Draw a Sierpinski triangle using Chaos game

sierpinski.c

#include <gd.h>

// C doesn't allow variable length arrays (even using a constant)
#define N_CORNERS 3

typedef struct {
  int x;
  int y;
} Point;

/**
 * Draw a Sierpinski triangle using Chaos game
 * - Js implementation: https://www.youtube.com/watch?v=7gNzMtYo9n4
 * - Description: https://mathigon.org/course/fractals/sierpinski
 * - GD documentation: https://libgd.github.io/manuals/2.1.1/files/preamble-txt.html
 *
 * How to build & run:
 * $ gcc -lgd sierpinski.c -o sierpinski
 * $ ./sierpinski image.png 500 500
 */
int main(int argc, char** argv) {
  if (argc != 4) {
    printf("USAGE: %s IMAGE WIDTH HEIGHT\n", argv[0]);
    return 1;
  }

  const char* filename = argv[1];
  FILE* file = fopen(filename, "wb");

  // rgb image
  const int WIDTH = atoi(argv[2]);
  const int HEIGHT = atoi(argv[3]);
  gdImagePtr image = gdImageCreateTrueColor(WIDTH, HEIGHT);

  // colors
  const int RED = gdTrueColor(0xff, 0, 0);
  const int GREEN = gdTrueColor(0, 0xff, 0);
  const int BLUE = gdTrueColor(0, 0, 0xff);
  int colors[N_CORNERS] = { RED, GREEN, BLUE };

  // three triangle corners
  Point corner0 = { WIDTH / 2, 0 };
  Point corner1 = { 0, HEIGHT };
  Point corner2 = { WIDTH - 1, HEIGHT - 1 };
  Point corners[N_CORNERS] = { corner0, corner1, corner2 };

  for (size_t i_corner = 0; i_corner < N_CORNERS; ++i_corner) {
    gdImageSetPixel(image, corners[i_corner].x, corners[i_corner].y, colors[i_corner]);
  }

  /* chaos game */
  const int N_ITERS = 100000;
  Point point = { rand() % WIDTH, rand() % HEIGHT };

  for (size_t i_iter = 0; i_iter < N_ITERS; ++i_iter) {
    int i_corner = rand() % N_CORNERS;
    Point corner = corners[i_corner];
    Point avg = { (point.x + corner.x) / 2, (point.y + corner.y) / 2 };
    point = avg;

    gdImageSetPixel(image, point.x, point.y, colors[i_corner]);
  }

  gdImagePng(image, file);

  // free assets
  fclose(file);
  gdImageDestroy(image);

  return 0;
}