canvas给图片加水印

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
<canvas id="myCanvas" width="1000" height="500" >
Your browser does not support the HTML5 canvas tag.
</canvas>
<script>
    //准备img对象 
    var img = new Image();   
    img.src = './img/demo.jpg';
 
    // 加载完成开始绘制
    img.onload=function(){
       //准备canvas环境 
       var canvas=document.getElementById("myCanvas");
       var ctx=canvas.getContext("2d");
       // 绘制图片
       ctx.drawImage(img,0,0);
       // 绘制水印
       ctx.font="20px microsoft yahei";
       ctx.fillStyle = "rgba(255,255,255,0.5)";
       ctx.fillText("my images",100,100);
    }
</script>
</body>
其实就是