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How to use JS to realize the parabolic trajectory motion of a small ball

亚连Original: 2018-06-19 15:58:291960browse

This article mainly introduces two methods of realizing the parabolic trajectory motion of a JS ball. It analyzes the relevant calculation and graphics drawing operation skills of javascript to realize the parabolic trajectory of a small ball in the form of examples. Friends in need can refer to the following

The examples in this article describe two methods of realizing the parabolic trajectory motion of the JS ball. Share it with everyone for your reference, the details are as follows:

The general idea of implementing the parabolic trajectory motion of the ball in js:

1. Use the setInterval() method to perform interval Refresh, update the position of the ball to achieve dynamic effects
2. Draw the ball and the sports area. The sports area can be vertically centered through flex layout
3. Use the physical formulaS(y)=1/ 2*g*t*t, S(x)=V(x)t to calculate the path
is now determinedV(x)=4m/s, The refresh interval is set to 0.1s. Originally, the conversion between px and meters is different for different sizes. In this example, a 17-inch monitor is used, which is about 1px=0.4mm. However, the browser is too small, so it can only simulate a parabolic trajectory. In this example, the distance between px and meters is reduced to 100 times.

The first method: draw the ball through border-radius

Idea: use border-radius: 50% Draw the ball, set relative to the ball, calculate the current position of the ball each time, and assign it to top and left. When calculating the motion trajectory, the variable can be incremented to calculate the number of setInterval calls, each time is 0.1s, so that the total time can be calculated. The code is as follows:

<!DOCTYPE>
<html>
<head>
  <meta http-equiv="Content-type" content="text/html; charset=utf-8" />
  <title></title>
  <style type="text/css">
  /*给body进行flex布局，实现垂直居中*/
  body {
  min-height: 100vh;/*高度适应浏览器高度*/
  display: flex;
  justify-content: center;/*水平居中*/
  align-items: center;/*垂直居中*/
    font-size: 20px;
    font-weight: bold;
  }
  /*设置运动区域*/
  #bg {
    width: 600px;
    height: 600px;
    border: 2px solid #e0e0e0;
    border-radius: 4px;/*给p设置圆角*/
    padding: 20px;
  }
  /*生成小球，并定义初始位置*/
  #ball {
    border-radius: 50%;/*可把正方形变成圆形，50%即可*/
    background: #e0e0e0;
    width: 60px;
    height: 60px;
    position: relative;
    top: 30px;
    left: 30px;
  }
  button {
    width: 80px;
    height: 30px;
    border-radius: 4px;
    color: #fff;
    background: #AA7ECC;
    font-size: 20px;
    font-weight: bold;
    margin-left: 20px;
  }
  </style>
</head>
<body>
<p id="bg">
  此时水平速度为：4<button onclick="start()">演示</button>
  <p id="ball"></p>
</p>
<script type="text/javascript">
function start(){
  var x,y,k=1,t;
  //x是水平方向移动路径；y是垂直方向的；k记录次数，可与0.1相乘得时间；t记录setInterval的返回id，用于clearInterval
  t = setInterval(function(){
  x = 30+0.1*k*4*100;
    //S(x)=S(0)+t*V(x)，100是自定义的米到px转换数
    y = 30+1/2*9.8*0.1*k*0.1*k*100;//S(y)=S(0)+1/2*g*t*t
  var j = document.getElementById("ball");
    //通过修改小球的top和left，修改小球的位置
    j.style.top = y;
    j.style.left = x;
  k++;//每次调用，k自增，简化计算
  if(x>480||y>480){
  clearInterval(t);//小球达到边界时，清除setInterval
  }
  },100);//每0.1s调用一次setInterval的function
}
</script>
</body>
</html>

Second type: canvas in h5 draws the ball and sports area

Idea: Use canvas to draw the ball, Since the ball cannot move through top and left, it needs to clear the canvas with clearRect every time it is called, and then draw the ball at the calculated position. The code is as follows:

<!DOCTYPE>
<html>
<head>
  <meta http-equiv="Content-type" content="text/html; charset=utf-8" />
  <title></title>
  <style type="text/css">
  body {
  min-height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  }
  #myCanvas {
  box-shadow: -2px -2px 2px #efefef,5px 5px 5px #b9b9b9;
  }
  </style>
</head>
<body>
<canvas id="myCanvas" width="600px" height="600px"></canvas>
<script type="text/javascript">
var x=50,y=50,k=1;
var c=document.getElementById("myCanvas");
var cxt=c.getContext("2d");
cxt.fillStyle="#e0e0e0";
cxt.beginPath();
cxt.arc(x,y,30,0,Math.PI*2,true);
cxt.closePath();
cxt.fill();
t=setInterval("parabola()",100);
function parabola(){
  cxt.clearRect(0,0,600,600);//清除原始图形
  cxt.beginPath();
  x=50+0.1*k*4*100;y=50+9.8*0.1*k*0.1*k*1/2*100;
  cxt.arc(x,y,30,0,Math.PI*2,true);
  cxt.closePath();
  cxt.fill();
  k++;
  if(x>520||y>520){
  clearInterval(t);
  }
}
</script>
</body>
</html>

The above is what I compiled for everyone. I hope it will be helpful to everyone in the future.

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