SpecIccForCairo

Why

In order for inkscape to produce CMYK PDFs and PSs with cairo, Cairo needs to support ICC. This is an intent to propose a Cairo API that would be suitable with inkscape needs.

Profiles

A set of profiles will apply to the whole Cairo surface. Cairo needs to be able to compute a transform from this set. We can get inspiration from the lcms api [1], in particular the cmsCreateMultiprofileTransform (more generic than cmsCreateTransform that handles only a single pair of color profiles).

So a proposal would be:

typedef enum cairo_color_intent {
  CAIRO_COLOR_INTENT_PERCEPTUAL,
  CAIRO_COLOR_INTENT_RELATIVE_COLORIMETRIC,
  CAIRO_COLOR_INTENT_SATURATION,
  CAIRO_COLOR_INTENT_ABSOLUTE_COLORIMETRIC
} cairo_color_intent_t;

typedef struct cairo_color_profile cairo_color_profile_t;

void cairo_surface_set_color_profile_srgb(cairo_surface_t *surf);

void
cairo_surface_set_color_profile(cairo_surface_t *surf,
                                cairo_color_profile_t *profile);

cairo_color_profile_t *
cairo_surface_get_color_profile(cairo_surface_t *surf);

void
cairo_surface_add_profile_mapping(cairo_surface_t *surf, 
                                  cairo_color_profile_t *in, 
                                  cairo_color_profile_t *out);

void
cairo_surface_remove_profile_mappings(cairo_surface_t *surf);

void
cairo_surface_set_color_intent(cairo_surface_t *surf,
                               cairo_color_intent_t intent);

cairo_color_intent_t
cairo_surface_get_color_intent(cairo_surface_t *surf);

void
cairo_set_source_icc_color(cairo_t *cr,
                           cairo_color_profile_t *profile,
                           const double *components,
                           int num_components);

Color transformations would be performed when painting from a pattern to a surface, based on the surface's profile, mappings, intent, etc.

Functional spec

To render a surface, cairo has several ways to accept color information (e.g. cairo_set_source_rgb).

Open issues

Should surface profile/color rendering changes be allowed once the surface is being actively used?

Why should we allow this ? Is there any use case ? The ones I can see are: render the same surface with two different profiles at the same time (e.g. on a multihead screen with different profiles, but is'nt X already handling that, batch export of the same file in many different colorspaces)

How should gradients work in this regime?

What is the issue here ? like we have a set_source_icc_color, we can have a set_stop_icc color, isnt'it ?

How should cairo_color_profile_ts be created/loaded?

lcms provides 2 ways, load from a file or from memory. I think cairo should provide the same ways, I dont see an additional one that could bring some benefit here, appart from direct reuse of a lcms handler for applications already using lcms like inkscape but that's not really clean - makes cairo API depend from lcms API.