void convert_grayscale(ALLEGRO_BITMAP *bmp) {
    ALLEGRO_BITMAP *dispbuf = al_get_target_bitmap();
    int x, y, w, h, lum;
    unsigned char r, g, b;
    w = al_get_bitmap_width(bmp);
    h = al_get_bitmap_height(bmp);

    al_set_target_bitmap(bmp);
    al_lock_bitmap(bmp, ALLEGRO_PIXEL_FORMAT_ABGR_8888, ALLEGRO_LOCK_READWRITE);

    for (y = 0; y < h; y++) {
        for (x = 0; x < w; x++) {
            al_unmap_rgb(al_get_pixel(bmp, x, y), &r, &g, &b);
            lum = (0.299*r + 0.587*g + 0.114*b);
            al_put_pixel(x, y, al_map_rgb(lum, lum, lum));
        }
    }
    al_unlock_bitmap(bmp);
    al_set_target_bitmap(dispbuf);
}