set_difference_blender - Man Page

#include <allegro.h>

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

Enables a difference blender mode for combining translucent or lit  truecolor pixels. This makes an image which has colours calculated by the  difference between the source and destination colours.

set_trans_blender(3), drawing_mode(3)

set_trans_blender(3).