global exotica seed

[christian-rauch]

I added a KinematicTree::SetSeed method in #487 to allow reproducible sampling of configurations.

A call to KinematicTree::BuildTree resets the random number generator:

exotica/exotica_core/src/kinematic_tree.cpp

Line 283 in 7905a39

generator_ = std::mt19937(rd_());

and requires to set the seed again.

Q1: Is this reset of the generator_ required? Can it be removed or should we replace rd_() by the user set seed?

I think it would be good if EXOTica would keep a global seed for all random number generator that may be used in solvers, tasks, etc., to enable reproducible results and easier debugging. This seed would be determined once at the beginning and all random number generators would need to use the same seed, e.g. an exotica::seed() that either returns a random or static user chosen seed.

Q2: Does this sound feasible? What would be a good approach to make sure that all random number generators use the same seed?

[wxmerkt]

A call to KinematicTree::BuildTree resets the random number generator:

A call to KinematicTree::BuildTree happens only on Instantiate, which happens only when you create a new Scene, which happens only when you create a problem. As thus, does it really reset? [I may be missing something here]

[christian-rauch]

Indeed, I create the problem more than once. Sometimes I do this to obtain properties of the kinematic tree, before configuring the actual task. As a workaround, you could reset the seed every time the problem is created.

However, I think in my particular problem the issue is that the seed for the KinematicTree is only set after Setup::CreateProblem has been called. Solvers, etc. that are Instantiated during CreateProblem are using an unseeded KinematicTree. I am working around this by providing the seed manually to the initialiser and seeding KinematicTree before using KinematicTree::SetSeed.

This does not scale well, if you have many more random number generators in exotica. I am therefore wondering if EXOTica should have a global seed that is determined when it is instantiated. This global-seed would then be used to draw new seeds for any generator in EXOTica. This way, you can reset generators, but deterministically.

[wxmerkt]

Makes sense - where would you suggest to place it?

[christian-rauch]

Since there are many entry points to initialise exotica, I am not sure about this. It appears to me that CreateSolver is always called first, i.e. the Setup class should store that seed and make it available via a getter Setup::GetSeed().
From a quick search, std::random_device is only used in three places: Renderable, KinematicTree and RandomColor. Do you have an idea where randomness is used elsewhere?

[wxmerkt]

OMPL solvers. I think Setup may be the best place to store. You can also create Problems and Scenes without a Solver, so Setup is the place to go.