truncating ros time

asked 2018-10-09 14:25:48 -0600

lahiruherath
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updated 2018-10-09 14:52:33 -0600

Hi, Is there anyway i could truncate the ros time value to certain length, lets say i need the time value to be only up to a certain decimal points

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So you want to reduce the precision of timestamps by truncating them?

If so, could I ask you for a topic title change? Right now it seems like you're asking about the precision of rostime and/or ROS timestamps. But that is not actually the case.

gvdhoorn ( 2018-10-09 14:45:00 -0600 )edit

And to avoid an xy-problem: can you describe why you'd want to do this?

gvdhoorn ( 2018-10-09 14:45:25 -0600 )edit

Hi, I want to stop tf from looking into the exact time stamp to find a transform

lahiruherath ( 2018-10-09 14:51:30 -0600 )edit

And why do you want to do that? What is it exactly that you are trying to achieve?

gvdhoorn ( 2018-10-09 14:57:31 -0600 )edit

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answered 2018-10-09 16:23:55 -0600

PeteBlackerThe3rd

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updated 2018-10-10 03:56:44 -0600

I'll jump in here because I think exactly how the TF system works needs to be clarified.

TF doesn't look for a transform with exactly the right time stamp when you lookup a transform, it almost always interpolates the two nearest TF samples. It can also extrapolate into the future a short time based on previous data but this is not recommended.

Are you getting the Lookup would require extrapolation at time X error? In this case adding a timeout Duration to the lookupTransform call would be the preferred method of solving this.

But as @gvdhoorn says, it would be helpful to know the actual problem you're trying to solve before we can suggest the best solution for you.

Update:

Thanks for clarifying the problem you're trying to solve, I see two possible solutions you can look into.

Use message filters to synchronise the messages as @tfoote has suggested
You could request the most recent data when looking up the transform by setting the time to zero. This achieves exactly what you suggested initially via a different mechanism.

Hope this helps.

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Hi Peter,

I know this, lemme explain my specific problem.

I have depth image from a sensor which i actually get using a another API and I convert to a ROS message. The odometry is also taken similarly and tf is broadcasted, I assign the time stamps myself. They are running on two separate threads.

lahiruherath ( 2018-10-09 16:37:22 -0600 )edit

The time stamps do not match because of this, I convert the depth to PCL and i need to transform it to another frame. The lookup gives an error. I cannot hang my conversion until a transform is taken. I just need to find a way to transform it in some viable rate. My sensors run on 15-25 Hz

lahiruherath ( 2018-10-09 16:40:44 -0600 )edit

So a time stamp in this range is okay for me. So i thought i would cap the initial stamp at some decimal place. Don't know whether it is viable or possible. Or maybe I'm just shooting in some uselss direction.

lahiruherath ( 2018-10-09 16:44:59 -0600 )edit

From your use case you should not be truncating your timestamps. What you're trying to do is basically extrapolate into the future by a small bit, but rounding all your timestamps. This has been explored many times and almost always leads to unintended consequences.

tfoote ( 2018-10-09 19:20:20 -0600 )edit

You cannot apply a transform until the data is available. It can actually make a difference when during the 40ms between a 25Hz signal is processed. I'd strongly recommend that you look at using a Message Filter in your pipeline to hold the data for the few milliseconds until the data is available.

tfoote ( 2018-10-09 19:22:08 -0600 )edit

http://wiki.ros.org/tf2/Tutorials/Usi...

tfoote ( 2018-10-09 19:22:17 -0600 )edit

And this was exactly why I asked about the xy-problem.

gvdhoorn ( 2018-10-10 01:22:34 -0600 )edit

@tfoote, Thanks didn't know about this message filter. Another small question what's your advice use tf since pcl_ros still has tf in its conversions, or shift to tf2?

lahiruherath ( 2018-10-10 05:11:25 -0600 )edit

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