[vtkusers] Persistent data across time step changes in programmable filter

[Sean Ziegeler]

I previously sent this to the ParaView list and got no answer, but I 
realize now that it's really more of a VTK pipeline question anyway.

I'm writing a programmable filter that makes changes to a vector field 
based on grid point properties (it's a polar orthographic transform 
using a highly accurate Earth shape model).

This works well.  However, when one changes the timestep, it recomputes 
those grid point properties to apply to the next timestep's vector 
field.  This is a very expensive operation, and is quite unnecessary 
since the grid doesn't change over time.  I'd like to somehow cache the 
grid point property array so that I could simply reuse it for future 
timesteps if it's already been computed.

I thought about storing it as a Field Data array.  However, with some 
experimentation, I find that Field Data arrays are cleared from the 
filter output when the pipeline re-executes for a new timestep.  Is 
there a way to keep a Field Data array around for subsequent time steps? 
  If not, is there some other mechanism for a programmable filter to 
retain data that won't change after the first timestep?

Thanks!
Sean