[Openicc] CUPS Color Management under Linux gets into distros

[Hal V. Engel]

On Monday, February 21, 2011 11:13:22 AM Alastair M. Robinson wrote:
> 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?

configure --disable-cairo-output

Might make CMYK output available?  It definitly makes splash the default 
backend.

For the cmake build it appears that WITH_Cairo=NO turns off cairo output.  

Using the splash backend needs to be tested to see if it will enable CMYK 
rendering.

> 
> > 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
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