jquery如何生成图片验证码

jQuery(function($){
    
    /**生成一个随机数**/
    function randomNum(min, max) {
        return Math.floor(Math.random() * (max - min) + min);
    }
    /**生成一个随机色**/
    function randomColor(min, max) {
        var r = randomNum(min, max);
        var g = randomNum(min, max);
        var b= randomNum(min, max);
        return "rgb(" + r + "," + g + "," + b + ")";
    }
    var code=drawPic();
    document.getElementById("changeImg").onclick = function(e) {
        e.preventDefault();
        code=drawPic();
    }
    /**绘制验证码图片**/
    function drawPic() {
        var canvas = document.getElementById("canvas");
        var width = canvas.width;
        var height = canvas.height;
        //获取该canvas的2D绘图环境 
        var ctx = canvas.getContext('2d'); 
        ctx.textBaseline ='bottom';
        /**绘制背景色**/
        ctx.fillStyle = randomColor(180,240);
        //颜色若太深可能导致看不清
        ctx.fillRect(0,0,width,height);
        /**绘制文字**/
        var str ='ABCEFGHJKLMNPQRSTWXY123456789';

　　　　 var code="";

　　　　　//生成四个验证码
        for(var i=1;i<=4;i++) {
            var txt = str[randomNum(0,str.length)];
            code=code+txt;
            ctx.fillStyle = randomColor(50,160);
            //随机生成字体颜色
            ctx.font = randomNum(15,40) +'px SimHei';
            //随机生成字体大小
            var x =10 +i *25;
            var y = randomNum(25,35);
            var deg = randomNum(-45,45);
            //修改坐标原点和旋转角度
            ctx.translate(x, y); 
            ctx.rotate(deg * Math.PI /180); 
            ctx.fillText(txt,0,0);
            //恢复坐标原点和旋转角度
            ctx.rotate(-deg * Math.PI /180);
            ctx.translate(-x, -y);
        }
        
        /**绘制干扰线**/
        for(var i=0;i<3;i++) {
            ctx.strokeStyle = randomColor(40, 180);
            ctx.beginPath();
            ctx.moveTo(randomNum(0,width/2), randomNum(0,height/2));
            ctx.lineTo(randomNum(0,width/2), randomNum(0,height));
            ctx.stroke();
        }
        /**绘制干扰点**/
        for(var i=0;i <50;i++) {
            ctx.fillStyle = randomColor(255);
            ctx.beginPath();
            ctx.arc(randomNum(0, width), randomNum(0, height),1,0,2* Math.PI);
            ctx.fill();
        }
        return code;
    }
});

<a href="#" id="changeImg">
    <canvas class="show-captcha" id="canvas" width="150" height="40"></canvas>
</a>

在jquery的代码中，根据id或name获取html或freemarker的标签，如：var canvas = document.getElementById("canvas"); 然后设置画布的宽高，设置干扰点，干扰线。