[vtkusers] Update() method

[Elena Caraba]

Dear VTK Users,

I'm having quite some difficulties understanding the purpose of the Update()
method. I'm trying to time parts of an isosurface algorithm  and depending
on whether or not I use the Update() method on the reader, isosurface, etc,
the time varies significantly ... or is pretty much constant... for
different data sizes.

Just for reading the different data, it takes about 10^-4 for each of the
data sets with no call to Update(), so it's constant. However, if I call the
Update on the reader, as soon as I read the data, the time varies from 1
second to about 60 seconds for the sets.

It appears that the image is the same, with or without Update().

Can someone help me understand the "mystery"? What is VTK doing with the
data when Update is called on the reader or the geometry (e.g. isosurface)?

Thank you,
Elena
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