mouseWheel() fun

wheel.pde

float total = 0;

void setup() {
  size(400, 400);
}

void draw() {
  background(50);
  fill(total > 0 ? color(0, 255, 0) : color(255, 0, 0));
  ellipse(width/2, height/2, total, total);
} 

void mouseWheel(MouseEvent event) {
  float e = event.getAmount();
  total += e;
}