[vtk-developers] Interesting imaging pipeline information bug

[David Gobbi]

Some of the imaging filters (e.g. vtkImageReslice) need the origin and
spacing in order to compute the UpdateExtent. Without this information,
these filters would always have to set the UpdateExtent to the whole extent.

For vtkImageReslice, for example, the ResliceTransform is expressed in
spatial units (e.g millimeters) so figuring out the relationship between
the output extent and the input extent requires that the origin and spacing
are known.

So it seems that the solution is as follows, at least for vtkImageReslice:
1) if SPACING and ORIGIN are not available, then they are not passed
downstream (pretty obvious that injecting bogus information into the
pipeline is bad).
2) also, if they are not available, then the UPDATE_EXTENT required for
proper streaming cannot be computed, so the input update extent would have
to be set to the whole extent.

Also, for some filters, the origin and spacing are used to compute the
output extent if the caller doesn't set the output extent explicitly.

Long story short: these filters can be modified so that they don't crap out
when ORIGIN and SPACING are missing, but we cannot remove ORIGIN and
SPACING from the pipeline completely without removing some nice features
and seriously breaking backwards compatibility.

Cheers,
 - David

On Thu, Jan 21, 2016 at 10:50 AM, Berk Geveci <berk.geveci at kitware.com>
wrote:

> Interesting. This probably sort-of worked previously because the pipeline
> called vtkImageData::CopyInformationToPipeline() at some point. I am saying
> sort-of because the pipeline could still have the wrong information at
> certain stages. In theory, these keys are supposed to be available during
> RequestInformation(). Otherwise, one can simply used origin and spacing
> from vtkImageData. The only way for these to be there during
> RequestInformation() is if the source/filters provided it. So if the source
> didn't provide it, you would end up with default values during
> RequestInformation and correct but redundant data during RequestData.
>
> Since the pipeline only very partially supports these, I would like to get
> rid of them OR make them optional. Optional being that filters should check
> for their existence and not assume they will be always available.
>
> Best,
> -berk
>
>
> On Thu, Jan 21, 2016 at 12:29 PM, David Gobbi <david.gobbi at gmail.com>
> wrote:
>
>> Hi All,
>>
>> I was looking through the bugtracker this morning (I know, I shouldn't do
>> that :-)
>> and found an interesting one:
>>
>>   http://www.vtk.org/Bug/view.php?id=15901
>>
>> Basic issue is that most VTK image algorithms do this:
>>
>>   double spacing[3], origin[3];
>>   inputInfo->Get(vtkDataObject::ORIGIN(), origin);
>>   inputInfo->Get(vtkDataObject::SPACING(), spacing);
>>
>> This seems to have worked fine in VTK 5, but in VTK 6, is there any
>> guarantee
>> that the pipeline will have ORIGIN and SPACING for vtkImageData?
>>
>>  - David
>>
>
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