javascript - canvas 怎么实现为ajax返回的图片添加水印?

Question

axse (

    function (data) {
        var str = data.d;
        var obj = JSON.parse(str);
        var banNum = obj.data[0].ImageView;
        console.log(banNum);
        var canvas=document.getElementById("myCanvas");
        for (var i=0;i < banNum.length;i++) {
            var img_url = banNum[i].ImgUrl;
            $("<img class='my_img"+i+"' id='img"+i+"' style='display: none;' src='"+img_url+"'></img>").appendTo('body');
            var img_height = $('.my_img'+i+'').css('height');
            var img_width = $('.my_img'+i+'').css('width');
            $("#myCanvas").attr({'width':img_height,'height':img_height});
            var img_id = document.getElementById('img'+i+'');    
            var ctx=canvas.getContext("2d");
               // 绘制图片
               ctx.drawImage(img_id,0,0);
               // 绘制水印
               ctx.font="15px microsoft yahei";
               ctx.fillStyle = "rgba(255,255,255,.5)";
              ctx.fillText("my images",100,100);
            $("#imga").attr('src',canvas.toDataURL());
        }
    }
)