黑色是因为video没有自动播放导致的。
而且video必须设置muted（静音）属性，否则视频都无法播放；
如果不设置muted,也可以用设置x5-video-player-type="h5" 替代（意为兼容qq浏览器，解决在小程序中黑色的问题）

  const coverVideoTarget = document.getElementById('coverVideo');
  function initThree(){
    scene = new THREE.Scene();
    // camera = new THREE.Camera();
    camera = new THREE.PerspectiveCamera(75,window.innerWidth/window.innerHeight,0.1,10000);
    camera.matrixAutoUpdate = false;
    scene.add(camera);
    scene.add(new THREE.AmbientLight(0xffffff, 1.5));
  
    root = new THREE.Object3D();
    root.matrixAutoUpdate = false;
    scene.add(root);


    renderer = new THREE.WebGLRenderer({ canvas: canvas, alpha: true, antialias: true });
    renderer.setPixelRatio(window.devicePixelRatio);
    renderer.setSize(webglWidth, webglHeight);
    setVideo('./video/18912265449355602.mp4');
    // setModel();
  }
  function setVideo(videoUrl){
      console.log('setVideo')
      coverVideoTarget.src = videoUrl;
      const texture = new THREE.VideoTexture(coverVideoTarget);
      var mat = new THREE.MeshBasicMaterial({map: texture,transparent:true});
  
      const geo = new THREE.PlaneGeometry(400,240);
      // const geo = new THREE.PlaneGeometry(20,12);
      coverVideo = new THREE.Mesh(geo, mat);
      coverVideo.position.z = -50;
      coverVideo.position.x = 140;
      coverVideo.position.y = 200;
      root.add(coverVideo);
  }

  function setVideo2(videoUrl){
      console.log('setVideo2')
      coverVideoTarget.src = videoUrl;
      let vWidth = 400, vHeight = 240;
      coverVideoTarget.addEventListener('canplay', function () {
        vWidth = this.videoWidth;
        vHeight = this.videoHeight;
      });
  
       var canvas_process = document.createElement('canvas');// 未被加入body。用于绘制video帧图片并传入tracker识别使用
       var context_process = canvas_process.getContext('2d');
       const texture = new THREE.CanvasTexture(canvas_process);
       function update() {
        context_process.fillStyle = 'black';
        context_process.fillRect(0, 0, vWidth, vHeight);
        context_process.drawImage(coverVideoTarget, 0, 0, vWidth, vHeight);
        texture.needsUpdate = true;
        requestAnimationFrame(update);
      }
      update();
      var mat = new THREE.MeshBasicMaterial({map: texture,transparent:true});
  
      const geo = new THREE.PlaneGeometry(400,240);
      // const geo = new THREE.PlaneGeometry(20,12);
      coverVideo = new THREE.Mesh(geo, mat);
      coverVideo.position.z = -50;
      coverVideo.position.x = 140;
      coverVideo.position.y = 200;
      root.add(coverVideo);
  }
  
function initNFT(marker, videoUrl) {// video宽高比 240 : 360
    if (typeof(NFTworker) !== 'undefined') {
      NFTworker.terminate();
      NFTworker = undefined;
    }
    // setVideo(videoUrl);
    NFTworker = new Worker('../js/imgTracker.worker.js');
    NFTworker.postMessage({ type: "initNFT", pw: trackImgWidth, ph: trackImgHeight, trackData: './../examples/Data/camera_para.dat', marker: marker });
    var resultTime = new Date().getTime(),noResultTime = 0;
    NFTworker.onmessage = function (ev) {
      var msg = ev.data;
      switch (msg.type) {
        case "nftInitSuccess": {
          var proj = JSON.parse(msg.proj);
          proj[0] *= ratioW;
          proj[4] *= ratioW;
          proj[8] *= ratioW;
          proj[12] *= ratioW;
          proj[1] *= ratioH;
          proj[5] *= ratioH;
          proj[9] *= ratioH;
          proj[13] *= ratioH;
          setMatrix(camera.projectionMatrix, proj);
          nftProcess();
          break;
        }
        case 'found': {
          // console.log('found')
          coverVideo && (coverVideo.visible = true);
          coverVideoTarget.play();
          setMatrix(root.matrix, JSON.parse(msg.matrixGL_RH));
          renderer.render(scene, camera);
          resultTime = new Date().getTime();
          nftProcess();
          break;
        }
        case 'noResult': {
          coverVideo && (coverVideo.visible = false);
          coverVideoTarget.pause();
          renderer.render(scene, camera);
          var noResultTime = new Date().getTime();
          if (noResultTime - resultTime > 3000) {
            console.log('go track');
            // trackProcess('track');
            nftProcess();
          } else {
            nftProcess();
          }
          break;
        }
      }
    };
  };

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>ALVA Image Tracker</title>
  <meta name="viewport" content="width=device-width, initial-scale=1, minimum-scale=0.5, maximum-scale=1">
  <link rel="stylesheet" href="css/nft-style.css">
</head>
<style>
  #coverVideo{
    /* visibility: hidden; */
    position: fixed;
    /* left: 0px; */
    left: -1000000px;
    top:0px;
    z-index: 10000;
    width: 320px;
    height: 240px;
    background-color: pink;
  }
</style>
<body>
  <div id="loading" >
    <span class="loading-text">Loading, please wait</span>
  </div>
  <div id="app">
    <video
      loop
      autoplay
      muted
      playsinline
      id="cameraVideo">
    </video>
    <video id="coverVideo" loop controls  webkit-playsinline="true" x-webkit-airplay="true"
    playsinline="true" x5-video-player-type="h5" preload="auto" src='./video/18912265449355602.mp4'></video>

    <canvas id="canvas"></canvas>
  </div>

  <script src="../vconsole.min.js"></script>
  <script src="js/third_party/three.js/three.js"></script>
  <script src="js/third_party/three.js/GLTFLoader.js"></script>
  <script src="https://res2.wx.qq.com/open/js/jweixin-1.6.0.js"></script>

  <script src="threejs_worker.js"></script>

</body>

</html>

	var imgTracker;
	var cameraVideo = document.getElementById( 'cameraVideo' );
	if (navigator.mediaDevices && navigator.mediaDevices.getUserMedia) {
	  var hint = { audio: false, video: true };
	  if( window.innerWidth < 800 ) { // 宽高比为2/3或者3/2
	    var width = ( window.innerWidth < window.innerHeight ) ? 240 : 360;
	    var height = ( window.innerWidth < window.innerHeight ) ? 360 : 240;
	    width = width * window.devicePixelRatio;
	    height = height * window.devicePixelRatio;
	
	    hint = {
	      audio: false,
	      video: {
	        facingMode: 'environment',// 调用后置摄像头
	        width: { min: width, max: width }
	      },
	    };
	  }
	  navigator.mediaDevices.getUserMedia( hint ).then( function( stream ) {
	    cameraVideo.srcObject = stream;
	    cameraVideo.addEventListener( 'loadedmetadata', function() {
	      cameraVideo.play();
	      start(cameraVideo);
	    } );
	  } ).catch(err => {
	    console.log('catch');
	  });
	} else {
	
	}