What ho, chaos game.

chaos.c

/* gcc -Wall -Wextra `allegro-config --libs` -lm w.c */

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <time.h>
#include <allegro.h>

#define POINTS  3
#define WIDTH   600
#define HEIGHT  600
#define REST    0
#define FNAME   "screen.pcx"

int main(int argc, char **argv)
{
    struct point { double x, y; } *pts;
    int n_pts = POINTS, w = WIDTH, h = HEIGHT, r = REST,
        i, white;
    double x, y;
    char *fname = FNAME;
    PALETTE pal;

    /* Parse options */
    if (argc > 1) n_pts = atoi(argv[1]);
    if (argc > 2) {
        w = atoi(argv[2]);
        if (argc == 3) h = w;
    }
    if (argc > 3) h = atoi(argv[3]);
    if (argc > 4) r = atoi(argv[4]);
    if (n_pts < 1 || w < 1 || h < 1 || r < 0 || argc > 5) {
        fprintf(stderr,
                "USAGE: %s [ N_POINTS [ WIDTH [ HEIGHT [ REST [ FILENAME ] ] ] ] ]\n",
                argv[0]);
        return 1;
    }

    /* Initialise Allegro */
    allegro_init();
    install_timer();
    install_keyboard();

    set_color_depth(32);
    if (set_gfx_mode(GFX_AUTODETECT_WINDOWED, w, h, 0, 0) != 0) return 2;

    white = makecol(255, 255, 255);
    get_palette(pal);

    set_trans_blender(0, 0, 0, 10);
    drawing_mode(DRAW_MODE_TRANS, 0, 0, 0);

    /* Determine attractor points (just on a circle (or ellipse)) */
    pts = malloc(sizeof(struct point) * n_pts);
    for (i = 0; i < n_pts; ++i) {
        pts[i].x = w / 2 + sin(2 * 3.14159 * i / n_pts) * w / 2;
        pts[i].y = h / 2 - cos(2 * 3.14159 * i / n_pts) * h / 2;
        putpixel(screen, pts[i].x, pts[i].y, makecol(255, 0, 0));
    }

    /* Random numbers */
    srandom((unsigned int)time(NULL));
    x = (double)(random() % w);
    y = (double)(random() % h);

    /* Main loop */
    while (!keypressed()) {
        int n = random() % n_pts;

        putpixel(screen, x, y, white);
        x = (x + pts[n].x) / 2;
        y = (y + pts[n].y) / 2;

        rest(r);
    }

    /* Save bitmap */
    save_pcx(fname, screen, pal);
    
    /* Unnecessary */
    free(pts);

    return 0;
} END_OF_MAIN()