function setup() {
  createCanvas(720, 720);
  noFill();
  background(255);
  strokeWeight(2);
  stroke(0, 25);
}
drawing with a changing shape by draging the mouse.

MOUSE
position x          : length
position y          : thickness and number of lines
drag                : draw

KEYS
del, backspace      : erase
s                   : save png