set_screen_blender - Man Page

#include <allegro.h>

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

Enables a screen blender mode for combining translucent or lit truecolor  pixels. This blender mode lightens the colour of the destination image by  multiplying the inverse of the source and destination colours. Sort of  like the opposite of the multiply blender mode.

set_trans_blender(3), drawing_mode(3)

set_trans_blender(3).