set_write_alpha_blender - Enables the special alpha-channel editing mode. Allegro game programming

       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_write_alpha_blender  -  Enables  the  special  alpha-channel  editing  mode. Allegro game programming
       library.

set_alpha_blender(3alleg4),    draw_trans_sprite(3alleg4),    drawing_mode(3alleg4),    exalpha(3alleg4),
       extrans(3alleg4)

Allegro                                           version 4.4.3                 set_write_alpha_blender(3alleg4)

#include<allegro.h>voidset_write_alpha_blender();