rq3.m

% RQ3   RQ decomposition of 3x3 matrix
%
% Usage: [R,Q] = rq3(A)
%
% Argument:  A - 3 x 3 matrix
% Returns:   R - Upper triangular 3 x 3 matrix
%            Q - 3 x 3 orthonormal rotation matrix
%    Such that  R*Q = A
%
% The signs of the rows and columns of R and Q are chosen so that the diagonal
% elements of R are +ve.
%
% See also: DECOMPOSECAMERA

% Follows algorithm given by Hartley and Zisserman 2nd Ed. A4.1 p 579

% Copyright (c) 2010 Peter Kovesi
% Centre for Exploration Targeting
% School of Earth and Environment
% The University of Western Australia
% peter.kovesi at uwa edu au
%
% Permission is hereby granted, free of charge, to any person obtaining a copy
% of this software and associated documentation files (the "Software"), to deal
% in the Software without restriction, subject to the following conditions:
% 
% The above copyright notice and this permission notice shall be included in 
% all copies or substantial portions of the Software.
%
% October  2010
% February 2014  Incorporated modifications suggested by Mathias Rothermel to
%                avoid potential division by zero problems

function [R,Q] = rq3(A)
    
    if ~all(size(A)==[3 3])
        error('A must be 3x3');
    end
    
    eps = 1e-10;
    
    % Find rotation Qx to set A(3,2) to 0
    A(3,3) = A(3,3) + eps;
    c = -A(3,3)/sqrt(A(3,3)^2+A(3,2)^2);
    s =  A(3,2)/sqrt(A(3,3)^2+A(3,2)^2);
    Qx = [1 0 0; 0 c -s; 0 s c];
    R = A*Qx;
    
    % Find rotation Qy to set A(3,1) to 0
    R(3,3) = R(3,3) + eps;
    c = R(3,3)/sqrt(R(3,3)^2+R(3,1)^2);
    s = R(3,1)/sqrt(R(3,3)^2+R(3,1)^2);
    Qy = [c 0 s; 0 1 0;-s 0 c];
    R = R*Qy;
    
    % Find rotation Qz to set A(2,1) to 0    
    R(2,2) = R(2,2) + eps;
    c = -R(2,2)/sqrt(R(2,2)^2+R(2,1)^2);
    s =  R(2,1)/sqrt(R(2,2)^2+R(2,1)^2);
    Qz = [c -s 0; s c 0; 0 0 1];
    R = R*Qz;
    
    Q = Qz'*Qy'*Qx';
    
    % Adjust R and Q so that the diagonal elements of R are +ve
    for n = 1:3
        if R(n,n) < 0
            R(:,n) = -R(:,n);
            Q(n,:) = -Q(n,:);
        end
    end