// Learning Processing
  // Daniel Shiffman
  // http://www.learningprocessing.com
  
  // Example 7-4: Using a function that returns a value, distance
  
  void setup() {
    size(200,200);
  }
  
  void draw() {
    background(0);
    stroke(0);
    // Top left square
    fill(distance(0,0,mouseX,mouseY)); // Our distance function is used to calculate a brightness value for each quadrant. 
                                       // We could use the built-in function dist() instead, but we are learning how to write our own functions.
    rect(0,0,width/2 - 1,height/2 - 1);
    // Top right square
    fill(distance(width,0,mouseX,mouseY));
    rect(width/2,0,width/2 - 1,height/2 - 1);
    // Bottom left square
    fill(distance(0,height,mouseX,mouseY));
    rect(0,height/2,width/2 - 1,height/2 - 1);
    // Bottom right square
    fill(distance(width,height,mouseX,mouseY));
    rect(width/2,height/2,width/2 - 1,height/2 - 1);
  }
  
  float distance(float x1, float y1, float x2, float y2) {
    float dx = x1 - x2;
    float dy = y1 - y2;
    float d = sqrt(dx*dx + dy*dy);
    return d;
  }