video标签自动播放音视频并绘制波形图

html中的<video>标签可以用来播放常见的音视频格式，支持的格式包括:MP3、Ogg、WAV、AAC、MP4、WebM、AVI等，当然支持的格式也和浏览器和操作系统有关。这里以一个可以自动播放音视频并绘制波形图的页面为例说明一下<video>标签的用法。

video标签想自动播放，需要设置三个可选属性分别是muted、autoplay、controls，muted负责让音视频播放静音，autoplay让音视频自动播放，controls属性负责显示对应的控制菜单。除了通过html页面设置标签属性之外我们还可以通过js脚本来设置对应的属性，设置方法如下所示:

<!DOCTYPE html>
<html>
<head><title>播放音视频</title><meta charset="UTF-8">
</head>
<body><video id="myVideo" autoplay muted controls><source src="./mysong.mp3" type="audio/mpeg"><!--source src="video.mp4" type="video/mp4"-->Your browser does not support the video tag.</video><script>var videoElement = document.getElementById('myVideo');// 自动静音播放videoElement.muted = true; videoElement.autoplay = true;videoElement.controls = true;</script>
</body>
</html>

浏览器为了防止页面自动播放音频干扰用户，不允许在用户没有进行交互操作的时候，网页自动以非静音的模式播放音视频。所以autoplay属性必须搭配muted属性一块使用。
如果想要绘制音视频播放过程中的音频波形图，我们需要拦截对应的音频上下文，分析绘制对应的音频数据。对应的实现如下所示:

<!DOCTYPE html>
<html>
<head><title>绘制音频波形图</title><meta charset="UTF-8">
</head>
<body><h1>绘制音频波形图</h1><video id="myVideo" controls><source src="./mysong.mp3" type="audio/mpeg">Your browser does not support the video tag.</video><canvas id="waveformCanvas"></canvas>
<script>// 获取video元素和canvas元素let video,analyser,ctx,canvas,audioContext,timerID,analyserNode;video = document.getElementById('myVideo');//播放的时候调用初始化操作video.addEventListener("play",initWaveDraw);//获取画布元素canvas = document.getElementById('waveformCanvas');ctx = canvas.getContext('2d');function initWaveDraw(){// 创建音频上下文if(!audioContext){audioContext = new(window.AudioContext || window.webkitAudioContext)();analyser = audioContext.createAnalyser();analyser.connect(audioContext.destination);analyserNode = audioContext.createMediaElementSource(video);analyserNode.connect(analyser);timerID = setInterval(drawWaveform,200); }}// 绘制波形图function drawWaveform() {// 获取波形数据var bufferLength = analyser.fftSize;console.log("drawing wave");var dataArray = new Uint8Array(bufferLength);analyser.getByteTimeDomainData(dataArray);// 清空画布ctx.clearRect(0, 0, canvas.width, canvas.height);// 绘制波形图ctx.lineWidth = 2;ctx.strokeStyle = 'rgb(0, 255, 255)';ctx.beginPath();var sliceWidth = canvas.width * 1.0 / bufferLength;var x = 0;for (var i = 0; i < bufferLength; i++) {var v = dataArray[i] / 128.0;var y = v * canvas.height / 2;if (i === 0) {ctx.moveTo(x, y);} else {ctx.lineTo(x, y);}x += sliceWidth;}ctx.lineTo(canvas.width, canvas.height / 2);ctx.stroke();}
</script>	
</body>
</html>

显示效果如下图所示:

在这里插入图片描述

如果想让播放器自动播放音频并放音，我们可以采用一些迂回策略，首先让播放器自动静音播放，然后设置一个定时器检测用户是否和页面发生了操作交互，如果产生了交互就播放音频并绘制波形图。(浏览器默认没有交互的时候不允许播放声音) 对应的实现如下所示:

<!DOCTYPE html>
<html>
<head><title>绘制音频波形图</title><meta charset="UTF-8">
</head>
<body><h1>绘制音频波形图</h1><video id="myVideo" controls><source src="./mysong.mp3" type="audio/mpeg">Your browser does not support the video tag.</video><canvas id="waveformCanvas"></canvas>
<script>// 获取video元素和canvas元素let video,analyser,ctx,canvas,audioContext,timerID, checktimerID, analyserNode;var hasUserInteracted = false;video = document.getElementById('myVideo');//获取画布元素canvas = document.getElementById('waveformCanvas');ctx = canvas.getContext('2d');video.muted = true; video.autoplay = true;video.controls = true;// 监听键盘按下事件function handleUserInteraction(){console.log("user has interacted");hasUserInteracted = true;}document.addEventListener('click', handleUserInteraction);checktimerID = setInterval(checkMouseClick, 1000);setTimeout(function(){ video.addEventListener("volumechange",handleUserInteraction);},2000);//定时检测鼠标事件,开启带声音的播放function checkMouseClick(){if(hasUserInteracted){			initWaveDraw();video.muted = false; video.play();				clearInterval(checktimerID);}}function initWaveDraw(){// 创建音频上下文if(!audioContext){audioContext = new(window.AudioContext || window.webkitAudioContext)();analyser = audioContext.createAnalyser();analyser.connect(audioContext.destination);analyserNode = audioContext.createMediaElementSource(video);analyserNode.connect(analyser);timerID = setInterval(drawWaveform,200); }}// 绘制波形图function drawWaveform() {// 获取波形数据var bufferLength = analyser.fftSize;console.log("drawing wave");var dataArray = new Uint8Array(bufferLength);analyser.getByteTimeDomainData(dataArray);// 清空画布ctx.clearRect(0, 0, canvas.width, canvas.height);// 绘制波形图ctx.lineWidth = 2;ctx.strokeStyle = 'rgb(0, 255, 255)';ctx.beginPath();var sliceWidth = canvas.width * 1.0 / bufferLength;var x = 0;for (var i = 0; i < bufferLength; i++) {var v = dataArray[i] / 128.0;var y = v * canvas.height / 2;if (i === 0) {ctx.moveTo(x, y);} else {ctx.lineTo(x, y);}x += sliceWidth;}ctx.lineTo(canvas.width, canvas.height / 2);ctx.stroke();}
</script>	
</body>
</html>