[Insight-developers] Bug in itkShrinkImageFilter

[Bill Lorensen]

Hans,

I'll look at the ShrinkImage problem and also look at Brad's bug.

Bill

On Wed, Dec 10, 2008 at 11:20 AM, Hans Johnson <hans-johnson at uiowa.edu> wrote:
> Bill,
>
> THANKS!  These comments are exactly what I was hoping for.
>
> So this now becomes two separate issues:
> 1) Change multiresolution pyramid filter to use ResampleImageFilter.
> 2) Make the Shrink image filter preserve object physical locations better,
> but without using interpolation.
>
> ====================
>
> I'll be working on a solution to #1 above today, but since this eliminates
> my immediate need to solve problem #2 with the ShrinkImageFilter I'll likely
> not be addressing that problem anytime soon.  Could that be assigned to
> someone else?
>
> ====================
> With regards to point number 2 above:
>
> I feel quite strongly that an 8x8 white square in the middle of a 32x32
> space should retain it's center of mass after the shrink image is applied
> with a shrink factor of 2,4, or 8.  This is one of the test cases in the new
> failing test I added last night.  In the current implementation, the center
> of mass is not preserved.
>
> In the case of shrinking a 64x64 image to an 8x8 image, the index locations
> map as follows
>
> Fill     Current    Proposed    Object Space Preserving
> Output   Input      Input       Input ContinuousIndex
> [0,0]    [0,0]      [4,4]       [3.5,3.5]
> [1,1]    [8,8]      [12,12]     [11.5,11.5]
> ...
> ...
> [7,7]    [56,56]    [60,60]     [59.5,59.5]
>
> By reading from the current input there is a bias towards shifting the
> objects towards the origin in the current implementation.  Perhaps the best
> way to deal with this would be to add the difference between columns 3 and 4
> in the table above to the origin.
>
> * My proposed solution will need to better checking of boundary conditions
> (seg fault would occur on images of size 60x60 being sub-sampled to and
> image space of 8x8).
>
>
> Thanks,
> Hans
>
> On 12/10/08 7:21 AM, "Bill Lorensen" <bill.lorensen at gmail.com> wrote:
>
>> Hans,
>>
>> Another issue if you make your changes. If the new Shrink is run on a
>> labeled image, I believe your code will interpolate the new pixels.
>> This behavior is not desirable for subsampling a labeled image.
>>
>> Perhaps we should leave this filter alone and change the
>> multiresolution pyramid code to use ResampleImageFilter with the
>> proper values set to do subsampling.
>>
>> Bill
>>
>> On Wed, Dec 10, 2008 at 12:05 AM, Bill Lorensen <bill.lorensen at gmail.com>
>> wrote:
>>> Hans,
>>>
>>> The documentation for ShrinkImageFilter says that it does simple
>>> subsampling. So it is not really a bug according to the documentation.
>>> However, I think your implementation is more useful (I assume it is
>>> correctly done). You should probably fix the documentation in the .h
>>> file when you commit these changes.
>>>
>>> Also, beware that we have enabled a KWStyle commit check for some
>>> directories. These directories are Testing/Code/Review, Code/Review,
>>> Code/IO and Code/BasicFilters. All of these directories are currently
>>> clear of any style defects.
>>>
>>> Bill
>>>
>>> On Tue, Dec 9, 2008 at 11:55 PM, Bill Lorensen <bill.lorensen at gmail.com>
>>> wrote:
>>>> Hans,
>>>>
>>>> Two of the tests are seg faulting.
>>>>
>>>> Bill
>>>>
>>>> On Tue, Dec 9, 2008 at 11:38 PM, Hans Johnson <hans-johnson at uiowa.edu>
>>>> wrote:
>>>>> Luis and Bill,
>>>>>
>>>>> I think I've uncovered a bug in the itkShrinkImageFilter that has
>>>>> ramifications in any use of the itkMultiResolutionPyramidImageFilterTest.
>>>>>
>>>>> I was writing a program that aligns the mid-sagital plane with the center
>>>>> of
>>>>> an image, and found that as I went from low-resolution estimates to
>>>>> high-resolution estimates, the center of mass of the object in the
>>>>> different
>>>>> scale images would change.
>>>>>
>>>>> A new test (currently failing) has been added, and the patch that makes
>>>>> that
>>>>> test pass has been included in the Bug report:
>>>>> http://public.kitware.com/Bug/view.php?id=8275
>>>>>
>>>>> This change, however, will cause any of the tests that depend on the
>>>>> MultiResolutionPyramidImage (and prehaps others) to start failing.  I'd
>>>>> like
>>>>> your advice on how to get this bug fix resolved in the least obtrusive way.
>>>>>
>>>>> Thanks,
>>>>> Hans
>>>>>
>>>>>
>>>>
>>>
>
>