void setup() {
  size(400, 400);
  background(0);
  stroke(255);
  drawBranch(width / 2, height, -PI / 2, 80);
}

void drawBranch(float x, float y, float angle, float length) {
  if (length < 5) return;

  float x2 = x + cos(angle) * length;
  float y2 = y + sin(angle) * length;

  line(x, y, x2, y2);

  drawBranch(x2, y2, angle - PI / 6, length * 0.7);
  drawBranch(x2, y2, angle + PI / 6, length * 0.7);
}