BrushStroke bs;
PVector follow, pfollow;

void setup() {
  size(400,400,P3D);
  noStroke();

  bs = new BrushStroke();
  bs.count = 200;
  
  follow = new PVector();
  pfollow = new PVector();
}

void mousePressed() {
  bs = new BrushStroke();
  bs.count = 200;
}

void draw() {
  background(0);
  
  if(keyPressed) {
    noFill();
    stroke(255);
  } else {
    fill(255);
    noStroke();
  }
 
  follow.x = alerp(follow.x, mouseX, .05, .01);
  follow.y = alerp(follow.y, mouseY, .05, .01);
  ellipse(follow.x, follow.y, 5, 5);
  
  boolean following = 
    pfollow.x!=follow.x || pfollow.y!=follow.y;
  if (mousePressed && following)
    bs.addSegment(
      new PVector(follow.x, follow.y),
      PVector.dist(pfollow, follow));
  beginShape(QUADS);
  BrushStroke.Segment prev = null;
  for(BrushStroke.Segment cur : bs.segments) {
    if(cur.first) continue;
    if(prev == null) {
      prev = cur; 
      continue;
    }
    vertex(prev.right.x, prev.right.y);
    vertex(prev.left.x, prev.left.y);
    vertex(cur.left.x, cur.left.y);
    vertex(cur.right.x, cur.right.y);
    prev = cur;
//    line(cur.left.x, cur.left.y, cur.right.x, cur.right.y);
  }
  endShape();
  
  pfollow.x = follow.x;
  pfollow.y = follow.y;
}

float alerp(float in, float tar, float rate, float acc) {
  return abs(tar-in)<=acc?tar:in+(tar-in)*rate;
}

class BrushStroke {
  int count;
  LinkedList<Segment> segments;
  
  BrushStroke() {
    segments = new LinkedList<Segment>();
  }
  
  void addSegment(PVector center, float ww) {
    Segment end = null;
    try {
      end = segments.getLast();
    } catch(Throwable ex) {}
    if(end != null) end.last = false;
    
    segments.add(new Segment(end, center, ww));
//    if(end != null && end.first) 
//      end.angle = segments.getLast().angle;
    if(segments.size() > count) segments.removeFirst();
  }

  class Segment {
    Segment previous;
    float angle, ww;
    PVector center, left, right;
    boolean first, last;

    Segment(Segment previous, PVector center, float ww) {     
      this.previous = previous;
      this.center = center;
      this.ww = ww;

      if (previous != null)
        angle = PVector.sub(previous.center, center).heading2D();
      else first = true;
      
      last = true;
      
      updatePts();
    }

    void updatePts() {
      left = new PVector();
      left.x = center.x + cos(angle-HALF_PI)*ww*.5;
      left.y = center.y + sin(angle-HALF_PI)*ww*.5;

      right = new PVector();
      right.x = center.x + cos(angle+HALF_PI)*ww*.5;
      right.y = center.y + sin(angle+HALF_PI)*ww*.5;
    }
  }
}