size(300, 260);

background(#ffffff); // white

int x = floor(random(width));
int y = floor(random(height));

int colour = #ffffff;

for (int i=0; i<30000; i++) {
  int v = floor(random(3));
  switch (v) {
  case 0:
    x = x / 2;
    y = y / 2;
    colour = #00ff00; // green
    break;
  case 1:
    x = width/2 + (width/2 - x)/2;
    y = height - (height - y)/2;
    colour = #ff0000; // red
    break;
  case 2:
    x = width - (width - x)/2;
    y = y / 2;
    colour = #0000ff; // blue
  }
  set(x, height-y, colour);
}