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### Highlights from Kabsch algorithm

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# Kabsch algorithm

### Ehud Schreiber (view profile)

05 Nov 2009 (Updated )

Find the rigid transformation & Least Root Mean Square distance between two paired sets of points

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Description

% Find the Least Root Mean Square between two sets of N points in D dimensions
% and the rigid transformation (i.e. translation and rotation)
% to employ in order to bring one set that close to the other,
% Using the Kabsch (1976) algorithm.
% Note that the points are paired, i.e. we know which point in one set
% should be compared to a given point in the other set.
%
% References:
% 1) Kabsch W. A solution for the best rotation to relate two sets of vectors. Acta Cryst A 1976;32:9223.
% 2) Kabsch W. A discussion of the solution for the best rotation to relate two sets of vectors. Acta Cryst A 1978;34:8278.
% 3) http://cnx.org/content/m11608/latest/
% 4) http://en.wikipedia.org/wiki/Kabsch_algorithm
%
% We slightly generalize, allowing weights given to the points.
% Those weights are determined a priori and do not depend on the distances.
%
% We work in the convention that points are column vectors;
% some use the convention where they are row vectors instead.
%
% Input variables:
% P : a D*N matrix where P(a,i) is the a-th coordinate of the i-th point
% in the 1st representation
% Q : a D*N matrix where Q(a,i) is the a-th coordinate of the i-th point
% in the 2nd representation
% m : (Optional) a row vector of length N giving the weights, i.e. m(i) is
% the weight to be assigned to the deviation of the i-th point.
% If not supplied, we take by default the unweighted (or equal weighted)
% m(i) = 1/N.
% The weights do not have to be normalized;
% we divide by the sum to ensure sum_{i=1}^N m(i) = 1.
% The weights must be non-negative with at least one positive entry.
% Output variables:
% U : a proper orthogonal D*D matrix, representing the rotation
% r : a D-dimensional column vector, representing the translation
% lrms: the Least Root Mean Square
%
% Details:
% If p_i, q_i are the i-th point (as a D-dimensional column vector)
% in the two representations, i.e. p_i = P(:,i) etc., and for
% p_i' = U p_i + r (' does not stand for transpose!)
% we have p_i' ~ q_i, that is,
% lrms = sqrt(sum_{i=1}^N m(i) (p_i' - q_i)^2)
% is the minimal rms when going over the possible U and r.
% (assuming the weights are already normalized).
%

MATLAB release MATLAB 7.6 (R2008a)
05 Jul 2016 Justin Michael

### Justin Michael (view profile)

08 Jul 2013 Daniel

### Daniel (view profile)

For large N most of the time is spent in the two for loops.
To remove them bsxfun is useful. Replace the lines in Kabsch.m:
------------------------------------
Pdm = zeros(D,N) ;
for i=1:N
Pdm(:,i) = m(i)*P(:,i) ;
end
C = Pdm*Q' ;
------------------------------------
by
------------------------------------
C = bsxfun(@times,m,P)*Q';
------------------------------------
and the lines
------------------------------------
lrms = 0 ;
for i=1:N
lrms = lrms + m(i)*Diff(:,i)'*Diff(:,i) ;
end
lrms = sqrt(lrms) ;
------------------------------------
by
------------------------------------
lrms = sqrt(sum(sum(bsxfun(@times,m,Diff).*Diff))) ;
------------------------------------
and the execution speed will be increased by ~70 for N=3'000'000

22 Oct 2012 Andreas Martin

### Andreas Martin (view profile)

I found a little mistake that results in numerical jitter under some circumstances.
If you change the line:

"if (det(C) < 0)" into "if (det(W*V') < 0)"

all works fine.

Cheers, Andreas

25 Oct 2012 1.17

24/10/2012 : corrected the check of whether a reflection is needed from
if (det(C) < 0)
to the more numerically stable
if (det(V*W') < 0)
as suggested by Andreas.

09 Jul 2013 1.18

Replaced loops by bsxfun() for efficiency,
as suggested by Daniel Pfenniger (thanks!).