the "Bypass" feature in imaging

[Lisa Sobierajski Avila]

<x-flowed>Hello everyone,

As you may know, the VTK pipeline architecture is being reworked for VTK 
3.0. The main goal of this rework is to simplify the pipeline update 
mechanism. This is being done by making the update process occur mostly at 
the superclass level (vtkSource and vtkDataObject) and clearly defining 
what parts of the update process could / should be overridden. A byproduct 
should be that new filters will be easier to write - and writing a graphics 
filter won't be too much different than writing an imaging filter.

This leads to my question - how many people out there are using the Bypass 
feature in the imaging filter? And, if you are, how awful would it be if it 
goes away? Here are the pros and cons:

Why we should keep the bypass feature: This feature allows you to easily 
turn off a filter in the middle of an imaging pipeline without having to 
keep track of all the other filters.

Why we should eliminate the bypass feature: It complicates the execute 
method in the imaging filters. It is not implemented in every imaging 
filter now (sometimes it is just ignored). If you use it without thought to 
what two filters you would be connecting if you turn Bypass on, then you 
can cause an error or even a segmentation fault in your pipeline.

It is possible to argue that instead of taking it out of the imaging 
filters, we should make it a more general feature of the pipeline (for 
example vtkDataSetToDataSetFilter subclass could bypass, as could 
vtkPolyDataToPolyDataFilter subclasses). Of course, this would then 
complicate even more execute methods, and would mean that many filters 
would have to produce an error if you try to turn Bypass on.

My opinion is that it should be removed. It is a nice feature, but it does 
complicate the execute methods and therefore would make understanding / 
writing filters easier if it is gone. Also, you can simply reconnect your 
pipeline to achieve the same effect. Of course, I don't use this feature so 
I may not have a full understanding of why this feature is used and what 
impact its removal will have.

If you have an opinion, please express it in email to the user's list (and 
do it soon please - since I want to check in all these changes in the next 
few days) If enough people express a strong opinion that it should stay, 
then I will probably leave it in pretty much as is right now, with the goal 
of making it more general in the future.

Thanks,

Lisa



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