Anders Logg wrote: > On Thu, Nov 26, 2009 at 11:07:20PM +0000, Garth N. Wells wrote: >> >> Anders Logg wrote: >>> On Thu, Nov 26, 2009 at 10:55:14PM +0000, Garth N. Wells wrote: >>>> [email protected] wrote: >>>>> ------------------------------------------------------------ >>>>> revno: 4323 >>>>> committer: Anders Logg <[email protected]> >>>>> branch nick: dolfin-dev >>>>> timestamp: Thu 2009-11-26 11:15:01 +0100 >>>>> message: >>>>> Use Lagrange as default element instead of quadrature for expressions. >>>> I thought that we agreed that this is a bad idea. >>>> >>>> Garth >>> I don't see why it is such a bad idea. Either we approximate a >>> coefficient by interpolating it into a finite element space or we >>> evaluate it at quadrature points. Just two different approaches to >>> approximation. >>> >> We discussed this a few days ago and agreed that it's not good. > > I don't remember why I agreed really. I keep getting back to thinking > it's not that bad. :-) > >> How should the order be chosen? > > The order can be chosen to preserve the order of approximation, for > which it is enough to take the degree to be the same as that for the > test and trial functions. > > This is what we do in bench/fem/convergence which shows the expected > convergence rates for degree 1-5 (but I haven't checked in a while). > >> All the finite element codes that >> I'm aware of would evaluate an Expression at quadrature points, so >> that's what most users would expect. > > DOLFIN has been interpolating to a FEM space for the last 5 years and > that has worked fine... :-) >
Why did we add QuadratureElements then? It's not just a question of what works, it's what would one expect. Garth >>> Besides, it's the only thing that currently works because of the bug >>> in the quadrature code generation that I wasn't able to fix. >>> >> That's a different motivation ;). > > Yes, this is why I changed. > >>> The choice of element is controlled by the variable named "Family" in >>> the function _auto_select_element_from_shape in expression.py. >>> >>> We can make this a global parameter and set the default tor >>> "Quadrature" once it works, >> I'm happy is "Quadrature" is the default, but if it's not the default >> will we ever get around to fixing it? > > I think Kristian is looking into it. In the meantime, we can go ahead > and update all the demos and prepare for a release. This is a bug in > FFC, not DOLFIN, and it doesn't change the interface. > > We might also want to make some cleanups in expression.py. There are > still a bunch of references to function spaces which should not be > there anymore. > > -- > Anders -- Dr Garth N Wells Department of Engineering University of Cambridge Trumpington Street Cambridge CB2 1PZ United Kingdom tel. +44 1223 3 32743 e-mail [email protected] http://www.eng.cam.ac.uk/~gnw20/ _______________________________________________ Mailing list: https://launchpad.net/~dolfin Post to : [email protected] Unsubscribe : https://launchpad.net/~dolfin More help : https://help.launchpad.net/ListHelp
