[cairo] Spot colors (and CMYK)

[Francois Robert]

Chris Wilson wrote :
> And you've identified just why getting this right is difficult.
> We've been told in the past not to do colorspaces for printing, 
> as doing the conversion is ink dependent and is best left to the
> printers. However, the information on spot inks is fascinating 
> and is definitely beyond Cairo's current capabilities. It sounds
> like each ink should be in its own layer to overlaid onto of the
> base "RGB" layer, acting as a mask for the ink.
> Layer management should be above Cairo, but we would need additional
> hooks in the PDF backend (and friends?) to include the addition spot
ink + layer.

> Is this even close?
Well... Not sure about how this would help (or maybe I have
mistunderstood you).

The common use case I am aware of for spot inks is the last I mentionned
(economical production of bi-color documents). In most respects, this
behaves as a regular printout (the deposition sequence of colorants is
significant but not as critical as in the others cases. The sequence is
a production detail and the designer may safely be unaware of it)

I am unfortunately much less aware of other cases where deposition order
would be much more critical, in the sense that the document designer
would need to be aware of it (because of the non-standard
opacity/transparency of the 'ink')

In other words: The ink physical deposition sequence is mostly up to the
production process (complete with operator's manual intervention...),
even though there are some standardized sequences for process colors at
least. Don't know enough to tell if there are standardized sequences for
spot / process mix (I would think that depends on the inks' themselves).

Something I forget to mention before is that there are some situations
where even photographic, colored printing is not done with the usual
CMYK process inks but with some other mix. This occurs quite commonly in
packaging (unfold some your favorite orange juice Tetrapak or other
color package and look for the registrations marks / ink control
patches: I have alredy seen 6 inks jobs with no black but some dark blue
instead, plus orangish-yellow)