Image subscriber lag (despite queue = 1)

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I am having some lag issues with a rospy subscriber listening to image messages.

Overview:

I have a rosbag streaming images to /camera/image_raw at 5Hz. I also have an image_view node for displaying the images for reference. This image_view shows them at 5Hz.

In my rospy subscriber (initialized with queue = 1), I also display the image (for comparing lag time against the image_view node). The subscriber subsequently does some heavy processing.

Expected result:

Since queue size is 1, the subscriber should process the latest frame, and skip all other frames in the meanwhile. Once it completes processing, it should then move on to the next latest frame. There should be no queuing of old frames. This would result in a choppy, but not laggy video (low fps, but no "delay" wrt rosbag stream, if that makes sense)

Actual result:

The subscriber is lagging behind the published stream. Specifically, the image_view node displays the images at 5Hz, and the subscriber seems to queue up all the images and processes them one by one, instead of just grabbing the latest image. The delay also grows over time. When I stop the rosbag stream, the subscriber continues to process images in the queue (even though queue = 1).

Note that if I change the subscriber to have a very large buffer size, as below, the expected behavior is produced:

self.subscriber = rospy.Subscriber("/camera/image_raw", Image, self.callback,  queue_size = 1, buff_size=2**24)

However, this is not a clean solution.

This issue has also been reported in the following links, where I found the buffer size solution. The official explanation hypothesizes that the publisher may actually be slowing down, but this is not the case since the image_view subscriber displays the images at 5Hz.

https://github.com/ros/ros_comm/issue... , http://stackoverflow.com/questions/26... , http://answers.ros.org/question/50112...

Any help is appreciated. Thanks!

Code:

def callback(self, msg):
    print "Processing frame | Delay:%6.3f" % (rospy.Time.now() - msg.header.stamp).to_sec()
    orig_image = self.bridge.imgmsg_to_cv2(msg, "rgb8")
    if (self.is_image_show_on):
        bgr_image = cv2.cvtColor(orig_image, cv2.COLOR_RGB2BGR)
        cv2.imshow("Image window", bgr_image)
        cv2.waitKey(1)

    result = process(orig_image) #heavy processing task
    print result