[petsc-users] questions about dmdacreaterestriction

[Xiangdong]

Hi Barry,

Thanks for your message. When I use DMCreateInterpolation, I got the error
message like "[0]PETSC ERROR: Invalid argument
[0]PETSC ERROR: Coarsening factor in x must be 2".  The error will be gone
if I change refine_x = 3 to refine_x=2. Why must the coarsening factor be a
factor of 2?

Thank you.

Xiangdong

Below is my test code:

int main(int argc, char **argv)
{
  PetscErrorCode ierr;

  PetscInitialize(&argc, &argv, PETSC_NULL, PETSC_NULL);

  int Nx=6, Ny=4, Nz=1;

  DM da;

  ierr = DMDACreate2d(PETSC_COMM_WORLD,DM_BOUNDARY_NONE,DM_BOUNDARY_NONE,
DMDA_STENCIL_STAR,Nx,Ny,PETSC_DECIDE,
PETSC_DECIDE,1,1,NULL,NULL,&da);CHKERRQ(ierr);

  int refine_x=3, refine_y=2, refine_z=1;
  ierr = DMDASetRefinementFactor(da,refine_x,refine_y,refine_z);

  ierr = DMDASetInterpolationType(da,DMDA_Q0);

  ierr = DMSetFromOptions(da); CHKERRQ(ierr);
  ierr = DMSetUp(da); CHKERRQ(ierr);

  DM dac;
  ierr = DMCoarsen(da,NULL,&dac);

  Mat M;
  Vec V;

  ierr = DMCreateInterpolation(dac,da,&M,&V);

  MatView(M,PETSC_VIEWER_STDOUT_WORLD);


  ierr = PetscFinalize(); CHKERRQ(ierr);
  PetscFunctionReturn(0);
}

On Mon, May 7, 2018 at 6:16 PM, Smith, Barry F. <bsmith at mcs.anl.gov> wrote:

>
>
>
> > On May 7, 2018, at 3:25 PM, Xiangdong <epscodes at gmail.com> wrote:
> >
> > Hello everyone,
> >
> > I have a 2D DMDA with Nx=6 and Ny=4. If I set refine_x=3 and refine_y=2,
> I can get a coarse DMDA by DMCoarsen(). However, when I tried to call the
> DMCreateRestriction between these two DMDAs, I got error about " No support
> for this operation for this object type. DMCreateRestriction not
> implemented for this type."
> >
> > Can you help me get around this?
>
>    Us DMCreateInterpolation() and then use MatMultTranspose() for example
> to apply the restriction.
>
> >
> > Another simple question, where can I find the explanation/meaning of
> DMDA_Q0 and DMDA_Q1?
>
>     This is finite element terminology, Q0 means piecewise constant basis
> functions (and hence piecewise constant interpolation) while Q1 means
> linear interpolation.
> For Q0 you should think of the unknowns as living on the centers of each
> element while for Q1 the unknowns live on the vertices of the elements.
>
>    Barry
>
> >
> > Below is my test example.
> >
> > Thank you.
> >
> > Best,
> > Xiangdong
> >
> > #include <petsc.h>
> >
> > int main(int argc, char **argv)
> > {
> >   PetscErrorCode ierr;
> >
> >   PetscInitialize(&argc, &argv, PETSC_NULL, PETSC_NULL);
> >
> >   int Nx=6, Ny=4, Nz=1;
> >
> >   DM da;
> >
> >   ierr = DMDACreate2d(PETSC_COMM_WORLD,DM_BOUNDARY_NONE,DM_BOUNDARY_NONE,
> DMDA_STENCIL_STAR,Nx,Ny,PETSC_DECIDE, PETSC_DECIDE,1,1,NULL,NULL,&
> da);CHKERRQ(ierr);
> >
> >   int refine_x=3, refine_y=2, refine_z=1;
> >   ierr = DMDASetRefinementFactor(da,refine_x,refine_y,refine_z);
> >
> >   ierr = DMDASetInterpolationType(da,DMDA_Q0);
> >
> >   ierr = DMSetFromOptions(da); CHKERRQ(ierr);
> >   ierr = DMSetUp(da); CHKERRQ(ierr);
> >
> >   DM dac;
> >   ierr = DMCoarsen(da,NULL,&dac);
> >
> >   Mat M;
> >   Vec V;
> >
> >   ierr = DMCreateRestriction(dac,da,&M);
> >
> >   MatView(M,PETSC_VIEWER_STDOUT_WORLD);
> >
> >
> >   ierr = PetscFinalize(); CHKERRQ(ierr);
> >   PetscFunctionReturn(0);
> > }
> >
>
>
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