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

Ramón Casero Cañas rcasero at gmail.com
Wed Apr 15 19:56:53 CEST 2015

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

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 ...

Best regards,


Dr. Ramón Casero Cañas

Institute of Biomedical Engineering
Department of Engineering Science, University of Oxford
Old Road Campus Research Building, Headington

tlf         +44 (0) 1865 617716
twitter   @Ramon_Casero
web      http://www.cs.ox.ac.uk/people/Ramon.CaseroCanas/
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