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al_set_blender(3)                                            al_set_blender(3)

NAME
       al_set_blender - Allegro 5 API

SYNOPSIS

              #include <allegro5/allegro.h>

              void al_set_blender(int op, int src, int dst)

DESCRIPTION
       Sets the function to use for blending for the current thread.

       Blending means, the source and destination colors are combined in
       drawing operations.

       Assume the source color (e.g.  color of a rectangle to draw, or pixel
       of a bitmap to draw) is given as its red/green/blue/alpha components
       (if the bitmap has no alpha it always is assumed to be fully opaque, so
       255 for 8-bit or 1.0 for floating point): sr, sg, sb, sa.  And this
       color is drawn to a destination, which already has a color: dr, dg, db,
       da.

       The conceptional formula used by Allegro to draw any pixel then depends
       on the op parameter:

       o ALLEGRO_ADD

                 r = dr * dst + sr * src
                 g = dg * dst + sg * src
                 b = db * dst + sb * src
                 a = da * dst + sa * src

       o ALLEGRO_DEST_MINUS_SRC

                 r = dr * dst - sr * src
                 g = dg * dst - sg * src
                 b = db * dst - sb * src
                 a = da * dst - sa * src

       o ALLEGRO_SRC_MINUS_DEST

                 r = sr * src - dr * dst
                 g = sg * src - dg * dst
                 b = sb * src - db * dst
                 a = sa * src - da * dst

       Valid values for src and dst passed to this function are

       o ALLEGRO_ZERO

                 src = 0
                 dst = 0

       o ALLEGRO_ONE

                 src = 1
                 dst = 1

       o ALLEGRO_ALPHA

                 src = sa
                 dst = sa

       o ALLEGRO_INVERSE_ALPHA

                 src = 1 - sa
                 dst = 1 - sa

       Blending examples:

       So for example, to restore the default of using premultiplied alpha
       blending, you would use (pseudo code)

              al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_INVERSE_ALPHA)

       If you are using non-pre-multiplied alpha, you could use

              al_set_blender(ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA)

       Additive blending would be achieved with

              al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ONE)

       Copying the source to the destination (including alpha) unmodified

              al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ZERO)

SEE ALSO
       al_set_separate_blender(3), al_get_blender(3)

Allegro reference manual                                     al_set_blender(3)