Extract first image of input stream to jpg file.

    Args:
        cam: Input stream of raw rosbag messages.

    Returns:
        File instance for first image of input stream.

def image(cam):
    """Extract first image of input stream to jpg file.

    Args:
        cam: Input stream of raw rosbag messages.

    Returns:
        File instance for first image of input stream.
    """
    # Set output stream title and pull first message
    yield marv.set_header(title=cam.topic)
    msg = yield marv.pull(cam)
    if msg is None:
        return

    # Deserialize raw ros message
    pytype = get_message_type(cam)
    rosmsg = pytype()
    rosmsg.deserialize(msg.data)

    # Write image to jpeg and push it to output stream
    name = '{}.jpg'.format(cam.topic.replace('/', ':')[1:])
    imgfile = yield marv.make_file(name)
    img = imgmsg_to_cv2(rosmsg, "rgb8")
    cv2.imwrite(imgfile.path, img, (cv2.IMWRITE_JPEG_QUALITY, 60))
    yield marv.push(imgfile)