[petsc-users] Indexing/using a 3D DMDA as a 1D vector

[Phil Tooley]

Ok, So I have figured this out a bit better now, facepalm moment as I
finally realised that this is exactly what the DMDA exists to do...  I
need to create a vector using the DMCreate*Vector functions and can then
insert values using the assembly functions. 

Then, if I understand correctly, in order to have the state I need to
multiply with my operator I should make use of the
DMDAGlobalToNaturalBegin() and End() functions to ensure the correct
ordering.  Do I then need to ensure I have reverted it to global
ordering before I try to apply finite difference methods again?

Thanks

P


On 12/09/18 14:27, Phil Tooley wrote:
> I will preface this by saying I am new to PETSc and am still trying to
> get my head around all of the layout mapping that is done.  That means I
> may well have fundamentally misunderstood something, but hopefully
> someone will be able to to put me right.
>
> In my application I have some 3D pixel data which I want to manipulate
> using finite difference methods and then transform by viewing as a 1-D
> vector and multiplying by a large sparse matrix operator.
>
> I would assume that the correct way to do this is by creating a DMDA to
> hold the image data and ghosting appropriately to apply my finite
> difference operations.  Then I had hoped that I could use some form of
> application ordering to allow viewing the data as a vector that can be
> multiplied with my operator matrix.  This is where I have come unstuck,
> I may just be missing something obivous but I can't figure out how to do
> this.  Can anyone point me in the correct direction please?
>
> Many Thanks
>

-- 
Phil Tooley
Research Software Engineering
University of Sheffield