[petsc-users] Output non-zero structure of a matrix

[Theler German Guillermo]

Hi Justin

I have started a thread that was largely commented by Barry in March
this year, maybe you can take a look at the archives (i.e.
http://lists.mcs.anl.gov/pipermail/petsc-users/2015-March/024681.html
and previous messages in the thread). Tha main conclusion is that if you
have a matrix A, you should do something like this:

#define petsc_call(f) ierr=(f);CHKERRQ(ierr);

  petsc_call(PetscViewerDrawOpen(PETSC_COMM_WORLD, PETSC_NULL, title,
PETSC_DECIDE, PETSC_DECIDE, size, size, &viewer));
  petsc_call(PetscViewerDrawGetDraw(viewer, 0, &draw));
  petsc_call(PetscDrawSetFromOptions(draw));
  petsc_call(PetscDrawSetSaveFinalImage(draw, image));
  petsc_call(MatView(A, viewer));

There are at least three drawbacks with this approach:

1. If you have a small matrix (say 1000x1000) and wanted to have a
one-to-one bijection between a pixel and the elements of your matrix,
setting the image size equal to the matrix size would not do the trick
because the resulting image always includes a margin whose size depends
on the size of the matrix in a way I could not figure out
2. You cannot have images bigger than the resolution of your X display
3. You have to compile PETSc with --with-afterimage-lib

At the conference I asked Barry if there was another approach. He said
that to overcome issue #2 you may define a virtual X display with a
bigger resolution, but that seemed to complicate even more the issue.

I then came up with a different approach, which is to dump a text file
in the SNG (Scriptable Network Graphics) format. This is an idea
developed by Eric Raymond in which a text file describes a bitmap using
a base64 string (i.e. almost as ASCII art) which then is parsed and
converted into a PNG with a utility named sng
(http://sng.sourceforge.net/).

I created a repository in https://bitbucket.org/gtheler/mat2sng/ where
the main code loads a matrix from a binary file and calls a function
mat2sng(). In its current implementation, it divides the matrix into
blocks such that eack of these block corresponds to a single pixel in
the output image and its colors depends on the sign of the sum of all
the elements belonging to that block. If the block size is 1x1 then
there is a correspondence between the output pixel and the matrix
elements. It only works for SeqAIJ matrices because I copied the way the
elements are read from MatView_SeqAIJ_ASCII() in
src/mat/impls/aij/seq/aij.c. Given the base64 representation, one may
use up to 32 shadows of red and blue to mark not just the sign of the
blocks but also the relative values.

If this idea actually works and is usable for some people, probably a
new ASCII viewer of this type may be incorporated into next.


Hope this helps.
Greeting from Ginobilli's land.

--
Germán Theler :: CTO Ingeniería & TICs

CITES – Centro de Innovación Tecnológica Empresarial y Social S.A.
Dirección General Sancor Seguros
Grupo Sancor Seguros
tel +54 3493 –428 500 – Int.: 3374
gtheler at cites-gss.com
www.cites-gss.com - www.gruposancorseguros.com



On Wed, 2015-07-22 at 02:13 -0500, Justin Chang wrote:
> Hi everyone,
>
> How do I print out the non-zero structure of a Matrix? Preferably to
> an image file (e.g., png, eps, etc).
>
> Thanks,
> Justin
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