[ros-users] stereo calibration topic requirements?

[Patrick Mihelich]

On Tue, Apr 6, 2010 at 5:43 PM, Eric Perko <wisesage5001 at gmail.com> wrote:

> are your image_raw topics
> synced? I'm trying to use two individual cameras, but because each one
> gets its own camera1394 node, I don't think their timestamps will ever
> really be in sync even with external triggering or some other
> guarantee I can make about the frames themselves matching up.
>

This is most likely the problem. The stereo camera calibrator only
subscribes to the image topics, not camera_info, but it does expect the left
and right images to be synchronized. The wge100_camera driver goes to some
trouble to support synching two cameras with an external trigger, and of
course this is easy with a unified stereo camera that does the triggering
for you.

The more lax case of trying to use two unsynchronized cameras for stereo
isn't directly supported by the image_pipeline right now. message_filters is
gaining support for approximate time synchronization, so maybe I should
target that for the next image_pipeline release. It's better to have the
cameras synchronized, but clearly that poses some technical problems.

A hacky workaround would be to write a relay node that listens to the left
and right image topics, matches up pairs that are close enough, and
republishes them with the same timestamp.

What is the behavior of the stereo camera calibrator if the left and
> right image topics are not sycned up? Does it just hang waiting for a
> synchronized pair?


Yep...

If so, it might be really helpful to get some sort
> of warning that the two image topics are out of sync and therefore
> will not be used.
>

I opened ticket 3962 <https://code.ros.org/trac/ros-pkg/ticket/3962>.

Patrick
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