the "Bypass" feature in imaging

[David Gobbi]

Hi Lisa,

I never use the Bypass method myself.  Many of the imaging filters 
I use do geometrical operations (like vtkImagePermute,
vtkImageClip, etc) and a Bypass for these filters really makes
no sense.  Same with vtkImageMapToRGBA, and pretty much any
dual-input filter.

Perhaps it could be implemented on a class-by-class basis for
the filters where it really is useful.

 - David

--
  David Gobbi, MSc                    dgobbi at irus.rri.on.ca
  Advanced Imaging Research Group
  Robarts Research Institute, University of Western Ontario

On Thu, 16 Dec 1999, Lisa Sobierajski Avila wrote:

> 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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