% DECOMPOSECAMERA Decomposition of a camera projection matrix
%
% Usage: [K, Rc_w, Pc, pp, pv] = decomposecamera(P);
%
% P is decomposed into the form P = K*[R -R*Pc]
%
% Argument: P - 3 x 4 camera projection matrix
% Returns:
% K - Calibration matrix of the form
% | ax s ppx |
% | 0 ay ppy |
% | 0 0 1 |
%
% Where:
% ax = f/pixel_width and ay = f/pixel_height,
% ppx and ppy define the principal point in pixels,
% s is the camera skew.
% Rc_w - 3 x 3 rotation matrix defining the world coordinate frame
% in terms of the camera frame. Columns of R transposed define
% the directions of the camera X, Y and Z axes in world
% coordinates.
% Pc - Camera centre position in world coordinates.
% pp - Image principal point.
% pv - Principal vector from the camera centre C through pp
% pointing out from the camera. This may not be the same as
% R'(:,3) if the principal point is not at the centre of the
% image, but it should be similar.
%
% See also: RQ3
% Reference: Hartley and Zisserman 2nd Ed. pp 155-164
% 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 Original version
% November 2013 Description of rotation matrix R corrected (transposed)
function [K, Rc_w, Pc, pp, pv] = decomposecamera(P)
% Projection matrix from Hartley and Zisserman p 163 used for testing
if ~exist('P','var')
P = [ 3.53553e+2 3.39645e+2 2.77744e+2 -1.44946e+6
-1.03528e+2 2.33212e+1 4.59607e+2 -6.32525e+5
7.07107e-1 -3.53553e-1 6.12372e-1 -9.18559e+2];
end
% Convenience variables for the columns of P
p1 = P(:,1);
p2 = P(:,2);
p3 = P(:,3);
p4 = P(:,4);
M = [p1 p2 p3];
m3 = M(3,:)';
% Camera centre, analytic solution
X = det([p2 p3 p4]);
Y = -det([p1 p3 p4]);
Z = det([p1 p2 p4]);
T = -det([p1 p2 p3]);
Pc = [X;Y;Z;T];
Pc = Pc/Pc(4);
Pc = Pc(1:3); % Make inhomogeneous
% Pc = null(P,'r'); % numerical way of computing C
% Principal point
pp = M*m3;
pp = pp/pp(3);
pp = pp(1:2); % Make inhomogeneous
% Principal ray pointing out of camera
pv = det(M)*m3;
pv = pv/norm(pv);
% Perform RQ decomposition of M matrix. Note that rq3 returns K with +ve
% diagonal elements, as required for the calibration matrix.
[K Rc_w] = rq3(M);
% Check that R is right handed, if not give warning
if dot(cross(Rc_w(:,1), Rc_w(:,2)), Rc_w(:,3)) < 0
warning('Note that rotation matrix is left handed');
end