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

[Kai-Uwe B. <ku...@gm...>]

Am 29.08.2015 um 21:54 schrieb Wolthera:
> On Sat, Aug 29, 2015 at 7:42 PM, <mar...@li...> wrote:
>> Regarding the question on gamut, the "good" way to represent a gamut is by
>> using a 3D solid, just because our vision have 3 cones that gives us three
>> dimensions. The tonge chromaticity diagram can give an idea of the
>> primaries, but does not explain how the gray axis affects the color.

> I know that, but right now a lot of artists already find the color
> management terrifying, displaying a 3d gamut would be something I would
> like postpone until the general krita-user community has gotten used to the
> idea that colors have spaces. I've already had a lot of people freak out at
> the notion of giving an explanation of what certain properties mean for a
> profile, and this is outside the reactions we get on Krita having profiles
> at all(after all, there's no need for anything besides a radio button box
> with 'RGB' and 'CMYK' for proper artists according to these people). I
> imagine giving people a 3d color gamut at this current stage will cause
> heart-attacks. Still, it seems the same kind of problems need to be solved
> anyhow.

Just imagine at how Krita will want to present colour spaces in 3D
(hint: CIE*Lab) and implement a 2D CIE*Lab graph for now.

>> A way to paint the gamut would be to use the AtoB1 tag, transform a coarse
>> sampling of all RGB or CMYK space and then use some sort of convex hull
>> algorithm, or alpha shapes, to build the solid on Lab. This may also work
>> for the tonge, it is just a matter of converting to xyY and take the xy
>> coordinates. Making a color transform from the profile to XYZ identity does
>> most of the work. Then there is a function to convert XYZ -> xyY.
>>
>>
> 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?

In Oyranos is a more simple approch implemented [1]. This tool generates
a saturation line going around from the primary colours over the
secondary ones (R->RG->G->GB->B->BR->R ; same for Cmyk) and convert that
through the according RGB or Cmyk profile to CIE*Lab. The CIE*Lab
colours can then directly drawn as coordinates to the 2D graph. All math
is integer based to get the desired clipping.

-- 
kind regards
Kai-Uwe

www.oyranos.org

[1] search for oyranos-profile-graph; It is used in KolorManager and ICC
Examin