借鉴了网上的代码，使用画布，将图像重绘后，保存。试了两种保存方法，都返回空白图片。基础库：2.21.4

WXML:

JS: (确定了传入的img_src是对的，在开发者工具里似乎把本地图片链接都转为http链接了)

var that = this;
wx.getImageInfo({
      src: img_src,  
      success(res){
      //---------利用canvas压缩图片--------------
        var canvasHeight =300;
                var canvasWidth = 400;
        that.setData({
          cHeight: canvasHeight,
          cWidth: canvasWidth
        });
        wx.createSelectorQuery()
          .select('#myCanvas') // 在 WXML 中填入的 id
          .fields({ node: true, size: true })
          .exec((res) => {
              // Canvas 对象
              const canvas = res[0].node
              // Canvas 画布的实际绘制宽高
              const renderWidth = res[0].width
              const renderHeight = res[0].height
              // Canvas 绘制上下文
              const ctx = canvas.getContext('2d')
              // 初始化画布大小
              const dpr = wx.getWindowInfo().pixelRatio
              canvas.width = renderWidth * dpr
              canvas.height = renderHeight * dpr
              ctx.scale(dpr, dpr)
              var img = canvas.createImage();
              img.src= img_src;
              ctx.drawImage(img, 0, 0,canvasWidth,canvasHeight)
              //下面是两种保存方法，
              wx.canvasToTempFilePath({
                canvas: canvas,
                success(res){
                  that.setData({
                    temp:  res.tempFilePath
                  })
                  _img = res.tempFilePath;
                  console.log("chenggong")
                },
                fail(res){
                  console.log("shibai")
                  console.log(res.data)
                }
              })
              const path = canvas.toDataURL("image/jpeg", 0.7) //返回值是一个数据url，是base64组成的图片的源数据、可以直接赋值给图片的src属性
              that.setData({
                base64_img: path,
                base64: true
              })
              console.log(path)
        })
      }
    })