在微信小游戏中，把three.js的3D内容在离屏画布处理，然后复制到在屏画布，方法是：

let c_toolbarHeight=140;
 
let sysInfo=wx.getSystemInfoSync();
require('./js/libs/weapp-adapter.js');
var canvas_webGL=window.canvas;
canvas_webGL.width = sysInfo.screenWidth * sysInfo.pixelRatio;
canvas_webGL.height = (sysInfo.screenHeight-c_toolbarHeight) * sysInfo.pixelRatio;
var ctx_webGL=canvas_webGL.getContext('webgl');
 
let options={context: ctx_webGL}
let renderer = new THREE.WebGLRenderer(options);
renderer.setSize(sysInfo.screenWidth, sysInfo.screenHeight-c_toolbarHeight);
renderer.setPixelRatio(sysInfo.pixelRatio);
 
function render(){
  //清除canvas_bkg的3D区域
  wx.tmGlobal.eraseZone(0,
    c_toolbarHeight,
    sysInfo.screenWidth,
    sysInfo.screenHeight);
  renderer.render(scene, camera);
  wx.tmGlobal.ctx_bkg.drawImage(canvas_webGL,
    0,c_toolbarHeight*sysInfo.pixelRatio);
  //画一条横的红线
  wx.tmGlobal.ctx_bkg.strokeStyle = '#FF8C00';
  wx.tmGlobal.ctx_bkg.lineWidth = 2;
  wx.tmGlobal.ctx_bkg.beginPath();
  wx.tmGlobal.ctx_bkg.moveTo(0,
    (c_toolbarHeight)*sysInfo.pixelRatio);
  wx.tmGlobal.ctx_bkg.lineTo(
    sysInfo.screenWidth*sysInfo.pixelRatio,
    (c_toolbarHeight)*sysInfo.pixelRatio);
  //画游戏结束临界线
  wx.tmGlobal.ctx_bkg.moveTo(0,
    (c_toolbarHeight+c_yugaoHeight)*sysInfo.pixelRatio);
  wx.tmGlobal.ctx_bkg.lineTo(
    sysInfo.screenWidth*sysInfo.pixelRatio,
    (c_toolbarHeight+c_yugaoHeight)*sysInfo.pixelRatio);
  wx.tmGlobal.ctx_bkg.stroke();
  //把canvas_bkg画到在屏画布
  wx.tmGlobal.ctx_main.clearRect(0,0,
    wx.tmGlobal.canvas_main.width,wx.tmGlobal.canvas_main.height);
    wx.tmGlobal.ctx_main.drawImage(wx.tmGlobal.canvas_bkg,0,0);
}

在vivo和iphone手机都表现正常，但是，华为手机显示不出来：

https://developers.weixin.qq.com/community/develop/doc/00026c3c1c8eb010de384a82d51000?jumpto=

其它用户也提了好久了，腾讯或华为都没有解决，试来试去，终于找到了另一种写法：

renderer = new THREE.WebGLRenderer();
let target = new THREE.WebGLRenderTarget(
  sysInfo.screenWidth*sysInfo.pixelRatio, 
  (sysInfo.screenHeight-c_toolbarHeight)*sysInfo.pixelRatio);
renderer.setRenderTarget(target);
gl=renderer.getContext();
​
var canvas_huawei=wx.createCanvas();
canvas_huawei.width=sysInfo.screenWidth*sysInfo.pixelRatio;
canvas_huawei.height=(sysInfo.screenHeight-c_toolbarHeight)*sysInfo.pixelRatio;
var ctx_huawei=canvas_huawei.getContext('2d');
​
var canvas_huawei2=wx.createCanvas();
canvas_huawei2.width=sysInfo.screenWidth*sysInfo.pixelRatio;
canvas_huawei2.height=(sysInfo.screenHeight-c_toolbarHeight)*sysInfo.pixelRatio;
var ctx_huawei2=canvas_huawei2.getContext('2d');
​
var imageData = ctx_huawei.createImageData(
  sysInfo.screenWidth*sysInfo.pixelRatio,
  (sysInfo.screenHeight-c_toolbarHeight)*sysInfo.pixelRatio);
var pixels = new Uint8Array(imageData.data.length);
​
function render(){
  //擦除背景画布的webGL区域（因为webGL是用的透明绘制）
  wx.tmGlobal.eraseZone(0,
    c_toolbarHeight,
    sysInfo.screenWidth,
    sysInfo.screenHeight);
  renderer.render(scene, camera);
  gl.readPixels(
      0,
      0,
      gl.drawingBufferWidth,
      gl.drawingBufferHeight,
      gl.RGBA,gl.UNSIGNED_BYTE,pixels);
  imageData.data.set(pixels);
  ctx_huawei.putImageData(imageData,0,0);
  //清除   
  ctx_huawei2.clearRect(0,0,canvas_huawei2.width,canvas_huawei2.height);
  //上下镜像翻转
  ctx_huawei2.translate(0,canvas_huawei2.height);
  ctx_huawei2.scale(1, -1);
  ctx_huawei2.drawImage(canvas_huawei,0,0);
  //恢复
  ctx_huawei2.translate(0,canvas_huawei2.height);
  ctx_huawei2.scale(1, -1);
  wx.tmGlobal.ctx_bkg.drawImage(canvas_huawei2,
      0,c_toolbarHeight*sysInfo.pixelRatio);
  ......
}