al_set_blender(3alleg5)

💖 al_set_blender 💖

okay bestie, welcome to the Allegro 5 API ✨ this one lives in the Allegro reference manual and honestly? it's giving pixel chemistry 🧪🎀

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🌸 SYNOPSIS 🌸

#include <allegro5/allegro.h>

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

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✨ DESCRIPTION ✨

Sets the function to use for blending for the current thread. (yes girl, per-thread, that's the vibe 💅)

Blending means the source and destination colors get combined in drawing operations. Like, they're literally mixing together to make the pixel you see 🫶

So 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 — and if the bitmap has no alpha it's always assumed to be fully opaque, so 255 for 8-bit or 1.0 for floating point — so:

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 Allegro uses 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) (which is white by default, periodt 🤍)

• 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

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🎀 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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🌈 SEE ALSO 🌈

• al_set_separate_blender(3alleg5)
• al_set_blend_color(3alleg5)
• al_get_blender(3alleg5)

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💀 reminder bestie: ALLEGRO_CONST_COLOR and ALLEGRO_INVERSE_CONST_COLOR are not supported on OpenGLES 1.0, no cap. don't come crying to me.

okay that's the whole thing, you ate that 🍓✨ go blend something iconic, main character energy only 👑

this was brought to you by the Allegro reference manual, and yeah, this page is al_set_blender(3alleg5) — that's the version and the date, mwah 💅🎀