[cairo] Spot colors (and CMYK)

[Kai-Uwe Behrmann]

Am 13.01.10, 13:43 -0000 schrieb Chris Wilson:
> On Wed, 13 Jan 2010 23:51:43 +1030, Adrian Johnson <ajohnson at redneon.com> 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?

Consensus among colour people is that each colour needs to be specified. 
Rgb says nothing about appearance. That would be a fine start for Cairo 
and colour management.

Cmyk blending is often done while printing. As that is not supported 
in Cairo, previews of prints will be more or less unsatisfying. But I am 
not shure if that should be covered at once, while introducing colour 
management.

The reference to a explicitely defined colour space would allow to 
play with device and editing RGBs, CIE*XYZ, CIE*Lab, Yuv, ...
Simply use of ICC profiles in order to give a triple of numbers a 
meaning.


With Cairo's feature, to flatten a vector graphic out, would come a 
responsibility while adding colour space support. It must be clear which 
are the colour spaces during the following stages:
* input,
* blending / compositing
* output

Adrian has suggested APIs to specify the input colour spaces on a per 
component basis. Thats the best way. I would add to these the following:
/* explicitely specify a image color space */
cairo_public cairo_surface_t *
cairo_image_surface_create (cairo_format_t      format,
                             cairo_color_space_t*color_space,
                             int                 width,
                             int                 height);

For the other stages:
/* specify traditionally provided input rgb(a) colors as a fallback */
cairo_set_assumed_color( cairo_t             * cairo,
                          cairo_color_space_t * color_space );
/* specify for drawing / compositing operations */
cairo_set_compositing_color_space( cairo_t             * cairo,
                                    cairo_color_space_t * color_space );
/* specify for output */
cairo_set_output_color_space( cairo_t             * cairo,
                               cairo_color_space_t * color_space );


cairo_color_space_t can be contructed and internaly used according to the 
actual used colour management engine. The colour management system may 
vary from platform to platform. It would make sense to pass available 
platform specific colour space references as is to Cairo.


kind regards
Kai-Uwe Behrmann
-- 
developing for colour management 
www.behrmann.name + www.oyranos.org