float theta;   
  
  void setup() {
    size(640, 360);
    smooth();
  }
  
  void draw() {
    background(0);
    frameRate(30);
    stroke(255);
    // Let's pick an angle 0 to 90 degrees based on the mouse position
    float a = (mouseX / (float) width) * 90f;
    // Convert it to radians
    theta = radians(a);
    // Start the tree from the bottom of the screen
    translate(width/2,height);
    // Draw a line 120 pixels
    line(0,0,0,-120);
    // Move to the end of that line
    translate(0,-120);
    // Start the recursive branching!
    branch(120);
  
  }
  
  void branch(float h) {
    // Each branch will be 2/3rds the size of the previous one
    h *= 0.66;
    
    // All recursive functions must have an exit condition!!!!
    // Here, ours is when the length of the branch is 2 pixels or less
    if (h > 2) {
      pushMatrix();    // Save the current state of transformation (i.e. where are we now)
      rotate(theta);   // Rotate by theta
      line(0, 0, 0, -h);  // Draw the branch
      translate(0, -h); // Move to the end of the branch
      branch(h);       // Ok, now call myself to draw two new branches!!
      popMatrix();     // Whenever we get back here, we "pop" in order to restore the previous matrix state
      
      // Repeat the same thing, only branch off to the "left" this time!
      pushMatrix();
      rotate(-theta);
      line(0, 0, 0, -h);
      translate(0, -h);
      branch(h);
      popMatrix();
    }
  }