fff: function () {
    var that = this;
    const ctx = wx.createCanvasContext("myCanvas")
    wx.chooseImage({
      success: function (res) {
        ctx.drawImage(res.tempFilePaths[0], 0, 0, 150, 100)
        ctx.draw(false, () => {
         wx.canvasGetImageData({
            canvasId: "myCanvas",
            x: 0,
            y: 0,
            width: 100,
            height: 100,
            success(res) {
              console.log(res.width) // 100
              console.log(res.height) // 100
              console.log(res.data instanceof Uint8ClampedArray) // true
              console.log(res.data) // 100 * 100 * 4
              let base64 = wx.arrayBufferToBase64(res.data);
              //console.log(base64)
              that.data.userImageBase64 = "data:image/jpg;base64," + base64;
              console.log(that.data.userImageBase64)
            }
          })
        })
           }
    })
  },

the background of Mini Program"s development is shown in the figure

but this last big code can not convert the correct picture, ask for God"s guidance.