[vtkusers] Is there a way to avoid update an outputport of vtkImageAlgorithm subclass?

[Mengda Wu]

Hi all,

   I am writing a filter that will subclass of vtkImageAlgorithm, which has
several output ports. For some port, it is really expensive to
update/regenerate. So I added a flag in the class to control whether or not
to update that particular port. For other ports, I want them to update as
usual. In case of skipping the update of that particular port, I would like
to keep its old output.

   I found everytime RequestData runs, it always clear the output (set
extent zero size) first so makes the old output disappear. Is there a way
to avoid this behavior?

   I have difficulty in finding a detailed document explains how the
algorithm/executives work. Can anyone point to a good source?

Thanks,
Mengda
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