[scribus] Colour Profiles - make colour matching worse!

Hal V. Engel hvengel at astound.net
Tue Jul 28 18:50:24 CEST 2009

On Monday 27 July 2009 05:04:18 pm John Beardmore wrote:
> Yes -  I suppose my point is that if you can standardise on one or the
> other out of CIELab or CIEXYZ, fewer profiles need to be written which
> should simplify things ?

It would not reduce the number of profiles needed.  No one creates two 
profiles for a device - one for each PCS.  There is no reason to do this since 
the CMS handles any conversions needed between PCS types and does this without 
user involvement.  In fact even programmers writing software that uses the CMS 
do not have to be concerned about what type of PCS is being used when doing a 
transform since the CMS just works.

> >> or if there isn't an
> >> internal representation, each intended output device ?
> >
> > No you are not understanding this.  There are two basic types of
> > profiles. Those for input devices and those for output devices.  Those
> > for input devices are only required to have the information to go from
> > the device color space (like say the RGB values of a camera or scanner)
> > to the PCS and output device profiles have the information to go from the
> > PCS to the output device color space (the RGB values of a monitor or the
> > CMYK values of a printer).  Profiles can also have tables that go in the
> > other direction but this is not required. You need two profiles to do a
> > meaningful transform and the PCS is an intermediary between the input
> > device and the output device color spaces.  I think you are making it
> > more complicated than it actually is.
> No -  I understand your para above, but it does seem unfortunate that
> there are two PCSs in common use. Is either one better for any
> particular purpose ?

The choice of the PCS type can make a difference and it depends on the device 
characteristics which is better.   In general profiling software will default 
to the best PCS for the type of device being profiled although CIELab is the 
most common PCS.  But the PCS type is something that is completely hidden from 
normal users and the CMS just works no matter what type of PCS the profiles 
involved in a transform use even if those are mixed types.  The only group 
that PCS types matter to are those writing profiling software since they need 
to select the best PCS type when creating profiles.  This is a very small 
group of people (probably <100 on the face of the planet including those who 
work on commercial as well as open source software).

> :
> >>> There als also colors that can not be reproduced in i digital image.
> >>> Gold and silver are two.
> >>
> >> Not sure that gold is a colour. Reflectivity is perhaps more of a
> >> surface property ?
> >
> > Gold is most certainly a color
> With my science head on I'd always taking it to be a reflection with
> some bands of wavelengths missing ?
> Yes -  OK, there is a shade of yellow that some people call gold 

Inks that actually look like a gold bar or a silver coin are available from 
many sources including Pantone.

> which I
> can imagine could fall within many colour spaces, but it has little
> similarity to the appearance of polished edge connector which I doubt
> coulour spaces can describe well 

This assumption is incorrect. 

> as the metal surface transforms the
> light / virtual image that impinges on it.
> > and the Pantone system has color patches that
> > are gold and silver and variations on these.  You can also purchase inks
> > for these colors to use as spot colors.
> Yes -  but I assume that these metal colloids along with varnishes and
> textures fall outside colour gamuts. I'm not sure that I'd regard them
> as colours even if they are marketed as such. Maybe I should have done
> arts instead of science !

You are getting mixed up between normal device gamuts and the gamut of human 
vision.  Just because a normal CMYK printer can't reproduce a color does not 
mean that it isn't a color.  In addition it is fairly common for those using 
offset printers to use spot colors and these spot colors can be way outside of 
the normal gamut of the offset press being used and that includes metallic 
colors that look just like a gold bar, a silver coin, polished brass or 
bronze.   These metallic colors are not common spot color and the inks for 
these are expensive but they do exist and they do get used for offset printing 
for specialized print jobs.

> Cheers, J/.

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