set_invert_blender - Man Page

#include <allegro.h>

void set_invert_blender(int r, int g, int b, int a);

Enables an invert blender mode for combining translucent or lit truecolor  pixels. Blends the inverse (or negative) colour of the source with the  destination.

set_trans_blender(3), drawing_mode(3)

set_trans_blender(3).