<!DOCTYPE html>

<html>

<head lang="en">

    <meta charset="UTF-8">

    <title></title>

    <script src="js/jquery.js"></script>

    <script>

        $(function () {

            var x=10,y=10,WIDTH,HEIGHT,RADIUS=10;

            canvas=$("#canvas").get(0);

            d=canvas.getContext("2d");

            //小球方向，默认为1，向右下

            var direction=1;

            WIDTH= canvas.width;

            HEIGHT= canvas.height;

            //小球走

            setInterval(function () {

                //1.判断方向，决定小球的横纵坐标

                //2.判断方向变化

                if(direction==1){

//                    右下

                    x++;

                    y++;

                    if(y>=HEIGHT-RADIUS){

                        direction=2;

                    }

                    if(x>=WIDTH-RADIUS){

                        direction=4;

                    }

                }else if(direction==2){

                    //右上

                    x++;

                    y--;

                    if(x>=WIDTH-RADIUS){

                        direction=3;

                    }

                    if(y<=RADIUS){

                        direction=1;

                    }

                }else if(direction==3){

//                    左上

                    x--;

                    y--;

                    if(y<=RADIUS){

                        direction=4;

                    }

                    if(x<=RADIUS){

                        direction=2;

                    }

                }else if(direction==4){

//                    左下

                    x--;

                    y++;

                    if(x<=RADIUS){

                        direction=1;

                    }

                    if(y>=HEIGHT-RADIUS){

                        direction=3;

                    }

                }

                d.clearRect(0,0,WIDTH,HEIGHT);  // 清除之前的canvas

                d.fillStyle="blue";

                d.beginPath(); // 从新开始画，防止 冲突重叠

                d.arc(x,y,RADIUS,0,Math.PI*2,true); // x y 坐标 半径30

                d.closePath(); // 结束画布，防止冲突重叠

                d.fill(); // 结束渲染

            },20);

        });

    </script>

</head>

<body>

    <canvas id="canvas" width="400" height="300" style="border:1px solid green"></canvas>

<span><img src="direction.png" width="400"></span>

</body>

</html>

<!DOCTYPE html>

<html>

<head lang="en">

    <meta charset="UTF-8">

    <title></title>

    <script src="js/jquery.js"></script>

    <script>

        $(function () {

            var x=10,y=10; //  圆坐标

            var WIDTH,HEIGHT,RADIUS=10;

            xx=1;yy=1; // 步长

            canvas=$("#canvas").get(0);

            d=canvas.getContext("2d");

            WIDTH= canvas.width;

            HEIGHT= canvas.height;

            //小球走

            setInterval(function () {

                if(x < RADIUS || x >= WIDTH-RADIUS){   // 一开始坐标就相等了，要去掉=号 x = radius

                    xx *= -1;

                }

                //判断小球纵向边界

                if(y < RADIUS || y >= HEIGHT-RADIUS){

                    yy *= -1;

                }

                x += xx;

                y += yy;

                d.clearRect(0,0,WIDTH,HEIGHT);  // 清除之前的canvas

                d.fillStyle="blue";

                d.beginPath(); // 从新开始画，防止 冲突重叠

                d.arc(x,y,RADIUS,0,Math.PI*2,true); // x y 坐标 半径30

                d.closePath(); // 结束画布，防止冲突重叠

                d.fill(); // 结束渲染

            },20);

        });

    </script>

</head>

<body>

    <canvas id="canvas" width="400" height="300" style="border:1px solid green"></canvas>

<span><img src="direction.png" width="400"></span>

</body>

</html>