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:
• 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
• 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
• 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)
• ALLEGRO_ZERO
f = 0, 0, 0, 0
• ALLEGRO_ONE
f = 1, 1, 1, 1
• ALLEGRO_ALPHA
f = s.a, s.a, s.a, s.a
• ALLEGRO_INVERSE_ALPHA
f = 1 - s.a, 1 - s.a, 1 - s.a, 1 - s.a
• 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
• ALLEGRO_CONST_COLOR (since: 5.1.12, not supported on OpenGLES 1.0)
f = cc.r, cc.g, cc.b, cc.a
• 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
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