al_premul_rgba(1) Allegro 5 API


#include <allegro5/allegro.h>
ALLEGRO_COLOR al_premul_rgba(
   unsigned char r, unsigned char g, unsigned char b, unsigned char a)


This is a shortcut for al_map_rgba(3alleg5)(r * a / 255, g * a / 255, b * a / 255, a).

By default Allegro uses pre-multiplied alpha for transparent blending of bitmaps and primitives (see al_load_bitmap_flags(3alleg5) for a discussion of that feature). This means that if you want to tint a bitmap or primitive to be transparent you need to multiply the color components by the alpha components when you pass them to this function. For example:

int r = 255;
int g = 0;
int b = 0;
int a = 127;
ALLEGRO_COLOR c = al_premul_rgba(r, g, b, a);
/* Draw the bitmap tinted red and half-transparent. */
al_draw_tinted_bitmap(bmp, c, 0, 0, 0);