[Elastix] Coordinates of control points in B-spline transform

Coert Metz coert.metz at gmail.com
Wed Apr 15 22:15:33 CEST 2015


Dear Ramón,

The code that computes the grid origin and spacing is (if I am not
mistaking) in src/Common/Transforms/itkGridScheduleComputer.hxx of the
elastix sources. Also the TransformParameters.*.txt file should list the
GridSpacing, GridSize, GridIndex and GridOrigin from which you can deduce
this information. All points are equidistant for sure. If I remember
correctly the ordering of the parameters is indeed as you mention.

Regards,
Coert Metz

On Wed, Apr 15, 2015 at 7:56 PM, Ramón Casero Cañas <rcasero at gmail.com>
wrote:

> Dear all,
>
> I have a couple of questions about B-splines in elastix:
>
> 1) What are the coordinates of the control points in a B-spline transform?
>
> After registering an image to itself, the t.TransformParameters are all
> zero, as expected for no deformation.
>
> But how's the grid determined, e.g. for a cubic spline?
>
> Let's say e.g. that we have a 1D grid with 5 control points, plus 1 extra
> point before the grid, plus 2 extra points after the grid, in each
> dimension.
>
> Are the coordinates of the 5 control points determined by placing one at
> the centre of the first pixel, and another at the centre of the last pixel,
> and then the other 3 equidistant between those?
>
> 2) Are then the extra ones outside the grid, and equidistant too?
>
> 3) Is the convention for the TransformParameters [x1 x2 x3...xN y1 y2 ...
> yN]?
>
>
> Best regards,
>
> Ramon.
>
> --
> Dr. Ramón Casero Cañas
>
> Institute of Biomedical Engineering
> Department of Engineering Science, University of Oxford
> Old Road Campus Research Building, Headington
> Oxford
> OX3 7DQ
> UK
>
> tlf         +44 (0) 1865 617716
> twitter   @Ramon_Casero
> web      http://www.cs.ox.ac.uk/people/Ramon.CaseroCanas/
>
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