Hi there. How do I write complicated time dependent mathematical expressions to file and have them play in paraview as a movie? In my attached example I’m calculating the spectral decomposition of a tensor and then performing some algebra before writing to file. The problem is that each time iteration is saved indepently and doesn’t play as a movie. Additionally the whole process is quite slow because I’m using project. Any suggestions would be very welcome?

```
from fenics import *
def e1(A):
return (tr(A) - sqrt((A[1,1]-A[0,0])**2+4*A[1,0]**2))/2
def e2(A):
return (tr(A) + sqrt((A[1,1]-A[0,0])**2+4*A[1,0]**2))/2
def vmat(A):
lambdan = e1(A)
c = A[1,0]
d = A[1,1]
v11 = lambdan - d
nv11 = sqrt(v11**2 + c**2)
a1 = v11/nv11
c1 = c/nv11
tol = 10e-12
return conditional(gt(abs(c), tol),as_matrix(((a1,-c1),(c1,a1))),as_matrix(((1,0),(0,1))))
file_tau = XDMFFile('tau.xdmf')
file_tau.parameters['flush_output'] = True
STRESS = TensorFunctionSpace(mesh, 'CG', 2)
tau0 = Function(STRESS)
beta_ve=0.5
Wi_out=2
c = vmat(tau0)*as_matrix([[exp(e1(tau0)),0],[0,exp(e2(tau0))]])*vmat(tau0).T
tauff = ((1-beta_ve)/Wi_out)*(c-Identity(2))
t=0
tauf = project(tauff, STRESS)
file_tau.write(tauf, t)
```