Gradually fades the palette from black. allegro game programming library.
#include <allegro.h>
void fade_in(const PALETTE p, int speed);
Fades gradually from a black screen to the specified palette. The speed is from 1 (the slowest) up to 64 (instantaneous).
Note that this function will block your game while the fade is in effect, and it won't work right visually if you are not in an 8 bit color depth resolution.
fade_out(3alleg4), fade_from(3alleg4), fade_interpolate(3alleg4), fade_in_range(3alleg4)