I wrote an approximate_time_synchronizer:
/ros/pkgs/wg-ros-pkg-trunk/sandbox/approximate_time_synchronizer
It is being integrated into message_filters, but in the meantime you
can use it from there.
There is some overview of it at:
http://www.ros.org/wiki/message_filters/ApproximateTime

The nice thing is that it does not require an epsilon. It just returns
the "best match" between the two streams of messages (topics).

I hope this helps.

  Romain


On Mon, Mar 15, 2010 at 11:27 AM, Dan Lazewatsky
<lazewatskyd@cse.wustl.edu> wrote:
> Thanks Patrick,
> What you brought up doesn't fix my problem, but does elucidate what's
> actually going on. It turns out that there are two problems I'm having:
> 1) The images are being published independently by two nodes, so they never
> have the same timestamp, and thus will never get matched up. Is there a way
> to give TimeSynchronizer a threshold so it will match two message that are
> close enough?
> 2) Based on #1, using the raw transport actually shouldn't work. The reason
> it currently does is because cvToImgMsg doesn't fill in the header, so all
> timestamps are 0. I can manually set the timestamps/headers before sending
> off the message, but isn't cv_bridge supposed to do this?
>
> Thanks,
> -Dan
>
> On 3/12/10 9:22 PM, Patrick Mihelich wrote:
>
> Hi Dan,
>
> You might need to increase the size of the queues used by the subscriber
> filters and/or the synchronizer. If those queues fill up, messages can get
> bumped off before they form a complete synchronized set.
>
> For debugging purposes it can be useful to register another callback to the
> subscriber filters, just so you know when they get any data. I've attached
> some sample source code.
>
> "compressed" transport definitely should work with the time synchronizer.
> "theora" will not work reliably because it seems the plugin doesn't retain
> the timestamp of the original image. I've opened a ticket for that.
>
> HTH,
> Patrick
>
> On Fri, Mar 12, 2010 at 9:12 AM, Dan Lazewatsky <lazewatskyd@cse.wustl.edu>
> wrote:
>>
>> Thanks, that did the trick. One thing I'm still having trouble with is
>> getting transports other than raw to work with
>> image_transport::SubscriberFilter. If I pass image_transport::TransportHints
>> th(transport) to image_transport::SubscriberFilter, where transport is
>> "compressed" or "theora", I never seem to get any data. rxgraph -t shows
>> that my node is connected to my image publisher on the right topic, but the
>> callback never gets called.
>>
>> Thanks,
>> -Dan
>>
>> On 3/11/10 5:11 PM, Patrick Mihelich wrote:
>>
>> On Thu, Mar 11, 2010 at 2:55 PM, Dan Lazewatsky
>> <lazewatskyd@cse.wustl.edu> wrote:
>>>
>>> On a related note, is there a way to use TimeSynchronizer with
>>> image_transport rather than subscribing to the image directly?
>>
>> Use image_transport::SubscriberFilter as a drop-in replacement for
>> message_filters::Subscriber. It uses an image_transport subscriber under the
>> hood.
>>
>> Cheers,
>> Patrick
>>
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