dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Mon, Jun 06, 2011 at 10:17:27AM -0000, Garth Wells wrote:
> On 06/06/11 11:05, Martin Sandve Alnæs wrote:
> > On 6 June 2011 11:54, Garth Wells <785874@xxxxxxxxxxxxxxxxxx> wrote:
> >> On 06/06/11 10:41, Martin Sandve Alnæs wrote:
> >>> On 31 May 2011 00:24, Anders Logg <logg@xxxxxxxxx> wrote:
> >>>> On Mon, May 30, 2011 at 09:53:42PM -0000, Martin Sandve Alnæs wrote:
> >>>>> There's two, don't remember what they do:
> >>>>>   def estimate_max_polynomial_degree(e, default_degree=1):
> >>>>>   def estimate_total_polynomial_degree(e, default_degree=1):
> >>>>> in algorithms/transformations.py (should rather be in analysis.py I guess).
> >>>>>
> >>>>> ** Changed in: dolfin
> >>>>>        Status: New => Invalid
> >>>>
> >>>> And those include spatial coordinates?
> >>>
> >>> Turns out they didn't. Just checked the code.
> >>> But it was easy to add. I'm commiting changes
> >>> to estimate_total_polynomial_degree now which
> >>> incorporate the spatial degree. Maybe this should
> >>> be used for assembling rhs and functionals, while
> >>> looking at elements are enough for the bilinear form?
> >>>
> >>> PyDOLFIN could do something like
> >>>
> >>> d = estimate_total_polynomial_degree(expr)
> >>> d = max(d, 1)
> >>> d = min(d, 8)
> >>>
> >>> to limit the degree to some reasonable range in cases such as
> >>> expr = sin(x**5)*cos(y**5)
> >>> which would lead to a degree of (5+2)+(5+2)=14 with the current heuristics.
> >>> Look at the code and tests in the last commit for more details, it's
> >>> quite short.
> >>>
> >>
> >> We have the same issue of order blow-out for problems with lots of
> >> coefficients. I'm therefore inclined not to include any heuristics, and
> >> leave it up to the user.
> >
> > Do you mean we should actually crash and burn with this line?
> >   f = assemble(sin(triangle.x[0]), mesh=mesh)
> > With the current heuristic this will give degree 3,
> > 1 from x and +2 from sin.
> >
>
> We obviously need an approach for functions from non-polynomial spaces.
> What I'm not inclined towards is arbitrary thresholds for integrating
> polynomial products.

What if we by default set that threshold to the maximal element degree
+ k, where k is say 1? It would be enough to retain expected
convergence.

--
Anders

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https://bugs.launchpad.net/bugs/785874

Title:
  Projection of x is not accurate

Status in DOLFIN:
  Invalid

Bug description:
  I've tested that projecting x works without the scaling bug that was
  just fixed, using dimensions 1,2,3 and both DG and CG from 0 to 3
  degrees. I print the max and min values of the vector of the
  projection function, and the values are _close_ to 0 and 1 but not to
  machine precision. The script is below.

  There's up to 2.7% error in the 3D case. Is the projection form
  integrated accurately enough? All but the DG0 function space should be
  capable of representing x exactly. Not sure if this is a dolfin or ffc
  bug.

  
  from dolfin import *

  def mcx(dim):
      if dim == 1:
          mesh = UnitInterval(20)
          cell = interval
          x = cell.x
      if dim == 2:
          mesh = UnitSquare(20, 20)
          cell = triangle
          x = cell.x[0]
      if dim == 3:
          mesh = UnitCube(20, 20, 20)
          cell = tetrahedron
          x = cell.x[0]
      return mesh, cell, x

  for dim in range(1, 4):
      mesh, cell, x = mcx(dim)
      minval, maxval = 1.0, 0.0
      #print dim, "DG"
      for degree in range(3):
          V = FunctionSpace(mesh, "DG", degree)
          u = project(x, V)
          #print dim, degree, u.vector().min(), u.vector().max()
          minval = min(minval, u.vector().min())
          maxval = max(maxval, u.vector().max())
      #print dim, "CG"
      for degree in range(1, 4):
          V = FunctionSpace(mesh, "CG", degree)
          u = project(x, V)
          #print dim, degree, u.vector().min(), u.vector().max()
          minval = min(minval, u.vector().min())
          maxval = max(maxval, u.vector().max())
      print minval, maxval