[Insight-users] grid spacing with BSplineDeformableRegistration

[Kristina Grunewald]

Hi Luis,

thanks for your reply.

I thought the first solution was better because when doing it the other
way, the grid image doesn't overlap the fixed image.

If we consider the following case:
fixed image size = 44
fixed image spacing = 4
fixed image origin = 0

number of grid nodes within the image support = 10
number of grid nodes
   = 13 (after having added the 3 due to spline order 3)

Then the first solution leads to
a)
grid spacing = 19
grid origin = -19
The grid image then ranges from its origin to
(-19 + (numberOfGridNodes -1) * spacing) = 209

and the second to:
b)
grid spacing = 16
grid origin = -16
The grid image then ranges from its origin to
(-16 + (numberOfGridNodes -1) * spacing) = 176

The fixed image ranges from 0 to 44*4 = 176

So in b) the spatial extent of the grid image doesn't exceed the spatial
extent of the fixed image, as we intended it to.


This leads me to another question:
what number is to be added to the number of grid nodes within the image
support when using a bspline order different from 3?


Thanks a lot,
Kristina




Luis Ibanez wrote:

>
> Hi Kristina,
>
>
> Thanks for pointing this out.
>
> I would rather suggest you to use the second solution,
> since the purpose of the spacing computation is not
> to end up with an integer spacing in the BSpline grid,
> but to make sure that the spacing of the BSpline grid
> is an integer multiple of the image spacing.
>
> We should fix the computation in the BSplineWarping1.cxx
> example, since it will work badly if somebody uses an
> input image with pixel spacing = 0.001, and a BSpline
> grid that has 1 image pixels per grid square. (e.g. the
> grid spacing in that case will be wrongly computed
> as == zero ).
>
>
> Please let us know if you have further questions,
>
>
>
>    Thanks
>
>
>       Luis
>
>
>
> --------------------------
> Kristina Grunewald wrote:
>
>> Hello,
>>
>> I found two different ways how to compute the spacing of a bspline 
>> grid transform in the existing code.
>> In Examples/Registration/BSplineWarping1.cxx it is done like that:
>>
>> spacing[x] = floor( fixedSpacing[x] * (fixedImageSize[x] - 1) / 
>> (gridSizeOnImage[x] - 1) );
>>
>> Whereas in Examples/Registration/DeformableRegistration4.cxx like:
>>
>> spacing[x] = fixedSpacing[x];
>> spacing[x] *= floor( static_cast<double>(fixedImageSize[x] - 1)  /
>>      static_cast<double>(gridSizeOnImage[x] - 1) );
>>
>> The rounding in the first case is done after having multiplied with 
>> the fixed image spacing, in the latter before.
>> This may result in different values, set fixedImageSize[x] = 44, 
>> fixedSpacing[x] = 4, and gridSizeOnImage[x] = 10.
>>
>> I prefer the first solution but I am not quite sure whether this is 
>> correct.
>>
>> Thanks,
>> Kristina
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>>
>>
>
>
>
>