在微信小程序的开发过程，经常会使用自带的api(wx.chooseImage)进行图片拍照或选择图片进行上传，有时图片太大，造成上传和下载时过慢，现对图片进行压缩后上传，以下是流程和代码

一、小程序的版本选择了3.2.5，使用其它版本有时不知道错误

二、wxml代码，需要添加canvas

三、js代码

1、参数

cwidth:'375',

cheight:'667',

//上传图片时，当前数组下标，用来确认上传第几张图片，上传完一次就+１

tempNum: 0,

// 图片上传参数　

filePath: "",

images: [],

count: 1,

2、选择图片wx.chooseImage

// 选择图片chooseImage: function (e) {console.log('选择图片chooseImage')var that = this;wx.chooseImage({count: that.data.count, // 默认9sizeType: ['compressed'], // 可以指定是原图还是压缩图，默认二者都有sourceType: ['camera'], // 可以指定来源是相册还是相机，默认二者都有success: function (res) {console.log('图片大小='+res.tempFiles[0].size)if(res.tempFiles[0].size>500000){//图片大于500K才进行压缩，压缩后大约400Kthat.canvasImage(res.tempFilePaths[0], 0) } else{//图片添加水印that.addTimeWatermark(res.tempFilePaths[0])} },})},//压缩图片canvasImage:function(imgUrl,index){console.log('canvasImage压缩图片')wx.showLoading({title: '照片上传中...',})var that = thiswx.getImageInfo({src: imgUrl,success(res) {console.log("路径", res.path)console.log('获得原始图片大小', res.width, res.height)var originWidth, originHeight;originHeight = res.height;originWidth = res.width;// 最大尺寸限制   //压缩比例var maxWidth = originWidth >= originHeight ? 540 : 810,maxHeight = originWidth >= originHeight ? 810 : 540;// 目标尺寸var targetWidth = originWidth,targetHeight = originHeight;//等比例压缩，如果宽度大于高度，则宽度优先，否则高度优先if (originWidth > maxWidth || originHeight > maxHeight) {if (originWidth / originHeight > maxWidth / maxHeight) {// 要求宽度*(原生图片比例)=新图片尺寸targetWidth = maxWidth;targetHeight = Math.round(maxWidth * (originHeight / originWidth));} else {targetHeight = maxHeight;targetWidth = Math.round(maxHeight * (originWidth / originHeight));}}console.log("压缩后的图片大小", targetWidth, targetHeight)var ctx = wx.createCanvasContext('mycanvas');ctx.clearRect(0, 0, targetWidth, targetHeight);ctx.drawImage(res.path, 0, 0, targetWidth, targetHeight);ctx.draw();//更新canvas大小that.setData({cwidth: targetWidth,cheight: targetHeight});setTimeout(function () {wx.canvasToTempFilePath({canvasId: 'mycanvas',success: (res) => {wx.hideLoading()console.log("压缩后的临时路径：", res.tempFilePath)that.addTimeWatermark(res.tempFilePath)},fail: (err) => {wx.hideLoading()console.error(err)}}, this)}, 400); //延迟400毫秒为了等canvas画上}})},
// 添加时间水印addTimeWatermark: function (imagePath) {var that = thisconsole.log('addTimeWatermark=' + imagePath)var ctx = wx.createCanvasContext('mycanvas');this.roundRectColor(ctx, 0, 30, 375, 620, 16);ctx.drawImage(imagePath, 15, 120, 344, 400);ctx.save();// 设置水印文字const time = app.util.formatTime()console.log('time=' + time)ctx.beginPath(); //开始绘制   ctx.setFontSize(22);ctx.setFillStyle('#DC3545')ctx.fillText(time, 15, 450); // 根据实际情况调整位置ctx.draw();//将生成好的图片保存到本地，需要延迟一会，绘制期间耗时setTimeout(function () {wx.canvasToTempFilePath({canvasId: 'mycanvas',success: function (res) {var tempFilePath = res.tempFilePath;console.log('imagePath=' + tempFilePath)that.data.images.push(tempFilePath);console.log(that.data.images)that.setData({imagePath: tempFilePath,images: that.data.images});},fail: function (res) {console.log(res);}});}, 200);},roundRectColor: function (context, x, y, w, h, r) { //绘制圆角矩形（纯色填充）context.save();context.setFillStyle("#FFFFFF"); //小框context.setStrokeStyle('#FFFFFF') //大框// context.setLineJoin('round'); //交点设置成圆角context.setLineWidth(r);context.strokeRect(x + r / 2, y + r / 2, w - r, h - r);context.fillRect(x + r, y + r, w - r * 2, h - r);context.stroke();context.closePath();},
//上传图片
app.util.uploadImg(that.callBackClose, app, that.data.pid, that.data.images, that.data.tempNum, 'check')