[vtkusers] Update() method

[Clinton Stimpson]

It updates the data of a filter so you can read its output.
If you're not going to read the output, don't bother calling it.
Mappers and other connected filters read the output so Update() is 
called automatically for them.

If you're changing parameters after you manually call Update(), you'll 
most likely slow the program down.

Clint

Elena Caraba wrote:
> 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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