[petsc-users] accessing sequential data (e.g. image) at (i, j, k) in DM in parallel

[Bishesh Khanal]

Dear all,

I have a part of a code that does NOT use Petsc, and contains an image K of
dimensions mXnXr.
It also provides me with a function to access any value at (i,j,k), e.g.
using K.getVoxelAt(i,j,k).


Now in my petsc code I create a DMDA of size mXnXr to solve a pde. The
coefficients of the pde at each grid point should physically correspond to
the one in the image K.
The usual code I would use to create a matrix from DM is:

DMDALocalInfo info;
ierr = DMDAGetLocalInfo(da,&info);CHKERRQ(ierr);

for (PetscInt k = info.zs; k<info.zs+info.zm; ++k) {
        for (PetscInt j = info.ys; j < info.ys+info.ym; ++j) {
            for (PetscInt i = info.xs; i < info.xs+info.xm; ++i) {

//Here (i,j,k) refers to the indexing/ordering that is internally
organized/distributed to different processors.

            }
        }
 }

Now, the problem is that I cannot use getVoxel(i,j,k) within the above loop
because the ordering and distribution of the image K and the function
getVoxel(i,j,k) knows nothing about the "da" I create in my part of the
code. K might have been stored by each processor locally. So I believe if I
want to use the coefficients from the image K in my code within the above
loop, first I need to either:
1) create global vectors from the DMDA and copy all the values of image K
to this global vector. Then access the values from this global vector.
or
2) find a way to transform (i,j,k) within the above loop to a valid (i,j,k)
for the function getVoxel(i,j,k)

What should I do to resolve the problem ?

Thanks,
Bishesh
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