请问这种canvas背景如何变成透明呢

请问这种canvas背景如何变成透明呢请问这种canvas背景如何变成透明呢



<!DOCTYPE HTML>
<html>
<body>
<canvas id="canvas" width="600"height="400">    
         <p>Your browserdoes not support the canvas element!</p>    
</canvas>    
     
<script type="text/javascript">    
    var canvas =document.getElementById("canvas");    
    var context2D =canvas.getContext("2d"); 
    const TIME = 1    
    var factor = 0;  
    
    function changeAlpha() {    
         factor+= TIME/50;    
         context2D.globalAlpha= (Math.sin(factor) + 1) / 2;   //这里采用简单算法布置透明    
    }  
    
    function draw() { 
        context2D.clearRect(0,0, canvas.width, canvas.height);     
        changeAlpha();
        //将上面的圆填充为灰色          
        context2D.fillStyle ="#FF0000"; 
        context2D.arc(100,100, 60, 0, Math.PI*2 , false);    //注意这里的参数是弧度制，而不是角度制
        context2D.fill(); 
    }    
    
    setInterval(draw, TIME); 
     
</script>    
</body>
</html>