[Insight-users] Best optimizer for landmark registration with only 3 points

[Charlotte Curtis]

I'd like to use landmark registration to get a rough first guess prior to
image registration.  When I run IterativeClosestPoint2 with 4 landmarks, it
complains about the number of landmarks:

vnl_least_squares_function: WARNING: unknowns(6) > residuals(4)
vnl_levenberg_marquardt: Number of unknowns(6) greater than number of data
(4)
Solution = [0, 0, 0, 0, 0, 0]

I'm just wondering if there's an optimizer that would let me get away with a
minimum of three landmarks.  Since 3 points would define a unique location
in 3D space, it seems to me that there shouldn't need to be 6.  Indeed, if I
simply duplicate all of my landmarks and fool it into thinking it has 8
unique points, it comes up with a reasonable solution.  This is my backup
plan if nothing else works, but it seems a little messy.  Any suggestions?
Thanks,

Charlotte
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