mouseX experiments in Processing

mouseX_ex.java

void setup() {
  size(640, 360);
}

void draw() {
  float transitionVal = float(mouseX) / width * 255;
  // dragging the mouse from left to right changes the background color from white to black
  background(255 - transitionVal);
  
  rectMode(CENTER);
  // dragging the mouse from left to right decreases the width of the rectangle,
  // until it's zero at the center; at which point, it starts to increase again
  // as the mouse keeps moving right
  rect(width/2, height/2, abs(mouseX - width/2), 125);
}