js写验证码

　　html代码添加

<canvas id="canvas" width="100" height="40" alt="单击刷新"></canvas>

　　js添加，配置canvas样式，过滤掉较浅的颜色。r * 0.299 + g * 0.578 + b * 0.114>=192就是浅色。

$(function () {
            draw(show_num);
            $("#canvas").on('click',function (){draw(show_num)});
        });

 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,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,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 24px 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 sumrgb = 193, r = 0, g = 0, b = 0;
            while (sumrgb > 192) { //排除浅色

                 r = Math.floor(Math.random() * 256);
                 g = Math.floor(Math.random() * 256);
                 b = Math.floor(Math.random() * 256);
                 sumrgb = r * 0.299 + g * 0.578 + b * 0.114;
            }
            return "rgb(" + r + "," + g + "," + b + ")";
        }
    </script>
    <style>
        #canvas {
            display: inline-block;
            border: 1px solid #ccc;
            border-radius: 5px;
            cursor: pointer;
        }
    </style>