[Openicc] CUPS Color Management under Linux gets into distros

[Alastair M. Robinson]

Hi :)

On 21/02/11 18:07, Hal V. Engel wrote:

> I can't find anything in either the autotools or cmake build systems for
> poppler that appear to turn CM for CMYK on or off. CM is either on or
> off and it does not appear to have any way to only turn it on for subset
> of object types or for specific output types.

Well the default backend for poppler is Cairo, and Cairo (currently, as 
far as I know) only supports RGB.  Perhaps we'd need to use the other 
backend, Splash, for CMYK support?

> But the way to get this tested is to start using it now so that issues
> can be ironed out as sooner.

Yup

> I don't think this will do too much for us if you are rendering
> everything to sRGB first and then to the final colorspace.

I disagree - if you're rendering to sRGB as an intermediate step (which, 
agreed, we don't really want to do - but it seems to be necessary for 
CMYK output profiles at present) then using Relative Colorimetric when 
rendering to the printer profile will result in black shades getting 
crushed.
sRGB (normally, and in the case of the LCMS built-in sRGB which Poppler 
uses) has a "perfect" blackpoint, with L* of 0, which no real-world 
printer can achieve - so if you use relative colorimetric, any colours 
darker than the blackpoint of the printer's profile will end up as black 
as the printer can manage.

For example - original image:
http://www.npandd.co.uk/coppermine/albums/userpics/10002/Bird.jpg

Converted from sRGB to ISO Coated using Perceptual intent, then back to 
sRGB (using rel-col+bpc to soft-proof).

http://www.npandd.co.uk/coppermine/albums/userpics/10002/Bird_pc_p.jpg

Finally converted from sRGB to ISO Coated using Relative Colorimetric, 
then back to sRGB (again using rel-col+bpc for proof).

http://www.npandd.co.uk/coppermine/albums/userpics/10002/Bird_rc_p.jpg

Note how the rel-col version crushes the dark colours in the claws and 
background.

If we have to use sRGB (or some wider-gamut RGB space) as an 
intermediate step, then we need to use Perceptual intent when converting 
that to the printer profile.

This isn't an issue when the printer's profile is RGB - poppler can 
render directly to it - though my experiments last night suggest that's 
not working properly either as yet.

All the best
--
Alastair M. Robinson