let prev_y = 0;

let brush_size = 10;

​

function setup() {

  createCanvas(600, 500);

  //background(220);

}

​

function draw() {

  //To use it like a brush, put background(220) in the setup()

  background(220);

  fill(0);

  text(mouseX + "," + mouseY, 20, 20);

  strokeWeight(brush_size);

  circle(mouseX, mouseY, brush_size);

  strokeWeight(brush_size*2);

  line(mouseX, mouseY, prev_x, prev_y);

  prev_x = mouseX;

  prev_y = mouseY;

  if(mouseIsPressed) {background(220); }                 

}