Canvas矩形框滚动

要实现图片或形状在canvas画布上的移动：原理其实就是形状在画布上的x,y轴不停的呈现和擦除动作，造成在画布上移动的假象。

<!DOCTYPE html>
<html>
<body>

<canvas id="myCanvas" width="300" height="150" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.
</canvas>

<script>

var c=document.getElementById("myCanvas");
var ctx=c.getContext("2d");
ctx.fillStyle="red";
ctx.fillRect(0,0,300,150);
for(var i=0;i<=150;i++){
  setTimeout(function(i){
//      ctx.clearRect((20+i),20,100,50);

  },500);
}

var num = 0;
var test1 =  setInterval(function (){
  num++;

  console.log(num);
  ctx.clearRect((20+num),20,100,50);
  if (num > 150) {
    clearInterval(test1);
  }
},50);

</script>

</body>
</html>