ufl team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Fri, Mar 06, 2009 at 05:49:45PM +0100, Kristian Oelgaard wrote:
> Quoting "Garth N. Wells" <gnw20@xxxxxxxxx>:
> 
> >
> >
> > Kristian Oelgaard wrote:
> > > Quoting "Garth N. Wells" <gnw20@xxxxxxxxx>:
> > >
> > >> I just tested the new FFC generated code (with quadrature) from the UFL
> > >> input for Poisson. From the input code
> > >>
> > >>      element = FiniteElement("Lagrange", "triangle", 1)
> > >>      v = TestFunction(element)
> > >>      u = TrialFunction(element)
> > >>      f = Function(element)
> > >>
> > >>      a = dot(grad(v), grad(u))*dx(0, {"quadrature_order":1})
> > >>      L = v*f*dx(0, {"quadrature_order":2})
> > >>
> > >> I computed exactly the same solution as with the FFC .form code, but with
> > >>
> > >>      element = FiniteElement("Lagrange", "triangle", 1)
> > >>      v = TestFunction(element)
> > >>      u = TrialFunction(element)
> > >>      f = Function(element)
> > >>
> > >>      a = dot(grad(v), grad(u))*dx
> > >>      L = v*f*dx
> > >>
> > >> I see differences. How is the order of integration being selected?
> > >
> > > As far as I remember the UFL algorithm currently just looks at order of the
> > > basis functions:
> > >
> >
> > So it only looks at the test and trial functions, and not the other
> > functions? And it doesn't take into account derivatives?
> 
> Yes.
> 
> > What, if anything, specific will UFL provide to the compiler to help
> > decide on the integration order?
> 
> The algorithm to compute this is present in UFL now it *just* needs to be
> finished, but I don't know the priority of this.
> 
> Kristian

But it's strange that there should be a difference. The old code
presumably used quadrature of high enough order to integrate
exactly. Even if the new code uses quadrature of even higher order,
there shouldn't be any difference (to within machine precision).

By the way, is quadrature_order the polynomial degree of the
polynomials that should be integrated exactly, or is it the order of
convergence. In case it is the former, maybe we should rename it to
quadrature_degree to avoid confusion?

-- 
Anders

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