Capture Rectangle Image From WebCam In String to Send It To Server

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Below is my Code

    classifier.detectMultiScale(gray, faces, 1.1, 3, 0);
    // draw faces.
    for (let i = 0; i < faces.size(); ++i) {
        let face = faces.get(i);
        let point1 = new cv.Point(face.x, face.y);
        let point2 = new cv.Point(face.x + face.width, face.y + face.height);
        cv.rectangle(dst, point1, point2, [255, 0, 0, 255]);
    }
    cv.imshow('canvasOutput', dst);
    // schedule the next one.
    let delay = 1000 / FPS - (Date.now() - begin);
    $.post('image-data', {"imageInRectangle": ??????}, function(res){

    });

At the place of ?????? , I need to capture the face image in rectangle as string and send the Image to server .