[Insight-developers] DenseFiniteDifferenceImageFilter and Pipelines

[Tom Vercauteren]

Hi all,

Some time ago, I filed a bug report
(http://www.itk.org/Bug/view.php?id=3972) about a potential pipeline
problem in itk::DenseFiniteDifferenceImageFilter. There has been a
small discussion (see links below) but the issue has not been fixed. I
need some feedback to know if my proposed fix does not embarrass other
ITK developers.

Here's a summary.

itk::DenseFiniteDifferenceImageFilter basically loops over two function calls:
for i=1:N
    CalculateChange
    ApplyUpdate
end

CalculateChange uses ITK iterators (not filters) to modify the
filter's update buffer.
ApplyUpdate uses ITK iterators (not filters) to modify the filter's
output buffer for example by adding values from  the filter's update
buffer.

Up to now everything works fine. Now suppose I implement a filter
called SubclassFilter that derives from
itk::DenseFiniteDifferenceImageFilter and overrides ApplyUpdate.

Let's assume that SubclassFilter::ApplyUpdate does not uses iterators
to modify the filter's output but rather standard ITK filters.

The problem is that the ITK filters in SubclassFilter::ApplyUpdate
will only be executed during the first iteration of the
itk::DenseFiniteDifferenceImageFilter loop.

Indeed, since CalculateChange only uses ITK iterators, the modified
time of the filter's update buffer is not incremented in the loop.

The ITK filters in SubclassFilter::ApplyUpdate therefore see that
there output (e.g. the filter's output buffer) is more recent that
there input (e.g. the filter's update buffer). Updating the filters
therefore has no effect.


My proposed fix is simply to call:
- m_UpdateBuffer->Modified() at the end of
DenseFiniteDifferenceImageFilter::CalculateChange and
- this->GetOutout()->Modified() at the end of
DenseFiniteDifferenceImageFilter::ApplyUpdate

So even though the buffers are modified through iterators, there
ModifiedTime gets incremented in the
itk::DenseFiniteDifferenceImageFilter loop.


Please tell me if this makes sense or if my explanation is unclear.

Best regards,
Tom Vercauteren


P.S.: previous posts are here
http://www.itk.org/pipermail/insight-users/2006-August/018962.html
http://www.itk.org/pipermail/insight-users/2006-August/018983.html
http://www.itk.org/pipermail/insight-users/2006-August/018994.html