2009/2/25 Anders Logg <[email protected]>: > On Wed, Feb 25, 2009 at 10:05:34PM +0000, A Navaei wrote: >> I'm not sure if I'm making sense here. Both of you suggested using >> assemble which requires Form which, if I'm not wrong, requires >> Function and that needs FunctionSpace which requires Mesh, but I >> emphasised that this is no FEM problem :) I'm just trying to get the >> best of available LA in dolfin. >> >> Let me give you a concrete example, have a look at Eq 12 of this paper: >> >> http://www.mia.uni-saarland.de/Publications/brox-eccv04-of.pdf >> >> Is it possible to use assemble() to extract A and b out of this >> equation? Many of the coefficients in the equation are image data, I >> guess I should convert the image data to Matrix and use Variable for >> the unknowns? > > Yes, this would be fine. You just need to set the values in the matrix > manually using A.set().
It's nice to know the automation is also available for non-FE approaches. I'll give it a try now. > > But this looks a bit ugly. Why not apply finite elements to solve the > PDE? I'm not sure if the available non-linear solvers are able to solve the highly non-linear Eq 7 directly, can they? The method suggested in the paper involves a double-linearisation which needs two nested loops, so, solving the final double-linearised equation inside this nested loops could be expensive. I'm trying to compare the profiling of the finite-element approach and the approach in the paper, which could be classified as finite-difference as they approximate the equation, and then compare the speed and accuracy of the results. -Ali > > -- > Anders > > >> -Ali >> >> 2009/2/25 <[email protected]>: >> > >> > I'm not exactly sure what you are asking. But it is easy to obtain the >> > matrices >> > and vectors in the linear system to be solved. >> > >> > eg. >> > dolfin/demo/la/trilinos/python/demo.py >> > >> > shows how to solve the system using Trilinos. You can use dolfin >> > to make matrices and vectors in PETSc, uBlas, MTL, and Trilinos format. >> > You can choose to let dolfin solve it for you or you can get the underlying >> > matrix and solve it with the above LA libraries. >> > >> > Kent >> > >> > >> > >> >> Is the linear algebra support in dolfin capable of automatically >> >> generating the sparse linear system of equations? (I know that ffc >> >> effectively does this, but note that this question does not involve >> >> solving a PDE using FEM, but a linear system of equations). >> >> >> >> For example, assume we have some busy sets of linear equations and we >> >> wish to convert them to Ax = b so that we can feed them to the linear >> >> solvers. Is there any way of automatically obtaining A and b avoiding >> >> manual calculations and hard-coding? >> >> >> >> >> >> -Ali >> >> _______________________________________________ >> >> DOLFIN-dev mailing list >> >> [email protected] >> >> http://www.fenics.org/mailman/listinfo/dolfin-dev >> >> >> > >> > >> > >> _______________________________________________ >> DOLFIN-dev mailing list >> [email protected] >> http://www.fenics.org/mailman/listinfo/dolfin-dev > > -----BEGIN PGP SIGNATURE----- > Version: GnuPG v1.4.9 (GNU/Linux) > > iEYEARECAAYFAkmlwzMACgkQTuwUCDsYZdGgrQCeJt6cXdWcVYMqpN1dzWOcVR08 > qgUAoIVFdtPvb9uT5O7KRC39fO9Chkff > =Eo80 > -----END PGP SIGNATURE----- > > _______________________________________________ > DOLFIN-dev mailing list > [email protected] > http://www.fenics.org/mailman/listinfo/dolfin-dev > > _______________________________________________ DOLFIN-dev mailing list [email protected] http://www.fenics.org/mailman/listinfo/dolfin-dev
