Post-processing problems. Cell data and point data.

[s f1]

Guys I have no clue where to post (Gerris forum that is inactive or here) regarding the following problem and taught to post it in this forum for some active discussion.

I am trying to write an output file for a case. I tried all possible outputs available:

.gfs: Works as expected on the gfsview2D platform.

Gerris, Basilisk are built on "collocated grids". So, I trust the cell center values over the interpolated nodal values. Reason being, I am interested in looking at the numerical solution rather than on showing a pretty picture for others.
gfsview is flexible in reading the cell center data for all the fields and when required the interpolation can be done.

Here comes my problem: I am looking at a flow profile and would like to use a calculator to subtract the parabolic flow velocity within the channel which is not possible with gfsview. From my knowledge, I see paraview showing this feature which is relatively simple and straight forward. So, I try the vtk, tecplot outputs to visualize the data and do my calculations.

.VTK and Tecplot: unfortunately here the outputs are written on the nodal points rather than on the cell centers. (I am not sure why isnt this left as an option for the user to choose either cell center or point values).
 After looking at all the filters available in paraview, I came across a filter: POINTDATATOCELLDATA. However, I am not convinced to do the interpolation to get the cell data and for sure differs from real value obtained. If I use the paraviews filter it is two times I do the interpolation to get the cell data which changes the real value.

.text: Last option, I tried to write down the ASCII format values with information regarding the coordinates of the cell centers and variable fields located at cell centers. However, visualization of this is not straight forward and need some conversion to other formats where I am lost.

So, can anyone advice me;
1. How to write the computed cell center data in vtk format.
 (or)
2. How to visualize the ascii output in paraview.

Thanks and hope anyone can guide me;
SaiD