前端生成图形验证码

代码：

      //         生成随机字符图片
      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() * 20 * Math.PI) / 180; //产生0~20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 = 5 + i * 20; //文字在canvas上的x坐标
          var y = 20 + Math.random() * 10; //文字在canvas上的y坐标
          context.font = "bold 30px 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 + ")";
      }