Subject: [FFC-dev] Choice of quadrature points in ffc

From: "Martin Sandve Alnæs" <[email-masked]>
Date: Mon, 22 Oct 2007 12:58:17 +0200

Hi again,
how is the quadrature order chosen in ffc? What kind of quadrature
rule du you use? For a stiffness matrix with 2D cubic elements the
generated code uses 9 points, but a 4th order Gauss rule on a triangle
is only 6 points and a 6th order rule is 12 points.

I just need to be sure my benchmarks compare the same thing, so I need
the correct number of quadrature points.

(So far the benchmarks have alternated between ffc and sfc being
faster for non-quadrature code).

--
Martin

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From: Anders Logg <[email-masked]>
Date: Tue, 23 Oct 2007 01:01:18 +0200

> is only 6 points and a 6th order rule is 12 points.

We don't use Gauss points, we use the same quadrature as in FIAT,
which is based on 1-D Jacobi quadrature extended to the d-dimensional
cube, then "collapsing" the cube into a simplex. It's not optimal but
it's simple.

FFC chooses the lowest order such rule that integrates the polynomial
exactly.

> faster for non-quadrature code).

Interesting. Can you post the code (tabulate_tensor) for the simplest
possible case where SFC is faster?

/Anders