html实现随机验证码

代码：

<!DOCTYPE html>
<html>
<!-- head -->
<head>
  <title>图片登录验证</title>
</head>
<body>
     <input type="text" value="" placeholder="请输入验证码（区分大小写）" 
     style="height:43px;position: relative; top:-15px; font-size:20px;"id ="text">
     <canvas id="canvas" width="100" height="43" onclick="dj()" 
      style="border: 1px solid #ccc;
        border-radius: 5px;"></canvas>
     <button class="btn" onclick="sublim()">提交</button>
    </body>
<script>
 var show_num = [];
 draw(show_num);
function dj(){
 draw(show_num);   
 }
function sublim(){
var val=document.getElementById("text").value;  
            var num = show_num.join("");
            if(val==''){
                alert('请输入验证码！');
            }else if(val == num){
                alert('提交成功！');
                document.getElementById(".input-val").val('');
                draw(show_num);

            }else{
                alert('验证码错误！\n你输入的是:  '+val+"\n正确的是:  "+num+'\n请重新输入！');
                document.getElementById("text").value='';
                draw(show_num);
            }        
          }
function draw(show_num) {
        var canvas_width=document.getElementById('canvas').clientWidth;
        var canvas_height=document.getElementById('canvas').clientHeight;
        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,q,w,e,r,t,y,u,i,o,p,a,s,d,f,g,h,j,k,l,z,x,c,v,b,n,m";
        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;
            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 + ")";
    }
</script>  
</html>