al_set_blender man page

al_set_blender — Allegro 5 API


#include <allegro5/allegro.h>

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


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): s = s.r, s.g, s.b, s.a. And this color is drawn to a destination, which already has a color: d = d.r, d.g, d.b, d.a.

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

Valid values for the factors sf and df passed to this function are as follows, where s is the source color, d the destination color and cc the color set with al_set_blend_color(3) (white by default)

Blending examples:

So for example, to restore the default of using premultiplied alpha blending, you would use:


As formula:

r = d.r * (1 - s.a) + s.r * 1
g = d.g * (1 - s.a) + s.g * 1
b = d.b * (1 - s.a) + s.b * 1
a = d.a * (1 - s.a) + s.a * 1

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


Additive blending would be achieved with


Copying the source to the destination (including alpha) unmodified


Multiplying source and destination components


Tinting the source (like al_draw_tinted_bitmap(3))

al_set_blend_color(al_map_rgb(0, 96, 255)); /* nice Chrysler blue */

Averaging source and destination pixels

al_set_blend_color(al_map_rgba_f(0.5, 0.5, 0.5, 0.5));

As formula:

r = d.r * 0 + s.r * d.r
g = d.g * 0 + s.g * d.g
b = d.b * 0 + s.b * d.b
a = d.a * 0 + s.a * d.a

See Also

al_set_separate_blender(3), al_set_blend_color(3), al_get_blender(3)

Referenced By

al_get_blend_color(3), al_get_blender(3), al_set_blend_color(3), al_set_separate_blender(3).

Allegro reference manual