16Number col[16];
    cmsNamedColorInfo(d->namedColorList, i, name, prefix, suffix, pcs, col);


> Well, I am not very interested in the exact implementation either. As a
> Krita programmer, I need three things: A way to linearise values from a
> color in a given color space, a way to de-linearise linear values into
> colors in a given colorspace(both for luminance calculations in filters),

Okay, so you need 2 profiles. One describing the space linearized and
another describing the space delinearized. Use color transforms and floating
point to convert between spaces and forgot messing out with TRC.
Hacking the internal of profiles is always a bad idea.

For the use feedback, you can use softproofing, but it is hard to say...
if done porperly you should see no differences.

> Sorry if I am asking too many questions, but I am not very experienced with
> programming and in these particular cases it seems there's not many people
> who have come before me.

Your questions are interesting and you are welcome.
Regards
Marti