Re: [Groff] inconsistencies with color specification

[Ted Harding]

On 18-Oct-01 Stewart C. Russell wrote:
> Werner LEMBERG wrote:
>> 
>> I would argue exactly the opposite: Using RGB everywhere.
> 
> erm, I hope that wouldn't mean that grops would use setrgbcolor. This
> would effectively bar the route to colour separation. Most preflighting
> software would say that the resulting PS was not suitable for printing,
> and many print shops go by what their preflighter says these days.

I was about to add support to Werner's RGB standpoint, but in
view of Stewart's comment it might be best if groff supported
both RGB and CMYK. At least this would be the least-hassle option.

The issue is not quite straightforward, however (and apologies in
advance to all grandmothers with PhDs in sucking eggs).

Basically (and rather roughly), RGB is a good colour model when
the colour is to be created by light emission (typically monitor
screens and the like). On the other hand, CMYK is a better model
when colour is created by light absorption (as in the ink used
to print on paper). This may be what is behind what Stewart is
reporting.

Despite the exact-looking mathematical complementarity between
RGB and CMY which some people have been describing to the list,
for several reasons this is not exact in real life. Mainly, this
is due to the physiological response of the eye to different
regions of the optical spectrum: subtracting Cyan is not the same
as adding Red; but there can be other factors such as the paper
it's printed on (since colour in print also depends on the light
which is retransmitted back through the ink having been bounced
off the underlying paper, and also affects the absorbency of the
ink). So, even if groff supports both RGB and CMYK, don't expect
it to look the same when the CMYK is the exact formal conversion
from the RGB.

Colour printer drivers usually have a "colour transfer
function" table inside, which allows non-linear output
of colour intensity, with different transfer functions
for C, M, Y and K. See, for instance, the Epson Stylus
Colour drivers like stc600p.upp. The user can vary these
to produce an improved effect.

That being said, there is clearly a point in groff
being able to generate PostScript which, at choice,
uses setrgbcolor or setcmykcolor. The user will just
have to note that the CMYK version of an RGB document
may need to have its CMYK parameters set somewhat
independently of the RGB ones.

The alternative is that if groff uses only RGB, then
when CMYK is needed the file will need conversion.
There are not many programs which do this ...

Best wishes to all,
Ted.

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Date: 18-Oct-01                                       Time: 22:00:42
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