[Paraview] vtu: how to represent variation in cell end values?

[Berk Geveci]

Emily,

It sounds like you need point data not cell data. Point data arrays
contain a value per point (those values are interpolated to obtain
values that are in the interior of the cell), cell data array contain
a value per cell (which is assumed to be constant across the cell).
VTK will interpolate point arrays linearly over a cell of type Line.

-berk

On Thu, Mar 25, 2010 at 3:43 PM, Emily Leigh <ecl4b at hotmail.com> wrote:
> Hello,
>
> I am writing a vtu file to represent data for a simple dataset in Paraview
> and I have run into a problem.  What I would like to do is to be able to
> represent a variation in a field value over the length of a cell and look at
> this variation as a single layer in Paraview (not separately as the sets of
> end values for end 1 and end 2, respectively).  More simply, each cell is
> assigned a field value at each end point and I want to be able to visualize
> this variation in end values for each cell for all of the cells in my
> (small) model.
>
> As the simplest example of my problem, let's say I have 3 points and 2 cells
> between the points.  Cell 1 has point 1 at its left end and point 2 at its
> right end and cell 2 has point 2 at its left end and point 3 at its left end
> (in my field this might represent a beam finite element). 1---2---3
> Cell 1: 1=>2 (finite element 1)
> Cell 2: 2=>3 (finite element 2)
>
> I've figured out how to represent the field data at the points (physically
> these would be displacements, rotations, etc.).  However, I need to figure
> out what syntax to use to represent the variation in a physical quantity
> over the length of the cell.  Let's say the y-force for element 1 has values
> of 100 at point 1 and -500 at point 2, and that y-force for element 2 has
> values of +500 at point 2 and -200 at point 3.
>
> Locally this variation in y-force is:
> Cell 1: 100=>-500
> Cell 2: +500=>-200
>
> Summing the effect of the y-force at each point:
> Point 1: 100
> Point 2: -500+500 = 0
> Point 3: -200
>
> I have tried the following syntax:
> <CellData Scalars="scalars">
> <DataArray type="Float32" Name="y-force-el" NumberOfComponents="2">
> 100 -500
> 500 -200
> </DataArray>
> However, when I used the above syntax I found that in Paraview I can only
> view the x-points (100 and 500), the y-points (-500 and -200) or the vector
> magnitudes for each cell (509.9 and  538.5) at one time.
>
> Does anyone know what syntax I need?
>
> (Once I figure out the syntax, I'll be all set to modify the code I wrote to
> read in my data and output it to vtu format).
>
> Thanks!
>
> -Emily
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