void setup() {
  size(500, 400);
  fill(128, 128, 255);
}

void draw() {
  background(255);
  float w = 2*abs(mouseX - 0.5*width);
  float h = 2*abs(mouseY - 0.5*height);
  ellipse(0.5*width, 0.5*height, w, h);
}