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A diagram of your tree might be helpful to understand your setup. In general if you loose communication you will only be able to compute transform to information that is known locally, whether that information is available through tf transforms or through any other mechanism.

If you have both vehicles relative to a world or map frame. They can compute transform to anything that they have information about.

If a link in a tf tree is broken you can still query within the two resultant subtrees. So you should make sure that you structure your tree in a way that both vehicles can conduct their queries even if the information from the other vehicle is not available.

A diagram of your tree might be helpful to understand your setup. In general if you loose communication you will only be able to compute transform to information that is known locally, whether that information is available through tf transforms or through any other mechanism.

If you have both vehicles relative to a world or map frame. They can compute transform to anything that they have information about.

If a link in a tf tree is broken you can still query within the two resultant subtrees. So you should make sure that you structure your tree in a way that both vehicles can conduct their queries even if the information from the other vehicle is not available.

Update based on diagram:

Yes you can always query some subpart of the tree, it doesn't matter what the rest of the tree looks like. tf only computes the minimum spanning set.

So the classic diagram would be

world -> robot1 -> tracking target
      \-> robot2

If both robots are in communications everything can be transformed. If robot1 and robot2 cannot communicate then robot1 can transform relative to tracking or world, but robot 2 only knows about itself and world.

So for your circled part as long as that transform is being published on that vehicle you can query for that transform.