动画原理——加载图片

书籍名称：HTML5-Animation-with-JavaScript

书籍源码：https://github.com/lamberta/html5-animation

1.实时加载图片

要实现实时加载图片，先创建一个图片对象设置它的src为URL地址。利用onload方法设置当图片对象加载完成执行回调函数。

在下面这个例子中，我们在图片加载完成后实时插入一张图片。

<!doctype html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Load Image</title>
    <link rel="stylesheet" href="../include/style.css">
  </head>
  <body>
    <header>
      Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a>
    </header>
    <canvas id="canvas" width="400" height="400"></canvas>
    
    <script>
    window.onload = function () {
      var canvas = document.getElementById('canvas'),
          context = canvas.getContext('2d'),
          image = new Image();

      image.src = "./assets/picture.jpg";
      image.onload = function() {
        context.drawImage(image, 0, 0);
      };
    };
    </script>
  </body>
</html>

View Code

2.插入页面中的图片元素

在页面加载的时候加载图片隐藏，使用DOM获取图片元素，然后调用canvas。

<!doctype html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Embed Image</title>
    <link rel="stylesheet" href="../include/style.css">
    <style>
      #picture {
        display: none;
      }
    </style>
  </head>
  <body>
    <header>
      Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a>
    </header>
    <canvas id="canvas" width="400" height="400"></canvas>
    <img id="picture" src="./assets/picture.jpg">
    
    <script>
    window.onload = function () {
      var canvas = document.getElementById('canvas'),
          context = canvas.getContext('2d'),
          image = document.getElementById('picture');

      context.drawImage(image, 0, 0);
    };
    </script>
  </body>
</html>

View Code

3.使用视频元素

和画静态动画一样，在一个时间点，只有一个画面。需要用到requestAnimationFrame

而插入页面中的视频元素和插入图片一样，下面看代码：

<!doctype html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Video Frames</title>
    <link rel="stylesheet" href="../include/style.css">
    <style>
      #movieclip {
        display: none;
      }
    </style>
  </head>
  <body>
    <header>
      Example from <a href="http://amzn.com/1430236655?tag=html5anim-20"><em>Foundation HTML5 Animation with JavaScript</em></a>
    </header>
    <canvas id="canvas" width="600" height="360"></canvas>
    <aside>Video file may take a moment to load.</aside>

    <video id="movieclip" width="640" height="360" autoplay>
      <source src="./assets/movieclip.mp4" type="video/mp4"/>
      <source src="./assets/movieclip.webm" type="video/webm"/>
      <source src="./assets/movieclip.ogv" type="video/ogg"/>
    </video>
  
    <script src="../include/utils.js"></script>
    <script>
    window.onload = function () {
      var canvas = document.getElementById('canvas'),
          context = canvas.getContext('2d'),
          video = document.getElementById('movieclip');

      (function drawFrame () {
        window.requestAnimationFrame(drawFrame, canvas);

        context.drawImage(video, 0, 0);
      }());
    };
    </script>
  </body>
</html>

View Code