var py = 0; 

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  if (mouseIsPressed){

    background("DodgerBlue");

  } else {

    background(30, 144, 255, 20); 

  }

  var A = 0.90;   // The weight of history

  var B = 1.0-A; 

  px = A*px + B*mouseX; 

  py = A*py + B*mouseY; 

  noStroke(); 

  fill("white"); 

  circle (px, py, 40); 

}

​

​