al_set_blender

void al_set_blender(int op, int src, int dst) Introduced in 5.0.0

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
```

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)

Most helpful discussions:

Other recent discussions:

Manual

Graphics routines

al_set_blender