How to write out a greyscale cairo surface to PNG

Question

I have a cairo_surface_t of format CAIRO_FORMAT_A8. I want to write out the surfe as a greyscale image, so every pixel has a single byte value of type uchar.

If I use cairo_surface_write_to_png directly on the CAIRO_FORMAT_A8 surface, all I get is an all-black image. I think this is how cairo internally treats the A8 surface - as alpha values, not as greyscale data. I want a single greyscale image, however.

I'd be enough if somebody count point out how to copy the A8 format to all 3 layers of an RGB24 image.

Any help appreciated!

Answer 1

Untested code below. The idea is to create an ARGB-surface and "copy" the A8 surface there via cairo_mask_surface(). If the colors are "swapped", swap the two cairo_set_source_rgb() calls.

cairo_surface_t *s = YOUR_A8_SURFACE;
cairo_t *cr = cairo_create(s);
cairo_push_group_with_content(cr, CAIRO_CONTENT_COLOR_ALPHA);
cairo_set_source_rgb(cr, 1, 1, 1);
cairo_paint(cr);
cairo_set_source_rgb(cr, 0, 0, 0);
cairo_mask_surface(cr, cairo_get_target(cr), 0, 0);
cairo_surface_write_to_png(cairo_get_group_target(cr), "/tmp/foo.png");
/* If you want to continue using the context:
   cairo_pattern_destroy(cairo_pop_group(cr)); */
cairo_destroy(cr);