[petsc-users] dumb questions about orientations of entities in dmplex

[Wang]

Hello, I'm new user of PETSc. I'm a little confused of the orientations of
those meshing entities in DM. Here are my questions.

1. When creating DM from a given mesh, such as DMPlexCreateGmshFromFile,
will the ordering of nodes for each element be changed? If so, even for
mesh which has been oriented (for example, in 2D mesh, the natural order is
already clockwise-oriented)?

2. What do those orientations of points in closure of one cell mean? Why do
we use those positive or negative integers, like s or -(s+1)? Isn't a bool
flag enough?

let's look at one example, closure (given by DMPlexGetTransitiveClosure) of
a 2D cell { f1, o1, e4, o2, e3,o3, e5,o4, v1,o5, v8,o6, v9,o7} where f, e
or v denotes face, edge or vertex respectively.

If the cone of each point in that list is uniquely defined as given
by DMPlexGetCone, then, with my understanding, the possible orientation for
each point in that closure is either 1 or -1. What does it mean by having
orientation equal to 0, s or  -(s+1), respectively,  for certain point in
the list, such as edge e4? My question is why we need  a start in the
orientation of a sieve point if it is already defined.

3. No matter what the answer to question 1 is, I'm
assuming DMPlexDistribute only renumbers things without changing the
orientation. Am I correct?

Thanks in advance.

Wang
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