[Insight-developers] Managing Abort in Filters

[Luis Ibanez]

Hi Jim,

My mistake, I misspelled the name of the Exception
in the email.  The actual name in the code is
"ProcessAborted", since this is done at the level
of the ProcessObject.

We could certainly add a hierarchy of more detailed
exceptions under the "Abort" category...

The filters modifies so far are catching the exception,
cleaning memory when necessay and then rethrowing the
exception with the correct LINE and FILE data.

The reason for using exception as a mechanism of
communication between the progressReporter and the
Filter is that we don't want to keep asking for
(is AbortGenerateData() == true ?), at each pixel
processed.  Placing this question in the Progress
Reporter schedules the question every (TotalPixels/100)
pixels.

Alternative options include:


1) make the Iterator itself know about the filter
    and inside the iterator ++ check for the Abort Flag


2) let the progress Reporter hold a pointer to the
    iterator and le it move the iterator to the End
    position with the aim of ending the while loop.
    This options is ok for filters with a single while
    loop, but when there are severeal nested while loops,
    it will start looking like good old COBOL code.


-----------------


The question at this point is:

if we have filters

       A ----->   B

and we cancel "A" while it is in its GenerateData()
method...


1) what will "B" do ?

2) Is the AbortGenerateData flag automatically propagated
    to all the filters downstream ?

3) How is indicated that the output of all pending filters
    is now invalid ?


In the meantime, the current implementation is letting
the ProcessAborted exception propagate up to the level
of the applicatoin GUI, where, after all, the abort
action originated when the user clicked in a "Cancel"
button.



   Luis



------------------------------------
Miller, James V (Research) wrote:
> Is "ProgressAborted" the right name for the exception?
> I don't think of it as the "progress" being aborted as 
> much as the filter being aborted. 'Course the only thing
> I can come up with right now is "AbortGenerateDataException".
> I think we want something more specific than just "AbortException".
> Looking at our other exceptions, some end in the word
> "Exception" and some end in the word "Error". (Rambling)
> 
> This approach is using exceptions as a means to 
> communicate between the ProgressReporter object and 
> the filter.  We are basically trying to hide duplicate
> code in the ProgressReporter class. To date, however, 
> I think we have tried to restrict the use of exceptions 
> to "errors" as opposed to "state".  But I guess, this is 
> kind of in the gray area.  I am not worried about the 
> overhead of the exception since it is only thrown when 
> the user select "abort".
> 
> The bottom line is that if a filter uses a ProgressReporter
> it had better catch this exception.  We never want this 
> exception to propagate up to the application.
> 
> 
> 
> 
> 
> 
>>-----Original Message-----
>>From: Luis Ibanez [mailto:luis.ibanez@kitware.com]
>>Sent: Tuesday, April 08, 2003 10:36 AM
>>To: Insight Developers List
>>Subject: [Insight-developers] Managing Abort in Filters
>>
>>
>>Hi,
>>
>>The following approach for managing Abort calls
>>has been added (tentatively) to filters:
>>
>>1) The ProgressReporter tests for the
>>      AbortGenerateData flag after calling
>>      ProgressUpdate(). That gives the user
>>      a chance to click in an "Abort/Cancel"
>>      button when the GUI is refreshed after
>>      the progress update.
>>
>>2) If the progress reporter finds the AbortGenerateData
>>     flag ON, it throws a (newly added to the toolkit)
>>     "ProgessAborted" exception.
>>
>>3) Filters should catch this "ProgressAborted" exception
>>     outside the outer loop of their GenerateData() and
>>     ThreadedGenerateData() methods.
>>
>>
>>
>>This logic has been added to the following
>>filter classes:
>>
>>
>>    - UnaryFunctorImageFilter
>>    - BinaryFunctorImageFilter
>>    - TernaryFunctorImageFilter
>>    - MedianImageFilter
>>
>>The first three cover most of the pixel-wise filters.
>>
>>
>>
>>   Luis
>>
>>
>>
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> 
>