[ros-users] stereo calibration topic requirements?

Patrick Beeson beeson.p at gmail.com
Wed Apr 7 02:22:58 UTC 2010

Along these lines, I know how to do software triggering (in theory) for 
lib1394, so I could add that to the camera1394 node (along with 
configuration options to connect to and publish data from multiple 
cameras that will be sync'ed.  But, this won't be for a while.

I noted that roscpp latest has a new feature in message_filters for 
ApproximateTime syncing, that adapts as needed to syncing multiple 
incoming messages.  Unfortunately, rospy doesn't implement this, so it's 
not a simple line edit to fix the calibration package, which uses rospy.

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