offset = []

function setup() {

  createCanvas(400, 400);

}

function calcOff(i){

  dx = points[i][0]-points[i + 1][0]

  dy = points[i][1]-points[i + 1][1]

  vx = 1

  vy = -vx * dx /dy

  m = sqrt(vx*vx + vy*vy)

  vx = vx / m * 10

  vy = vy / m * 10

  offset.push([points[i][0] + vx, points[i][1] + vy])

}

function draw() {

  background(255);

  points.push([mouseX, mouseY]);

  if(points.length >= 2){

    calcOff(points.length - 2)

  }

  if(points.length > 100){

    points.shift();

  }

  if(offset.length > 100){

    offset.shift();

  }

  for(var i = 0; i < points.length - 1; i ++){

    stroke(0);

    line(points[i][0], points[i][1], points[i + 1][0], points[i + 1][1])

  }

  for( i = 0; i < offset.length - 1; i ++){

    stroke(200);

    line(offset[i][0], offset[i][1], offset[i + 1][0], offset[i + 1][1])

  }

}