set_write_alpha_blender (3) - Linux Man Pages
set_write_alpha_blender: Enables the special alpha-channel editing mode. Allegro game programming library.
NAMEset_write_alpha_blender - Enables the special alpha-channel editing mode. Allegro game programming library.
DESCRIPTIONEnables 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.
SEE ALSOset_alpha_blender(3), draw_trans_sprite(3), drawing_mode(3), exalpha(3), extrans(3)