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## Copyright (C) 2024 David Legland
## All rights reserved.
##
## Redistribution and use in source and binary forms, with or without
## modification, are permitted provided that the following conditions are met:
##
## 1 Redistributions of source code must retain the above copyright notice,
## this list of conditions and the following disclaimer.
## 2 Redistributions in binary form must reproduce the above copyright
## notice, this list of conditions and the following disclaimer in the
## documentation and/or other materials provided with the distribution.
##
## THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS ''AS IS''
## AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
## IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
## ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
## ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
## DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
## SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
## CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
## OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
## OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
##
## The views and conclusions contained in the software and documentation are
## those of the authors and should not be interpreted as representing official
## policies, either expressed or implied, of the copyright holders.
function [axis, theta] = rotation3dAxisAndAngle(mat)
%ROTATION3DAXISANDANGLE Determine axis and angle of a 3D rotation matrix.
%
% [AXIS, ANGLE] = rotation3dAxisAndAngle(MAT)
% Where MAT is a 4-by-4 matrix representing a rotation, computes the
% rotation axis (containing the points that remain invariant under the
% rotation), and the rotation angle around that axis.
% AXIS has the format [DX DY DZ], constrained to unity, and ANGLE is the
% rotation angle in radians.
%
% Note: this method use eigen vector extraction. It would be more precise
% to use quaternions, see:
% http://www.mathworks.cn/matlabcentral/newsreader/view_thread/160945
%
%
% Example
% origin = [1 2 3];
% direction = [4 5 6];
% line = [origin direction];
% angle = pi/3;
% rot = createRotation3dLineAngle(line, angle);
% [axis angle2] = rotation3dAxisAndAngle(rot);
% angle2
% angle2 =
% 1.0472
%
% See also
% transforms3d, vectors3d, angles3d, eulerAnglesToRotation3d
%
% ------
% Author: David Legland
% E-mail: david.legland@inrae.fr
% Created: 2010-08-11, using Matlab 7.9.0.529 (R2009b)
% Copyright 2010-2023 INRA - Cepia Software Platform
% extract the linear part of the rotation matrix
A = mat(1:3, 1:3);
% extract eigen values and eigen vectors
[V, D] = eig(A - eye(3));
% we need the eigen vector corresponding to eigenvalue==1
[dummy, ind] = min(abs(diag(D)-1)); %#ok<ASGLU>
% extract corresponding eigen vector
vector = V(:, ind)';
% compute rotation angle
t = [A(3,2)-A(2,3) , A(1,3)-A(3,1) , A(2,1)-A(1,2)];
theta = atan2(dot(t, vector), trace(A)-1);
% If angle is negative, invert both angle and vector direction
if theta<0
theta = -theta;
vector = -vector;
end
% try to get a point on the line
% seems to work, but not sure about stability
[V, D] = eig(mat-eye(4)); %#ok<ASGLU>
origin = V(1:3,4)'/V(4, 4);
% create line corresponding to rotation axis
axis = [origin vector];
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