function draw(show_num) {
    var canvas_width=$('#canvas').width();
    var canvas_height=$('#canvas').height();
    var canvas = document.getelementbyid("canvas");//获取到canvas的对象，演员
    var context = canvas.getcontext("2d");//获取到canvas画图的环境，演员表演的舞台
    canvas.width = canvas_width;
    canvas.height = canvas_height;
    var scode = "a,b,c,e,f,g,h,j,k,l,m,n,p,q,r,s,t,w,x,y,z,1,2,3,4,5,6,7,8,9,0";
    var acode = scode.split(",");
    var alength = acode.length;//获取到数组的长度

    for (var i = 0; i <= 3; i++) {
        var j = math.floor(math.random() * alength);//获取到随机的索引值
        var deg = math.random() * 30 * math.pi / 180;//产生0~30之间的随机弧度
        var txt = acode[j];//得到随机的一个内容
        show_num[i] = txt.tolowercase();
        var x = 10 + i * 20;//文字在canvas上的x坐标
        var y = 20 + math.random() * 8;//文字在canvas上的y坐标
        context.font = "bold 23px 微软雅黑";

        context.translate(x, y);
        context.rotate(deg);

        context.fillstyle = randomcolor();
        context.filltext(txt, 0, 0);

        context.rotate(-deg);
        context.translate(-x, -y);
    }
    for (var i = 0; i <= 5; i++) { //验证码上显示线条
        context.strokestyle = randomcolor();
        context.beginpath();
        context.moveto(math.random() * canvas_width, math.random() * canvas_height);
        context.lineto(math.random() * canvas_width, math.random() * canvas_height);
        context.stroke();
    }
    for (var i = 0; i <= 30; i++) { //验证码上显示小点
        context.strokestyle = randomcolor();
        context.beginpath();
        var x = math.random() * canvas_width;
        var y = math.random() * canvas_height;
        context.moveto(x, y);
        context.lineto(x + 1, y + 1);
        context.stroke();
    }
}

function randomcolor() {//得到随机的颜色值
    var r = math.floor(math.random() * 256);
    var g = math.floor(math.random() * 256);
    var b = math.floor(math.random() * 256);
    return "rgb(" + r + "," + g + "," + b + ")";
}