set_write_alpha_blender - Man Page

#include <allegro.h>

void set_write_alpha_blender();

Enables the special alpha-channel editing mode, which is used for drawing  alpha channels over the top of an existing 32-bit RGB sprite, to turn it  into an RGBA format image. After calling this function, you can set the  drawing mode to DRAW_MODE_TRANS and then write draw color values (0-255)  onto a 32-bit image. This will leave the color values unchanged, but  alter the alpha to whatever values you are writing. After enabling this  mode you can also use draw_trans_sprite() to superimpose an 8-bit alpha  mask over the top of an existing 32-bit sprite.

set_alpha_blender(3), draw_trans_sprite(3), drawing_mode(3), exalpha(3), extrans(3)

draw_trans_sprite(3), exalpha(3), extrans(3), makeacol(3), makeacol32(3), set_alpha_blender(3), set_trans_blender(3).