Everything is good (though there's certain problems with matplotlib's triangulation sometimes) but in this case I had a hole in a flow field. So if I have a file with (x, y, value) tuples, the hole will be also triangulated with a value equal to zero and a final tricontourf figure will be quite far from the original flow field. Also as OpenFOAM's sample utility will anyway create a triangulation of the mesh, why do it again. So I've decided to try plotting VTK file with a triangular grid. And it was more or less easy. Though there's almost no documentation on python-vtk module. Well, I guess this module is just a wrapper around C++ so one need to write C++-python instead of python.

Surely this script can be improved to handle vectors and scalars automatically, to handle polymesh files in more general way (currently I suppose, all polygons in the mesh are triangles).

Here is a script for loading a vector field and plotting its X component:

```
#!/usr/bin/env python
import numpy as N
import vtk
import matplotlib.pyplot as P
def load_velocity(filename):
if not os.path.exists(filename): return None
reader = vtk.vtkPolyDataReader()
reader.SetFileName(filename)
reader.ReadAllVectorsOn()
reader.Update()
data = reader.GetOutput()
cells = data.GetPolys()
triangles = cells.GetData()
points = data.GetPoints()
point_data = data.GetPointData()
Udata = point_data.GetArray(0)
ntri = triangles.GetNumberOfTuples()/4
npts = points.GetNumberOfPoints()
nvls = Udata.GetNumberOfTuples()
tri = N.zeros((ntri, 3))
x = N.zeros(npts)
y = N.zeros(npts)
ux = N.zeros(nvls)
uy = N.zeros(nvls)
for i in xrange(0, ntri):
tri[i, 0] = triangles.GetTuple(4*i + 1)[0]
tri[i, 1] = triangles.GetTuple(4*i + 2)[0]
tri[i, 2] = triangles.GetTuple(4*i + 3)[0]
for i in xrange(points.GetNumberOfPoints()):
pt = points.GetPoint(i)
x[i] = pt[0]
y[i] = pt[1]
for i in xrange(0, Udata.GetNumberOfTuples()):
U = Udata.GetTuple(i)
ux[i] = U[0]
uy[i] = U[1]
return (x, y, tri, ux, uy)
P.clf()
levels = N.linspace(-0.5, 1.5, 64)
x, y, tri, ux, uy = load_velocity('U-file.vtk')
P.tricontour(x, y, tri, ux, levels, linestyles='-',
colors='black', linewidths=0.5)
P.tricontour(x, y, tri, ux, levels)
P.gca().set_aspect('equal')
P.minorticks_on()
P.gca().set_xticklabels([])
P.gca().set_yticklabels([])
P.title('$U_x$ / Gamma')
P.savefig("Ux.png", dpi=300, bbox_inches='tight')
P.savefig("Ux.pdf", bbox_inches='tight')
```

(

**Note**: updated version of the script adapted for VTK 6.2.0 is posted here)

And as a result of this script we'll get this nice picture:

And here is a link to the genereated PDF.