Cairo only render to specific color component - cairo

Question

I am using Cairo and would like to render one color component at a time. For example, if I render a set of blue rectangles and then render a set of red rectangles, I want where they overlap to be purple rather than red.
Using set_source_rgb(ctx, 0.0, 1.0, 0.0) doesn't work, because it will overwrite the other channels with zeros. Using transparency doesn't work either, as it equally effects all channels. I would like a way to only render to one channel.
Is that possible? Thank you.

Answer 1

Use CAIRO_OPERATOR_ADD instead of CAIRO_OPERATOR_OVER (the default):
#include <cairo.h>
int main() {
cairo_surface_t *s = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, 20, 20);
cairo_t *cr = cairo_create(s);
cairo_set_operator(cr, CAIRO_OPERATOR_ADD);
/* Render blue */
cairo_set_source_rgb(cr, 0, 0, 1);
cairo_rectangle(cr, 0, 0, 15, 15);
cairo_fill(cr);
/* Render red */
cairo_set_source_rgb(cr, 1, 0, 0);
cairo_rectangle(cr, 5, 5, 15, 15);
cairo_fill(cr);
cairo_surface_write_to_png(s, "out.png");
cairo_destroy(cr);
cairo_surface_destroy(s);
return 0;
}