set_write_alpha_blender - Enables the special alpha-channel editing mode.
Allegro game programming library.
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.
drawing_mode(3alleg4), exalpha(3alleg4), extrans(3alleg4)