var theta;

var strokeW = 7;

​

function setup() {

  createCanvas(windowWidth, windowHeight);

}

​

function draw() {

  background(220);

  theta = map(mouseX,0,width,0,PI/2);

  len = height / 3;

  translate(width/2, height);

  stroke(0);

  branch(len, strokeW);  

}

​

function branch(len, strokeW) {

  strokeWeight(strokeW);

  line(0, 0, 0, -len);

  translate(0, -len);

  len *= 0.66;

  strokeW--;

  if(strokeW < 1)

    strokeW = 1;

​

  if (len > 2) {

    push();

    rotate(theta);    

    branch(len, strokeW);

    pop();

​

    push();

    rotate(-theta);

    branch(len, strokeW);

    pop();

  }

}