Cloud Vision上的base64数据URL（错误：无图像）

Question

我将通过xhr请求从画布获取的base64代码发送到Firebase Functions，并尝试在Cloud Vision上使用它，但出现此错误：

Error: No image present.
    at _coerceRequest (/user_code/node_modules/@google-cloud/vision/src/helpers.js:69:21)
    at ImageAnnotatorClient.<anonymous> (/user_code/node_modules/@google-cloud/vision/src/helpers.js:224:12)
    at ImageAnnotatorClient.wrapper [as annotateImage] (/user_code/node_modules/@google-cloud/vision/node_modules/@google-cloud/promisify/build/src/index.js:44:35)
    at ImageAnnotatorClient.<anonymous> (/user_code/node_modules/@google-cloud/vision/src/helpers.js:141:17)
    at PromiseCtor (/user_code/node_modules/@google-cloud/vision/node_modules/@google-cloud/promisify/build/src/index.js:71:28)
    at ImageAnnotatorClient.wrapper [as labelDetection] (/user_code/node_modules/@google-cloud/vision/node_modules/@google-cloud/promisify/build/src/index.js:56:16)
    at exports.hello.functions.https.onRequest (/user_code/index.js:23:8)
    at cloudFunction (/user_code/node_modules/firebase-functions/lib/providers/https.js:57:9)
    at /var/tmp/worker/worker.js:783:7
    at /var/tmp/worker/worker.js:766:11

这是将画布图像转换为base64并将其发送的方法：

const base64Img = canvas.toDataURL();
const data = base64Img.replace(/^data:image\/(png|jpg);base64,/, "");
const blob = new Blob([data], {type: 'text/plain'});

let xhr = new XMLHttpRequest();
xhr.open("POST", "hello", true);
xhr.send(blob);

这就是我试图将其发送到Cloud Vision on Cloud Functions的方式：

const functions = require('firebase-functions');
const vision = require('@google-cloud/vision');

exports.hello = functions.https.onRequest((req, res) => {
    let imageBuffer = Buffer.from(req.body).toString('base64');
    const request = {
      "requests":[
        {
          "image":{
            "content": imageBuffer
          },
          "features":[
            {
              "type":"LABEL_DETECTION",
            }
          ]
        }
      ]
    }
    const client = new vision.ImageAnnotatorClient();
    client
        .labelDetection(request)
        .then(results => {
            res.send(results);
        })
        .catch(err => {
        console.error('ERROR:', err);
            res.send(err);
        });
});