Hi Lars,<br><br>Thanks for pointing this out.<br><br>The origins are actually computed correctly,<br><br>Although it is understandable that you may be surprised by the<br>way the origin of the image changes at every resolution level.<br>
<br>Here is the rationale for recomputing the image origin:<br><br>We want to make sure that if the image contains an object,<br>the center of mass of that object doesn't change its physical<br>coordinates at every resolution level. <br>
<br>The sub-pixel refinement of the image origin must be done<br>at every resolution level in order to ensure this stability<br>property. (It actually took a lot of hard work to get it to<br>behave this way) <br><br><br>
The image will "look" shifted if you look at them with<br>a viewer that doesn't take into account the coordinates<br>of the image origin.<br><br>You may want to look at them with:<br><br>A) Paraview (<a href="http://www.paraview.org">www.paraview.org</a>), and/or<br>
B) Slicer (<a href="http://www.slicer.org">www.slicer.org</a>)<br><br><br> Regards,<br><br><br> Luis<br><br><br>-------------------------------------------------------------------------------------<br><div class="gmail_quote">
On Fri, Apr 16, 2010 at 2:59 AM, Lars Friedrich Lars <span dir="ltr"><<a href="mailto:lars-friedrich@gmx.net">lars-friedrich@gmx.net</a>></span> wrote:<br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
Hello,<br>
<br>
when using itk::MultiResolutionPyramidImageFilter for shrinking a 2D image over 3 levels (schedule 4 4 / 2 2 / 1 1) I recognized that the resultant output origins of the images in level 0 (4 4) and level 1 (2 2) seem to be 'incorrect'.<br>
<br>
original image (=level 2):<br>
origin=+205,-205 dimension=410,410 spacing=1,1 direction=1 0 0 1<br>
<br>
--> pyramid output:<br>
level 0:<br>
ORIGIN=+206.5,-203.5 dimension=102,102 spacing=4,4 direction=1 0 0 1<br>
level 1:<br>
ORIGIN=+205.5,-204.5 dimension=205,205 spacing=2,2 direction=1 0 0 1<br>
<br>
To my mind the outputs at levels 0 and 1 seem to be incorrectly shifted by 0.5*originalspacing.<br>
--> desired output origins:<br>
level 0:<br>
ORIGIN=+206,-204<br>
level 1:<br>
ORIGIN=+205,-205<br>
<br>
When I compare the output images from level 0 and 1 to the original image, I can see the shift visually. Manually changing the output to the desired values 'fixes' the problem, and the overlay of the compared images shows a better visual agreement.<br>
<br>
This is the code fragment from itkMultiResolutionPyramidImageFilter.txx which is responsible for the shift:<br>
//Now compute the new shifted origin for the updated levels;<br>
const typename OutputImageType::PointType::VectorType outputOriginOffset<br>
=(inputDirection*(outputSpacing-inputSpacing))*0.5;<br>
<br>
<br>
Is this a bug or a feature? Does the shift account for smoothing-effects of the internally used discrete gaussian smoothing filter or shrinking-effects of the shrink image filter?<br>
<br>
Thank you!<br>
<br>
regards,<br>
<br>
lars<br>
<br>
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