function particle:update()
  if self.age < self.lifespan then
    if self.jitter > 0 then
      local curr_jitter = rnd(self.jitter)-self.jitter/2
      local curr_angle = atan2(self.dx,self.dy)
      local curr_mag = sqrt(self.dx^2+self.dy^2)
      self.dx = cos(curr_angle+curr_jitter)*curr_mag
      self.dy = sin(curr_angle+curr_jitter)*curr_mag
    end
    self.dx = (self.dx+self.xgrav)*self.drag
    self.dy = (self.dy+self.ygrav)*self.drag
    local next_x = self.x+self.dx
    local next_y = self.y+self.dy

    local line_pts = {{self.x, self.y}}
    line_pts.segments = 0

    while self.collide do
      -- left collision
      if next_x < 0 and btwn(self.y+((next_y-self.y)*self.x/(self.x-next_x)),0,screen) then
        self.y  += (next_y-self.y)*self.x/(self.x-next_x)
        self.x  = 0
        self.dx *= -self.bounce
        next_x  *= -self.bounce

      -- right collision
      elseif next_x > screen and btwn(self.y+((next_y-self.y)*(screen-self.x)/(next_x-self.x)),0,screen) then
        self.y  += (next_y-self.y)*(screen-self.x)/(next_x-self.x)
        self.x  = screen
        self.dx *= -self.bounce
        next_x  = screen-(next_x-screen)*self.bounce

      -- top collision
      elseif next_y < 0 and btwn(self.x+((next_x-self.x)*self.y/(self.y-next_y)),0,screen) then
        self.x  += (next_x-self.x)*self.y/(self.y-next_y)
        self.y  = 0
        self.dy *= -self.bounce
        next_y  *= -self.bounce

      -- bottom collision
      elseif next_y > screen and btwn(self.x+((next_x-self.x)*(screen-self.y)/(next_y-self.y)),0,screen) then
        self.x  += (next_x-self.x)*(screen-self.y)/(next_y-self.y)
        self.y  = screen
        self.dy *= -self.bounce
        next_y  = screen-(next_y-screen)*self.bounce

      else -- no collision
        break
      end
      add(line_pts, {self.x, self.y})
      line_pts.segments += 1
    end

    self.x = next_x
    self.y = next_y
    add(line_pts, {self.x, self.y})
    line_pts.segments += 1
    add(self.lines, line_pts)

  elseif self.maxlength >= 1 then
    self.maxlength -= 1
  end
  -- trim the lines
  while #self.lines > self.maxlength do
    del(self.lines, self.lines[1])
  end
  -- get older
  self.age += 1
end