[cairo] Spot colors (and CMYK)

[Francois Robert]

[OT: Because I have only been lurking here for a *very* long time -my
last post was about choosing an name for Cairo-, I am really not
familiar with the proper etiquette / mail settings / mail address
stripping / etc... for posting here. ]
[OT2: Hopefully this will make it to the list, after much fiddling with
aliased mailboxes...]

On mercredi 13 janvier 2010 14:44, Chris Wilson wrote :
> On Wed, 13 Jan 2010 23:51:43 +1030, Adrian Johnson wrote:
>> 5) Add CMYK image format support to the image surface.

> The consensus always seems to be that we always do blending in RGB 
> space, so what advantages does being able to specify colours in CMYK
offer?
> -ickle

A few of observations, obviously skewed toward the printing business
(after all those are the CMYK / spot users):

- Wasn't the goal of Cairo to use the PDF transparency model ? The PDF
spec say that the blending colorspace can be /DeviceGray, /DeviceRGB,
/DeviceCMYK, /CalGray, /CalRGB and /ICCBased equivalent to those above.
Mind you, the result you obtain in many of the PDF blending modes and/or
colorspace is not necessarily very intuitive. I guess I'am just saying
that RGB is not the only choice for conducting those calculations.

- In theory, color management should make it possible to use full RGB or
even L*a*b* workflows in the printing industry and not care about the
inks details. In practice, however, this is still not very common and
many shops are still using CMYK (at least at some point, if only when
dealing with archived publications). In my opinion, therefore, having
CMYK support (and spot color too) makes sense if the PDF and the PS
backend are to be of any use in a print shop context.

- regarding spot colors specifically: 
It may help if we speak of "spot ink" instead. Spot inks are colorants
placed on paper for viusal effect otherwise unachievable or not
economical. For instance : 
 * metallic color (which cause specular reflection)
 * out of gamut color (eg. high visibility fluorescent orange)
 * transparent varnish (mixing the ink's specular reflection with the
paper's diffuse one)
 * special effects (eg irridescent effect, such as banknotes security
features)
 * lower production cost : An ad placed in an otherwise black & white
newspaper page, is using a single color (think the Coca Cola red or the
Schweppes yellow). Full quadrichromy is quite common nowdays but many
small (local) newspaper still have black-only pages, so using a spot
color is necessary for satisfying an advertizer. On a side note, the
exact spot color name ("Pantone CVU 1234") is part of the certain brand
definition.

- When dealing with print-oriented colors (both spots and CMYK), it is
probably a good idea to support "overprinting" (a crude and
device-dependent way of specifying an optical mix between inks). PDF and
PS both support this feature.

- Spot ink/colors are certainly non trivial to handle, especially if
they are optically mixed with other (process or spot) colorants. Some
optical properties of those inks must be known to infer how to render
them (or an optical mix of them) on a non-printing device or on a device
that does not have the specified colorant. The order of colorant
deposition may be important too (depositing a process ink on top of an
opaque one does not yield the same result as the other way round...)

Hope this helps