freebsd man page for set_alpha_blender

set_alpha_blender(3alleg4)					  Allegro manual					set_alpha_blender(3alleg4)

NAME

       set_alpha_blender - Enables a special alpha-channel blending mode. Allegro game programming library.

SYNOPSIS

       #include <allegro.h>

       void set_alpha_blender();

DESCRIPTION

       Enables	the  special  alpha-channel blending mode, which is used for drawing 32-bit RGBA sprites. After calling this function, you can use
       draw_trans_sprite() or draw_trans_rle_sprite() to draw a 32-bit source image onto any hicolor or truecolor destination.	The  alpha  values
       will  be  taken	directly  from the source graphic, so you can vary the solidity of each part of the image. You can't use any of the normal
       translucency functions while this mode is active, though, so you should reset to one of the normal blender modes (eg.  set_trans_blender())
       before drawing anything other than 32-bit RGBA sprites.

SEE ALSO

       set_trans_blender(3alleg4), draw_trans_sprite(3alleg4), draw_trans_rle_sprite(3alleg4), set_write_alpha_blender(3alleg4), exalpha(3alleg4),
       exrotscl(3alleg4), extrans(3alleg4)

Allegro 							   version 4.4.2					set_alpha_blender(3alleg4)