国庆8天长假，重庆之行因故未成，偶得闲，用three.js结合cannon.js写个3D小游戏耍耍。
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);
......
}
