[Canvas]用透明PNG图在背景上画前景能不遮挡背景

欲看动态效果请点击下载并用Chrome/Firefox浏览器打开index,html。

图例：

从效果可以明显的看到，五角星边缘和中心都没有对背景遮挡。

代码：

<!DOCTYPE html>
<html lang="utf-8">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<head>
     <title>动态背景3 19.3.4 12:43 by:逆火狂飙 horn19782016@163.com</title>

     <style>
     #canvas{
        background:#ffffff;
        cursor:pointer;
        margin-left:10px;
        margin-top:10px;
        -webkit-box-shadow:3px 3px 6px rgba(0,0,0,0.5);
        -moz-box-shadow:3px 3px 6px rgba(0,0,0,0.5);
        box-shadow:3px 3px 6px rgba(0,0,0,0.5);
     }

     #controls{
        margin-top:10px;
        margin-left:15px;
     }
     </style>

    </head>

     <body onload="init()">
        <div id="controls">
            <input id='animateBtn' type='button' value='运动'/>
        </div>

        <canvas id="canvas" width="1200px" height="562px" >
            出现文字表示你的浏览器不支持HTML5
        </canvas>
     </body>
</html>
<script type="text/javascript">
<!--
var paused=true;
animateBtn.onclick=function(e){
    paused=! paused;

    if(paused){
        animateBtn.value="运动";
    }else{
        animateBtn.value="暂停";
        window.requestAnimationFrame(animate);
    }
}

var ctx;// 绘图环境
var bg;// 背景
var icon1;
var icon2;

var bgOffset;
var bgVelocity;

function init(){
    // init Canvas
    var canvas=document.getElementById('canvas');
    canvas.width=1200;
    canvas.height=562;
    ctx=canvas.getContext('2d');

    // init varialbles
    bg=new Image();
    bg.src="greenBG.jpg";

    icon1=new Image();
    icon1.src="fiveStar1.png";

    icon2=new Image();
    icon2.src="fiveStar2.png";

    bgOffset=0;
    bgVelocity=10;
};

function update(){

}

var index=0;
function draw(){
    ctx.clearRect(0,0,ctx.canvas.width,ctx.canvas.height);
    bgOffset+=bgVelocity;
    if(bgOffset>ctx.canvas.width){
        bgOffset=0;
    }

    ctx.drawImage(bg,bgOffset,0,ctx.canvas.width-bgOffset,canvas.height,0,0,ctx.canvas.width-bgOffset,canvas.height);
    ctx.drawImage(bg,0,0,bgOffset,canvas.height,ctx.canvas.width-bgOffset,0,bgOffset,canvas.height);

    index++;
    if(index>100){
        index=0;
    }

    if(index % 2 ==0){
        ctx.drawImage(icon1,20,60);
    }else{
        ctx.drawImage(icon2,20,60);
    }

}

function animate(){
    if(!paused){

        update();
        draw();

        setTimeout( function(){
            window.requestAnimationFrame(animate); /// 让浏览器自行决定帧速率
        }, 0.20 * 1000 );// 延时执行
    }
}
//-->
</script>

2019年3月4日13点47分