[cairo] Spot colors (and CMYK)

[ecir hana]

On Thu, Feb 18, 2010 at 6:51 AM, Kai-Uwe Behrmann <ku.b at gmx.de> wrote:
> Am 17.02.10, 21:55 +0100 schrieb ecir hana:
>>
>> Ok, so but why do I need to allocate each new color? Just set the
>> working space and call:
>>
>> cairo_set_source_native_channels(cairo_t *cr, double *native_channels);
>
> Well I have decided in Oyranos to reference all colour data with a profile.
> This makes communication very explicite. The overhead is minimal as profiles
> are typically referenced not copied.
> A profile should not change during building cairos drawing graph. Following
> this a backend A would imediately draw with the colour and forget
> cairo_color_t. Backend B had to copy the structure for remembering. The
> overhead for backend B would be the cairo_color_t + the channels doubles and
> a call to reference cairo_profile_t.
> (The cairo_color_t * argument is owned by the caller and can be changed on
> user side at any time.)
>
> /* a small sequence */
> cairo_color_t * colour = cairo_color_create( /**/ );
> cairo_source_color_set( cr, color );
> cairo_rectangle( cr, /**/ );
> cairo_color_set_native_channels( color, 0.2, 0.2, 0.2, 1.0 );
> cairo_set_source_color( cr, color );
> cairo_rectangle( cr, /**/ );
> cairo_color_set_native_channels( color, 0.5, 0.5, 0.5, 1.0 );
> cairo_set_source_color( cr, color );
> cairo_rectangle( cr, /**/ );
>
>
> With your model backend B had to track colour space changes itself. It would
> end in eigther do the same as I outlined above behind the scenes. Or cairo
> had to maintain a internal table for storing each colour value plus the
> according cairo_profile_t reference. So eigther way some overhead would
> remain.

I'm not sure I understood. You don't have to store every color +
profile. Just use the latest set profile. Think of it as path building
works. Cairo maintains the last x, y position as well and every new
position is calculated from there. If I rewrite your sequence above:

cairo_profile_t * profile = cairo_profile_create( /**/ );
cairo_set_profile(cr, profile)
cairo_rectangle(cr, /**/); // default stroke and fill?
cairo_set_source_tints(0.2, 0.2, 0.2, 1.0);
cairo_rectangle(cr, /**/);
cairo_set_source_tints(0.2, 0.2, 0.2, 1.0);
cairo_rectangle(cr, /**/);

And setting another profile once more would either cause everything
already drawn to convert to new space (pixel surfaces) or just change
the current color space (PDF "cs" and "CS" operator)

> Hooks would be part of cairos API. The CMS would be on the user side. In
> comparision Pango is the same way independent.

Yes but even without Pango you are able to write some basic text.