将Blob发布到JS中的认知人脸检测API

Question

我尝试将请求发送到Cognitive Face Detect API，但始终收到空响应。我使用json发送请求没有问题，但是使用blob发送请求。我寻找并发现有人提供将base64转换为blob的代码。这是我的代码：

    var myCanvas = document.getElementById('myCanvas');
    var ctx = myCanvas.getContext('2d');
    var img = new Image;
    img.onload = function(){
      ctx.drawImage(img,0,0); // Or at whatever offset you like
    };
    img.src ='https://pmcvariety.files.wordpress.com/2017/04/willsmith_actor.jpg';
    var imgurl = myCanvas.toDataURL('image/jpeg');


    makeblob = function (dataURL) {
        var BASE64_MARKER = ';base64,';
        var parts = dataURL.split(BASE64_MARKER);
        var contentType = parts[0].split(':')[1];
        var raw = window.atob(parts[1]);
        var rawLength = raw.length;

        var uInt8Array = new Uint8Array(rawLength);

        for (var i = 0; i < rawLength; ++i) {
            uInt8Array[i] = raw.charCodeAt(i);
        }

        return new Blob([uInt8Array], { type: contentType });
    }

    body = makeblob(imgurl);

    var xhr = new XMLHttpRequest();

    xhr.open('POST', 'https://canadacentral.api.cognitive.microsoft.com/face/v1.0/detect?returnFaceId=true', true);
    xhr.setRequestHeader("Content-Type", "application/octet-stream");
    xhr.setRequestHeader("Ocp-Apim-Subscription-Key", "my key here");
    xhr.onload = function (oEvent) {
        if (this.status == 200) {
            alert(this.response);
        }
    }
    xhr.send(body);