[ros-users] confused by interaction of rosbag and image_transport

[Patrick Mihelich]

On Fri, Apr 16, 2010 at 8:26 AM, Jack O'Quin <jack.oquin at gmail.com> wrote:

> The problem came when writing an image_transport subscriber to process
> and display the images. Just subscribing to the compressed image
> published by the bag does not work.


image_view is an image_transport subscriber, so your node should be able to
work with the bag just as well. Remember that you need to subscribe to
'/right_front/camera/image_raw' (even though that topic does not exist in
the bag) and instruct your image_transport::Subscriber to use 'compressed'
transport either through the ~image_transport parameter or in code with
TransportHints.

The solution you found using 'republish' is a good one if you have multiple
nodes on the same machine listening to an image topic, as it avoids the CPU
overhead of each subscriber node doing the decompression.

I suspect the decompression happens in the
> publisher. How does image_view deal with that? (Guess I should read
> the code.)
>

Decompression happens in the subscriber - the original motivation of
image_transport was to save bandwidth. image_view simply uses an
image_transport::Subscriber, which loads the appropriate plugin to
decompress the images. I do suggest looking at image_view.cpp, it is a very
straightforward application of image_transport. The image_transport subscriber
tutorial<http://www.ros.org/wiki/image_transport/Tutorials/SubscribingToImages>actually
implements a slightly simplified version of image_view.

What are the recommended Best Practices for saving and using large
> files like these? In minutes, these devices produce gigabytes of data,
> and we plan to add four more similar cameras with a 360 degree field
> of view.
>

We're all still figuring that out. Recording the compressed topics is a good
method. We're working on adding compression to bag files, but
domain-specific compression like JPEG or PNG will always beat
general-purpose algorithms.

If you don't mind "thinning out" your data, you can also use
topic_tools<http://www.ros.org/wiki/topic_tools>to limit your data
topics to a maximum bandwidth or rate. Although, it
occurs to me that these don't have good support for synchronized topics like
image_raw/camera_info yet.

Patrick
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