function VALS = solve26( L, option_flag,N)
% function VALS = solve26( L, option_flag,N)
%
% Code for solving the generalized camera relative pose problem 
% for 2 stations and 6 observed points. 
%
% L is a cell structure with two elements each being a 6x6 matrix. 
%   The matrices describres (in plucker coordinates) the 
%   observation at corresponding time. Ech observation is a column. 
%
% option_flag controls how the elimination is performed. 
%  
% N is usually set to 3. 
%
% VALS is a 3x64 matrix where each column is the 3 first components of 
%  a quaternion parameterizing this rotation. One way to work with these 
%  is to download the "Quaternion Toolbox" from Mathworks homepage 
%  (search for dcm2q if you are unable to find the toolbox. 
%
%
%% TEST AND EXAMPLE CODE: 
%%
%load solve26_testvec
%N= length( Lvec);
%% N=60;
%errvec = zeros(1,N);
%tic
%for i=1:N
%      VALS = solve26( Lvec{i}, 0, 3 );
%      [err1,I] = min(abs( VALS(1,:) - Qtrue_vec{i}(1) ));
%      errvec(i)= err1;
%      errvec2(i)= norm( Qtrue_vec{i} - VALS(:,I));
%end
%fprintf('Time for solving %i instances was:', N); 
%toc 
%
%
%
%
%Please refer to: 
%
%@INPROCEEDINGS{stewenius-etal-omnivis-2005,
%  AUTHOR = {H. Stew\'enius and  D. Nist\'er and  M. Oskarsson and  K. {\AA}str{\"o}m},
%  TITLE = {Solutions to Minimal Generalized Relative Pose Problems},
%  BOOKTITLE = {Workshop on Omnidirectional Vision},
%  YEAR = 2005,
%  MONTH = OCT,
%  ADDRESS = {Beijing China},
%  PDF = {http://www.vis.uky.edu/~stewe/publications/stewenius_05_omnivis_sm26gen.pdf}
%}
%
%and/or 
%
%@PHDTHESIS{stewenius-pdh-2005,
%     AUTHOR = {H. Stew\'enius},
%     TITLE = {Gr{\"o}bner Basis Methods for Minimal Problems in Computer Vision},
%     SCHOOL = {Lund University},
%     YEAR = 2005,
%     MONTH = APR,
%     PDF = {http://www.maths.lth.se/matematiklth/personal/stewe/THESIS/stewenius_phd_2005.pdf}
%}
%
%
% If you use this program




%
% 
%% Henrik Stewenius 200407?? 
%%
%%
%%
%%
%%


if( not((exist('build_eq2')==3) && (exist('first_row')==3) && (exist('initM')==3) && (exist('last_gb')==3) && (exist('mult_w_all')==3) ))
    fprintf(['On or more of the neded mexfiles are unavailable.\n'...
             'Please compile all needed files and then try again\n'...
             'Suggested way to do this in matlab (on Linux) is:\n'...
     '   mex -DMAIN_BUILD_EQ2 solve26.c -output  build_eq2\n'...
'   mex -DMAIN_MULT_WITH_ALL solve26.c  -output  mult_w_all\n'...
'   mex -DMAIN_INITM solve26.c -output  initM\n'...
'   mex -DMAIN_LASTGB solve26.c -output  last_gb\n'...
'   mex -DMAIN_FIRST_ROW solve26.c -output  first_row\n']);

if( not(exist('solve26.c')))
    error('File solve26.c is missing. You will not be able to compile'); 
end 

if(not(exist('solve26AFILE.c')  ))
    error('File solve26AFILE.c is missing. You will not be able to compile');    
end 
    error('Unable to Run'); 
end 
A = build_eq2(L,N);
if option_flag
   B =  build_modmult2( A);
   U =B(1:56,:);
else
   [L,U] = lu(A ) ;
   U = U(1:15,1:15)\U(1:15,:); 
   B =  mult_w_all( U);
   [L,U] = lu( B ) ;
   U = U(1:56,1:56)\U(1:56,:);
end
M= initM;
last_gb_restrow = last_gb( U);
restrow = first_row( U(1:56,:) , last_gb_restrow );
restrow = restrow( 57:end);
M(1,:)= restrow;
M(2:26,:)=-U([25 27 28 29 31 32 34 36 37 38 39 40 41 43 44 45 46 47 49 ...
	      50 51 52 54 55 56], 57:end);
   
[V,D] = eig( M ) ;
VALS = V( 61:63,:) ./(ones(3,1)*V(end,:));