<div dir="ltr">Hi Matt,<div><br></div><div style>Thanks a lot for this explanation :)</div><div style><br></div><div style>AtB</div><div style><br></div><div style>Anja</div><div style><br></div><div class="gmail_extra"><br>
<br><div class="gmail_quote">On 10 April 2013 20:28, Matt McCormick <span dir="ltr"><<a href="mailto:matt.mccormick@kitware.com" target="_blank">matt.mccormick@kitware.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi Anja,<br>
<br>
Yes, sampling and the grid play an important role.<br>
<br>
The sampling locations of the output must be chosen, and they are not<br>
necessarily the sampling locations of the input.<br>
<br>
This is what the ResampleImageFilter and its Transform may operator in<br>
the "opposite" direction what is expected -- what is always computed<br>
is the output pixel location.<br>
<br>
HTH,<br>
Matt<br>
<div class="HOEnZb"><div class="h5"><br>
On Wed, Apr 10, 2013 at 7:17 PM, Anja Ende <<a href="mailto:anja.ende@googlemail.com">anja.ende@googlemail.com</a>> wrote:<br>
> Oh yes, of course! That is a subtle point.<br>
><br>
> May I ask a follow up question then? In inverting a deformation field then,<br>
> how do we decide the sampling of this grid of the inverse field?<br>
><br>
> Say our original deformation field has a displacement at every voxel and<br>
> these displacement vectors can point to any real location. So, is the<br>
> inverse deformation field going to be calculated at these real locations?<br>
> However, we need to have a uniform grid and this is not guaranteed by this<br>
> scheme now. Can someone explain this to me on how this is done?<br>
><br>
> Sorry for asking these naive questions. I could use the ITK filters out of<br>
> the box but I would like to know some of these basics.<br>
><br>
> Thanks,<br>
> Anja<br>
><br>
><br>
> On 10 April 2013 20:06, Matt McCormick <<a href="mailto:matt.mccormick@kitware.com">matt.mccormick@kitware.com</a>> wrote:<br>
>><br>
>> Hi Anja,<br>
>><br>
>> If displacement vector field gives the displacement at location x, but<br>
>> the negative of the displacement at location x is not the inverse;<br>
>> what is required is negative of the displacement vector at x + \delta<br>
>> x.<br>
>><br>
>> HTH,<br>
>> Matt<br>
>><br>
>> On Wed, Apr 10, 2013 at 5:47 PM, Anja Ende <<a href="mailto:anja.ende@googlemail.com">anja.ende@googlemail.com</a>><br>
>> wrote:<br>
>> > Hello all,<br>
>> ><br>
>> > This is a very naive question and I would be very grateful if someone<br>
>> > can<br>
>> > clarify this doubt for me.<br>
>> ><br>
>> > I see that there are quite a few filters and some very sophisticated<br>
>> > methods<br>
>> > for inverting deformation fields.<br>
>> ><br>
>> > If I have a dense deformation field, why is it not enough to simply<br>
>> > negate<br>
>> > the displacement vector to compute the inverse? I am sure there is a<br>
>> > simple,<br>
>> > logical explanation for this but I am having trouble figuring it out.<br>
>> ><br>
>> > Thanks a lot for your help!<br>
>> ><br>
>> > Anja<br>
>> ><br>
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></div></div></blockquote></div></div></div>