微信小遊戲中three.js離屏畫布

國慶8天長假，重慶之行因故未成，偶得閒，用three.js結合cannon.js寫個3D小遊戲耍耍。

在微信小遊戲中，把three.js的3D內容在離屏畫布處理，然後複製到在屏畫布，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區域
    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);
    //畫一條橫的紅線
    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);
    //畫遊戲結束臨界線
    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畫到在屏畫布
    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手機都表現正常，但是，華為手機顯示不出來：

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

其它使用者也提了好久了，騰訊或華為都沒有解決，試來試去，終於找到了另一種寫法：

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(){
    //擦除背景畫布的webGL區域（因為webGL是用的透明繪製）
    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);
    //上下映象翻轉
    ctx_huawei2.translate(0,canvas_huawei2.height);
    ctx_huawei2.scale(1, -1);
    ctx_huawei2.drawImage(canvas_huawei,0,0);
    //恢復
    ctx_huawei2.translate(0,canvas_huawei2.height);
    ctx_huawei2.scale(1, -1);
    wx.tmGlobal.ctx_bkg.drawImage(canvas_huawei2,
            0,c_toolbarHeight*sysInfo.pixelRatio);
    ......
}