al_set_separate_blender - Allegro 5 API

       Like  al_set_blender(3alleg5), but allows specifying a separate blending operation for the alpha channel.
       This is useful if your target bitmap also has an alpha channel and the two  alpha  channels  need  to  be
       combined in a different way than the color components.

       al_set_separate_blender - Allegro 5 API

al_set_blender(3alleg5), al_get_blender(3alleg5), al_get_separate_blender(3alleg5)

Allegro reference manual                                                        al_set_separate_blender(3alleg5)

              #include <allegro5/allegro.h>

              void al_set_separate_blender(int op, int src, int dst,
                 int alpha_op, int alpha_src, int alpha_dst)