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): 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:

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ALLEGRO_ADD

 r = d.r * df.r + s.r * sf.r
 g = d.g * df.g + s.g * sf.g
 b = d.b * df.b + s.b * sf.b
 a = d.a * df.a + s.a * sf.a

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ALLEGRO_DEST_MINUS_SRC

 r = d.r * df.r - s.r * sf.r
 g = d.g * df.g - s.g * sf.g
 b = d.b * df.b - s.b * sf.b
 a = d.a * df.a - s.a * sf.a

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ALLEGRO_SRC_MINUS_DEST

 r = s.r * sf.r - d.r * df.r
 g = s.g * sf.g - d.g * df.g
 b = s.b * sf.b - d.b * df.b
 a = s.a * sf.a - d.a * df.a

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(3alleg5) (white by default)

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ALLEGRO_ZERO

 f = 0, 0, 0, 0

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ALLEGRO_ONE

 f = 1, 1, 1, 1

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ALLEGRO_ALPHA

 f = s.a, s.a, s.a, s.a

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ALLEGRO_INVERSE_ALPHA

 f = 1 - s.a, 1 - s.a, 1 - s.a, 1 - s.a

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ALLEGRO_SRC_COLOR (since: 5.0.10, 5.1.0)

 f = s.r, s.g, s.b, s.a

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ALLEGRO_DEST_COLOR (since: 5.0.10, 5.1.8)

 f = d.r, d.g, d.b, d.a

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ALLEGRO_INVERSE_SRC_COLOR (since: 5.0.10, 5.1.0)

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

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ALLEGRO_INVERSE_DEST_COLOR (since: 5.0.10, 5.1.8)

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

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ALLEGRO_CONST_COLOR (since: 5.1.12, not supported on OpenGLES 1.0)

 f = cc.r, cc.g, cc.b, cc.a

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ALLEGRO_INVERSE_CONST_COLOR (since: 5.1.12, not supported on OpenGLES 1.0)

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

Blending examples:

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

al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_INVERSE_ALPHA);

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

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)

Tinting the source (like al_draw_tinted_bitmap(3alleg5))

al_set_blender(ALLEGRO_ADD, ALLEGRO_CONST_COLOR, ALLEGRO_ONE);
al_set_blend_color(al_map_rgb(0, 96, 255)); /* nice Chrysler blue */

Averaging source and destination pixels

al_set_blender(ALLEGRO_ADD, ALLEGRO_CONST_COLOR, ALLEGRO_CONST_COLOR);
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