Provided by: allegro5-doc_5.0.10-2_all bug

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:

       · ALLEGRO_ADD

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

       · ALLEGRO_DEST_MINUS_SRC

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

       · 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

       · ALLEGRO_ZERO

                 src = 0
                 dst = 0

       · ALLEGRO_ONE

                 src = 1
                 dst = 1

       · ALLEGRO_ALPHA

                 src = sa
                 dst = sa

       · ALLEGRO_INVERSE_ALPHA

                 src = 1 - sa
                 dst = 1 - sa

       · ALLEGRO_SRC_COLOR (since: 5.0.10, 5.1.0)

                 f = s.r, s.g, s.b, s.a

       · ALLEGRO_DEST_COLOR (since: 5.0.10, 5.1.8)

                 f = d.r, d.g, d.b, d.a

       · ALLEGRO_INVERSE_SRC_COLOR (since: 5.0.10, 5.1.0)

                 f = 1 - s.r, 1 - s.g, 1 - s.b, 1 - s.a

       · ALLEGRO_INVERSE_DEST_COLOR (since: 5.0.10, 5.1.8)

                 f = 1 - d.r, 1 - d.g, 1 - d.b, 1 - d.a

       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)

       Multiplying source and destination components

              al_set_blender(ALLEGRO_ADD, ALLEGRO_DEST_COLOR, ALLEGRO_ZERO)

SEE ALSO

       al_set_separate_blender(3alleg5), al_get_blender(3alleg5)