dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

We've heard about this work, but never seen it ;).

Can you post it somewhere? It could provide a discussion basis on how to
move this along.

I think that we should be careful in asking FFC to do too much. It may
better to compute J, det(J), etc, outside tabulate_tensor(...) functions.

Garth

On 23/03/11 21:05, Peter Brune wrote:
> I saw this thread and had already started running the tests I had for
> this with the latest FFC to see if anything had changed recently.  I
> never made isoparametry through UFL public because I could never get the
> efficiency to be reasonable.  The situation appears to have improved a
> little bit from a year ago, but not much.  With the latest FFC, it's
> about 10x slower (in 2D) to assemble Poisson with P1 for the test/trial
> space and P2 for the coordinates than the same problem with affine
> coordinates.  It's even worse if you include things like
> Piola-transformed surface normals into the mix.  When I was working on
> this I only assembled a very small fraction of the elements (around a
> cylinder in a flow) with the parametric Jacobian, so it worked OK for
> small problems.
> 
> I believe that it would do much, much better if the optimizing
> quadrature compiler in FFC supported fractions.  This is necessary
> because you need to apply the inverse isoparametric Jacobian, which
> includes 1 / |J|, to any basis function derivatives in the form.
> 
> For reference, here are the assembly times for the iso(super, I suppose
> is more accurate)-parametric, optimized, and non-optimized Poisson
> problems in 2D on a 256x256 square (with the parametric coordinates just
> being the regular coordinates passed through). 
> 
> isoparametric assembly      |        3.2017      3.2017     1
> optimized assembly          |       0.34515     0.34515     1
> regular assembly            |       0.36524     0.36524     1
> 
> I got really deep in FFC trying to make this work with no success, but
> this was before the rewrite.  I'd be willing to declare victory on this
> one and submit my code if someone else were willing to make it fast
> enough to use.  There's also the issue of how exactly to extend the
> interface to support this in an elegant fashion.  Right now I just call
> a function that takes a form and a parametric Jacobian, runs a
> Transformer on the form, and spits out a new form.
> 
> - Peter
> 
> 
> On Wed, Mar 23, 2011 at 3:30 PM, Anders Logg <logg@xxxxxxxxx
> <mailto:logg@xxxxxxxxx>> wrote:
> 
>     Peter Brune claims to have solved this by a small addition to the form
>     language that automatically expresses the curved elements as a mapping
>     and expands appropriately (and invisible to the user) those mappings
>     to yield a form that may then be assembled. The higher order geometry
>     is then expressed as a vector-field on the mesh.
> 
>     Perhaps Peter can be pushed to polish up on his code and submit it.
> 
>     --
>     Anders
> 
> 
>     On Wed, Mar 23, 2011 at 07:46:57PM +0000, Garth N. Wells wrote:
>     > We haven't really looked at this. It was discussed a while back,
>     but no
>     > one has committed much time to it. We struggled to settle on an
>     > appropriate abstraction to push on with.
>     >
>     > Garth
>     >
>     > On 23/03/11 18:40, Douglas Arnold wrote:
>     > > What is the status of curved (e.g., isoparametric) elements in
>     dolfin?
>     > > I gather they are not implemented in the main branch.   Has anyone
>     > > done anything with this can be used?  Is there any example code?
>     > > (For example, if you want to
>     > > solve the Poisson problem in a disc and get better than 2nd order
>     > > convergence, you need to do better than polygonal approximation of
>     > > the disc.)
>     > >
>     > >  -- Doug
>     > >
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