为什么这个画布动画会留下痕迹？怎么预防呢？

Question

我开始处理一些画布动画，并且难以理解CodePen中发生的情况。这是有问题的动画：https://codepen.io/towc/pen/mJzOWJ

var w = c.width = window.innerWidth,
    h = c.height = window.innerHeight,
    ctx = c.getContext( '2d' ),
    
    opts = {
      
      len: 20,
      count: 50,
      baseTime: 10,
      addedTime: 10,
      dieChance: .05,
      spawnChance: 1,
      sparkChance: .1,
      sparkDist: 10,
      sparkSize: 2,
      
      color: 'hsl(hue,100%,light%)',
      baseLight: 50,
      addedLight: 10, // [50-10,50+10]
      shadowToTimePropMult: 6,
      baseLightInputMultiplier: .01,
      addedLightInputMultiplier: .02,
      
      cx: w / 2,
      cy: h / 2,
      repaintAlpha: .04,
      hueChange: .1
    },
    
    tick = 0,
    lines = [],
    dieX = w / 2 / opts.len,
    dieY = h / 2 / opts.len,
    
    baseRad = Math.PI * 2 / 6;
    
ctx.fillStyle = 'black';
ctx.fillRect( 0, 0, w, h );

function loop() {
  
  window.requestAnimationFrame( loop );
  
  ++tick;
  
  ctx.globalCompositeOperation = 'source-over';
  ctx.shadowBlur = 0;
  ctx.fillStyle = 'rgba(0,0,0,alp)'.replace( 'alp', opts.repaintAlpha );
  ctx.fillRect( 0, 0, w, h );
  ctx.globalCompositeOperation = 'lighter';
  
  if( lines.length < opts.count && Math.random() < opts.spawnChance )
    lines.push( new Line );
  
  lines.map( function( line ){ line.step(); } );
}
function Line(){
  
  this.reset();
}
Line.prototype.reset = function(){
  
  this.x = 0;
  this.y = 0;
  this.addedX = 0;
  this.addedY = 0;
  
  this.rad = 0;
  
  this.lightInputMultiplier = opts.baseLightInputMultiplier + opts.addedLightInputMultiplier * Math.random();
  
  this.color = opts.color.replace( 'hue', tick * opts.hueChange );
  this.cumulativeTime = 0;
  
  this.beginPhase();
}
Line.prototype.beginPhase = function(){
  
  this.x += this.addedX;
  this.y += this.addedY;
  
  this.time = 0;
  this.targetTime = ( opts.baseTime + opts.addedTime * Math.random() ) |0;
  
  this.rad += baseRad * ( Math.random() < .5 ? 1 : -1 );
  this.addedX = Math.cos( this.rad );
  this.addedY = Math.sin( this.rad );
  
  if( Math.random() < opts.dieChance || this.x > dieX || this.x < -dieX || this.y > dieY || this.y < -dieY )
    this.reset();
}
Line.prototype.step = function(){
  
  ++this.time;
  ++this.cumulativeTime;
  
  if( this.time >= this.targetTime )
    this.beginPhase();
  
  var prop = this.time / this.targetTime,
      wave = Math.sin( prop * Math.PI / 2  ),
      x = this.addedX * wave,
      y = this.addedY * wave;
  
  ctx.shadowBlur = prop * opts.shadowToTimePropMult;
  ctx.fillStyle = ctx.shadowColor = this.color.replace( 'light', opts.baseLight + opts.addedLight * Math.sin( this.cumulativeTime * this.lightInputMultiplier ) );
  ctx.fillRect( opts.cx + ( this.x + x ) * opts.len, opts.cy + ( this.y + y ) * opts.len, 2, 2 );
  
  if( Math.random() < opts.sparkChance )
    ctx.fillRect( opts.cx + ( this.x + x ) * opts.len + Math.random() * opts.sparkDist * ( Math.random() < .5 ? 1 : -1 ) - opts.sparkSize / 2, opts.cy + ( this.y + y ) * opts.len + Math.random() * opts.sparkDist * ( Math.random() < .5 ? 1 : -1 ) - opts.sparkSize / 2, opts.sparkSize, opts.sparkSize )
}
loop();

window.addEventListener( 'resize', function(){
  
  w = c.width = window.innerWidth;
  h = c.height = window.innerHeight;
  ctx.fillStyle = 'black';
  ctx.fillRect( 0, 0, w, h );
  
  opts.cx = w / 2;
  opts.cy = h / 2;
  
  dieX = w / 2 / opts.len;
  dieY = h / 2 / opts.len;
});

canvas {
  
  position: absolute;
  top: 0;
  left: 0;
}

<canvas id=c></canvas>

如果让动画运行一段时间，以便“火花”散开，则它们会留下淡淡的灰色痕迹。是什么导致了这种情况，有可能防止这种情况发生吗？

谢谢！