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

[Adrian Johnson]

James Cloos wrote:
>>>>>> "Adrian" == Adrian Johnson <ajohnson at redneon.com> writes:
> 
> Adrian> The tint transform will map tint values between 0 and 1 to the
> Adrian> linear interpolation between first_color and second_color.
> 
> The more general solution would be to specify the tint transform as a
> gradient.  The tint transform functions and gradient functions are at
> least essentially the same—perhaps even exactly the same—concept.

I like the idea of using a gradient. So then the API becomes:

  cairo_color_space_t *
  cairo_color_space_create_spot (const char      *name,
                                 cairo_pattern_t *pattern);

where pattern must be a linear gradient with at least one stop. The 
alternate color space will be the color space of the gradient. The tint 
transform will map tint values between 0 and 1 to the colors between 
offset 0 and 1 on the gradient.

> And a type7 gradient can make for a reasonably convincing alternate for
> things like metalic, opalescent, iridescent spots or glossy overcoats.
> (Not exact, of course, but not unreasonable either.)

How would the tint transform be derived from a type 7 gradient?

> Which begs the question of when the type6/7 gradient code will be added. ☺

Good question. My last update to the API and implementation is at [1]. 
The only feedback I got on the list was Behdad preferred some of the 
API functions to be renamed [2].

> 
> But anyway.
> 
> Since it takes several API calls to build up a gradient, I suspect that
> cairo should demand that the colour stops in a given gradient all be in
> the same colour space, thus the API for specifying the alternate colour
> should not need to specify the alternate's space explicitly.

I agree that gradient color stops should all be in the same color 
space. I don't see any benefit in allowing mixed color spaces for 
gradient stops. PDF requires all stops to be in the same color space.

> 
> -JimC

[1]http://lists.cairographics.org/archives/cairo/2009-September/018095.html
[2]http://lists.cairographics.org/archives/cairo/2009-September/018098.html