[cairo] [RFC] Color space API (partial proposal)

[ecir hana]

On Tue, Feb 23, 2010 at 12:35 PM, Adrian Johnson <ajohnson at redneon.com> wrote:
>
> I see the problem with my DeviceN API now. The PDF tint transform function
> transforms n tint colors to m color components in the alternate color space.
> I'm not sure of the best way to define a cairo API for this.

There are several things which can be done about this:

- ask the user to explicitly supply multi-dimensional table of
tintTransform samples, making it Not Your Problem. The format of such
table is defined here:
http://www.adobe.com/devnet/acrobat/pdfs/pdf_reference_1-7.pdf
3.9.1 Type 0 (Sampled) Functions, p. 169

- it is my understanding that is is possible to convert one type of
profile to another type and at least one type is basically a 8-bit
LUT. That is, you wouldn't specify the profile of DeviceN, only for
each of the colorants. Then, you consolidate the various spaces into
one common space and there you blend all the colorant's alternatives.
Next you somehow skew (not sure if there is support for this in CMS)
the space so that the final blend lays at its edge. LUT of such space
would equal to tintTransform.

- you would ask for LAB alternatives but I don't know how to to
calculate a tint out of LAB color, nor how to blend many LABs into
one. It would be nice to have LAB alternatives, but someone would need
to explain how to do those operations otherwise it is not very useful.
InDesign might very well use LAB and who knows what else but it might
also be the case that they are closely working with, say, Pantone and
the whole conversion process is proprietary.

- define just one CMYK profile for DeviceN and don't specify further
profiles for the colorants, just the tints values. Blending would be
done as I explained above, tint values as well. Think PDF/X-1a.

Naturally, I lean towards the last option as it can be implemented now
and covers reasonable portion of alternatives (which you have to
understand are just a substitute for the original spot, anyway).
Besides, as this "Color space API" is designed in a way it is as
orthogonal to the rest of Cairo as possible, you could add new kind of
fallback based on spectral and opacity data sometime in the future,
when they become more common.

(Again, for all the people demanding LAB - why not - how to do
blending and tints? And please note, even LAB wont cover all of the
possible spot colors.)