Hi David,
> I want to evaluate each point out of the cloud only once in case of a
distributed triangulation.
A quick (and dirty) solution would be to evaluate and perform the
communication non-uniquely and pick out one of the received values at
the end. See also VectorTools::evaluate_at_points() and the flag
VectorTools::EvaluationFlags::insert: although intentionally not
documented this function will take the first value of the process with
the lowest rank.
> I would probably need to go a step further and implement a consensus
algorithm in order to decide only for one specific process, which seems
not trivial to me.
You are right. One would need an additional communication step (with or
without CA) to make the evaluation/communication more efficient. I was
thinking about introducing an additional flag to the constructor of
RemotePointEvaluation, but have not implemented it yet, since in our use
cases we either need the average values (DG) or we set up the
communication pattern in every time step due to some very dynamic
systems (multi phase flow).
Anyway, I am happy to discuss how to implement the needed feature. The
code that would need to be touched is:
https://github.com/dealii/dealii/blob/654b14481eeff7e744297025ea2078ab115611ac/source/grid/grid_tools.cc#L5905-L5909,
i.e., a consistent modification of send_components and recv_components.
I am happy to write a draft if you want.
Hope this helps,
Peter
On 12.05.21 09:08, 'David' via deal.II User Group wrote:
Dear all,
I'm currently looking for a way to evaluate my global solution on an
arbitrary cloud of points and stumbled across the (new) class
`Utilities::MPI::RemotePointEvaluation<dim>`. In general, the class
does exactly what I want with one particular exception; I want to
evaluate each point out of the cloud only once in case of a
distributed triangulation. If I understand it correctly this property
is fulfilled in case the boolean function `is_map_unique()` returns
true, but I don’t see any option to filter or tell the class that I
actually want the map to be unique.
Assuming that the class cannot handle this case I tried to pre-filter
the vertices by hand: For me, the most natural choice to fulfill the
unique evaluation of the points is to assign them to a process, so
that I only work on ‘locally owned vertices’. I compute a bounding box
using the iterator filter `LocallyOwnedCell()`, filter the received
cloud of points on the process and finally plug them into the
`RemotePointEvaluation`. However, this does not work in case points of
the cloud are located at the boundary of the bounding box, because
they will technically be part of both bounding boxes of the involved
processes. I would probably need to go a step further and implement a
consensus algorithm in order to decide only for one specific process,
which seems not trivial to me.
Before starting now to implement a more advanced solution I would like
to ask if there is an obvious way I’m not aware of. Is it possible to
enforce a unique evaluation using one of the ‘PointEvaluation’ classes
or is it possible to let deal.II decide for an owner of an arbitrary
vertex? I have also seen the function
‘find_active_cell_around_point()’ which might help, but it might end
up in similar problem as well.
Thanks in advance and best regards,
David
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