Canvas/07_canvas_抛物线.html

<!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8" />
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <title>Page Title</title>
  <meta name="viewport" content="width=device-width, initial-scale=1">
</head>

<body>
  <canvas id="myCanvas" style="border:1px solid red;">
    当前浏览器不支持canvas，请更换浏览器后再试
  </canvas>
  <script>
    var ball = {
      x: 700,
      y: 100,
      r: 20,
      g: 2,
      vx: -4,
      vy: 0,
      color: 'red'
    }
    window.onload = function () {
      var canvas = document.getElementById('myCanvas')

      canvas.width = 800
      canvas.height = 400

      var ctx = canvas.getContext('2d')

      function step(n) {

        render(ctx)
        update()

        requestAnimationFrame(step)
      }

      requestAnimationFrame(step)
    }

    function update() {
      ball.x += ball.vx
      ball.y += ball.vy
      ball.vy += ball.g

      if (ball.y >= 400 - ball.r) {
        ball.y = 400 - ball.r
        ball.vy = -ball.vy * 0.75
      }
    }

    function render(ctx) {
      ctx.clearRect(0, 0, ctx.canvas.width, ctx.canvas.height)

      ctx.fillStyle = ball.color
      ctx.beginPath()
      ctx.arc(ball.x, ball.y, ball.r, 0, Math.PI * 2)
      ctx.fill()
    }


  </script>
</body>

</html>