当选项卡处于非活动状态/后台进行录制时，画布不会重新绘制 (WebGL)

Question

我正在构建在线Web应用程序，该应用程序在画布上呈现视频，然后使用canvas.captureStream((和mediaRecorder记录画布。问题是，当用户切换选项卡或最小化窗口时，画布会冻结。我的动画继续运行，因为我使用了webWorkerSetInterval(Hacktimer.js(。根据chrome，他们尚未提供解决方案 https://bugs.chromium.org/p/chromium/issues/detail?id=639105。

谁能建议解决方法？我尝试了不允许最小化的新窗口，但没有成功。切换窗口时录制不会停止。(仅在切换或最小化选项卡时停止(

Answer 1

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铌：
既然这个问题已经被专门编辑以处理 webgl 上下文，它可能不完全重复之前的答案，这确实不适用于 webgl 上下文;但是因为一个铬错误...

因此，此答案将向您展示如何在等待Chrome修复时解决此错误。

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链接的答案利用 WebAudio API 的计时方法来创建一个定时循环，与屏幕刷新率或窗口/选项卡的可见性无关。

但正如标题中所述，这目前不适用于 chrome 上的 webgl 上下文。

简单的解决方法是使用屏幕外的 2D 上下文作为流源，并将我们的 webgl 画布绘制到此 2D 上下文上：

function startRecording(webgl_renderer, render_func) {
  // create a clone of the webgl canvas
  var canvas = webgl_renderer.domElement.cloneNode();
  // init an 2D context
  var ctx = canvas.getContext('2d');
  function anim(){
    // render the webgl Animation
    render_func();
    // draw the wegbl canvas on our 2D one
    ctx.clearRect(0,0,canvas.width, canvas.height);
  	ctx.drawImage(webgl_renderer.domElement, 0,0);
  }
	var fps = 60;
  // start our loop @60fps
  var stopAnim = audioTimerLoop(anim, 1000 / fps);
  // maximum stream rate set as 60 fps
  var cStream = canvas.captureStream(fps);
  let chunks = [];
  var recorder = new MediaRecorder(cStream);
  recorder.ondataavailable = e => chunks.push(e.data);
  recorder.onstop = e => {
    // we can stop our loop
    stopAnim();
    var url = URL.createObjectURL(new Blob(chunks));
    var v = document.createElement('video');
    v.src = url;
    v.controls = true;
    document.body.appendChild(v);
  }
  recorder.start();
  // stops the recorder in 20s, try to change tab during this time
  setTimeout(function() {
    recorder.stop();
  }, 20000);
  btn.parentNode.removeChild(btn);
}
/*
    An alternative timing loop, based on AudioContext's clock
    @arg callback : a callback function 
        with the audioContext's currentTime passed as unique argument
    @arg frequency : float in ms;
    @returns : a stop function
*/
function audioTimerLoop(callback, frequency) {
  var freq = frequency / 1000;      // AudioContext time parameters are in seconds
  var aCtx = new AudioContext();
  // Chrome needs our oscillator node to be attached to the destination
  // So we create a silent Gain Node
  var silence = aCtx.createGain();
  silence.gain.value = 0;
  silence.connect(aCtx.destination);
  onOSCend();
  var stopped = false;       // A flag to know when we'll stop the loop
  function onOSCend() {
    var osc = aCtx.createOscillator();
    osc.onended = onOSCend; // so we can loop
    osc.connect(silence);
    osc.start(0); // start it now
    osc.stop(aCtx.currentTime + freq); // stop it next frame
    callback(aCtx.currentTime); // one frame is done
    if (stopped) {  // user broke the loop
      osc.onended = function() {
        aCtx.close(); // clear the audioContext
        return;
      };
    }
  };
  // return a function to stop our loop
  return function() {
    stopped = true;
  };
}
/* global THREE */
/* Note that all rAF loop have been removed
   since they're now handled by our 'audioTimerLoop' */
(function() {
    'use strict';
    var WIDTH = 500, HEIGHT = 500;
    var scene = new THREE.Scene();
    var camera = new THREE.PerspectiveCamera(75, WIDTH / HEIGHT, 0.1, 1000);
    var renderer = new THREE.WebGLRenderer();
    renderer.setSize(WIDTH , HEIGHT);
    document.body.appendChild(renderer.domElement);
    var geometry = new THREE.CubeGeometry(5, 5, 5);
    var material = new THREE.MeshLambertMaterial({
        color: 0x00fff0
    });
    var cube = new THREE.Mesh(geometry, material);
    scene.add(cube);
    camera.position.z = 12;
    
    var pointLight = new THREE.PointLight(0xFFFFFF);
    pointLight.position.x = 10;
    pointLight.position.y = 50;
    pointLight.position.z = 130;
    scene.add(pointLight);
    var render = function() {        
        var delta = Math.random() * (0.06 - 0.02) + 0.02;
        cube.rotation.x += delta;
        cube.rotation.y += delta;
        cube.rotation.z -= delta;
        renderer.render(scene, camera);
    };
    render();
    console.clear();
    
    btn.onclick = function(){startRecording(renderer, render);};
}());

body {
    margin: 0;
    background: #000;
}
button{
  position: absolute;
  top: 0;
  }

<script src="https://cdnjs.cloudflare.com/ajax/libs/three.js/85/three.min.js"></script>
<!-- Mobile devices need an user interaction to start the WebAudio API -->
<button id="btn">Start</button>