Bline_length = B_parameters(1) * 100 * [top_length, top_length, top_length, top_length, top_length, bottom_length, bottom_length, bottom_length, bottom_length, bottom_length, top_length, top_length, top_length, top_length, bottom_length, bottom_length, bottom_length, bottom_length ];
            
        otherwise
           spline_pos = [ -1 1 1; 2 0 -1; 2 2 0; 0 2 1; -2 2 2; -2 0 1; -2 -2 0; 0 -2 -1; 2 -2 -2; 1 1 -1; -2 0 -1; -2 2 0; 0 -2 1; -2 2 2; 2 0 1; 2 -2 0; 0 2 -1; 2 -2 -2 ]; % starting point of splines, which represent field lines    
           %spline_pos = [ 2.6,0,0;094,0,0.034202;0766,0,0.064279;05,0,0.086603;0174,0,0.098481;2.4826,0,0.098481;2.45,0,0.086603;2.4234,0,0.064279;2.406,0,0.034202;2.4,0,1.2246e-017;2.406,0,-0.034202;2.4234,0,-0.064279;2.45,0,-0.086603;2.4826,0,-0.098481;0174,0,-0.098481;05,0,-0.086603;0766,0,-0.064279;094,0,-0.034202];
           Bline_length = 0.2*[100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 100 ];
    end
    
    if get( handles.popMagField, 'Value' ) > 1    % if any of magnetic field is chosen
        for i = 1:length( spline_pos )
            if i <= length( spline_pos )/2     % second half of spline_pos point are the same like first half, but they are used to generate spline in other directions
                direction = 1;
            else
                direction = -1;
            end
            Bfield_lines(i) = struct( 'spline', Calculate_spline( spline_pos(i,1:3),handles, direction, BFIELD_LINE, i, Bline_length(i) ) );
        end
    else                                     % if no magnetic field is chosen
        for i = 1:length( spline_pos )
            Bfield_lines(i) = struct( 'spline', []);
        end
    end;
    
    % initialize electric field lines; the same like magnetic field
    
    %spline_pos = [ 0 0 0.1; 2 0 -2; 2 2 0; 0 2 2; -2 2 3; -2 0 2; -2 -2 0; 0 -2 -2; 2 -2 -3; 0 0 0.1; 2 0 -2; 2 2 0; 0 2 2; -2 2 3; -2 0 2; -2 -2 0; 0 -2 -2; 2 -2 -3 ];
    spline_pos = [ 0 0 0; 0 1 0; 0 1 1; 0 0 1; 0 -1 1; 0 -1 0; 0 -1 -1; 0 0 -1; 0 1 -1; 0 2 0; 0 2 2; 0 0 2; 0 -2 2; 0 -2 0; 0 -2 -2; 0 0 -2; 0 2 -2; 0 0 3 ]; % starting point of splines, which represent field lines 
    %spline_pos = [ -50 -100 -10.1; -40 -100 -10; -40 -101 -10; -50 -101 -10; -60 -101 -10; -60 -100 -10; -60 -100 -10; -50 -100 -10; -40 -100 -10; -50 -100 -10; -40 -100 -10; -40 -101 -10; -50 -101 -10; -60 -101 -10; -60 -100 -10; -60 -101 -10; -50 -100 -10; -40 -100 -10 ]; % starting point of splines, which represent field lines  
    Eline_length = [12640, 12640, 12640, 12640, 12640, 12640, 12640, 12640, 12640, 12640, 12640, 12640, 12640, 12640, 12640, 12640, 12640, 12640 ];

    if get( handles.popElecField, 'Value' ) > 1      % if any of electric field is chosen  
        for i = 1:length( spline_pos )
            if i <= length(spline_pos)/2             % second half of spline_pos point are the same like first half, but they are used to generate spline in other directions
                direction = 1;
            else
                direction = -1;
            end
            Efield_lines(i) = struct( 'spline', Calculate_spline( spline_pos(i,1:3),handles, direction, EFIELD_LINE, i, Eline_length(i) ) );
        end
    else                                            % if no electric field is chosen
        for i = 1:length( spline_pos )
            Efield_lines(i) = struct( 'spline', [] );
        end
    end;
end

% ******** calculates spline ***********************************************
function spline = Calculate_spline( position, handles, direction, field, line_num, line_length )
 % calculates one spline of magnetic or electric field. the parameter
 % direction is not in use atm
    global_variables;
    absolute_time = -1;
    
    switch field
        case EFIELD_LINE
            [l,spline] = ode15s( @calculate_E,[0 line_length],position );
            
        case BFIELD_LINE
            [l,spline] = ode15s( @calculate_B,[0 line_length],position );
    end
    
end

% *********** sets virtual reality world magnetic and electric field lines
function w = make_vr(simfile, Bfield_lines, Efield_lines);

    w = vrworld(simfile);  % updating virtual world
    open(w);
    temp = get(w,'Figures');
    if  length(temp) == 0
        view(w);
        set(w,'TimeSource','external');
        %view(w, '-web');
    end
    
    alpha = [0: pi./10 : 2*pi]';                        
    cross = 0.01*[cos(alpha) -sin(alpha)];

    for i = 1:length(Bfield_lines)              % updating magnetic field lines
        if i <= length(Bfield_lines)/2 
            tmp_num = i;
        else
            tmp_num = i+1;
        end
        name_line = strcat('B', num2str(tmp_num), '_spine');
        line = vrnode(w, name_line);
        if length(Bfield_lines(i).spline) > 0
            %temp1 = field_lines(i).spline;
            line.spine = Bfield_lines(i).spline;
        else
            line.spine = [];
        end;
        line.crossSection = cross;                        
        % TODO: crossSection of a magnetic field line could represent the
        % strength of the magnetic field in the particular area
        line.solid = true;            
    end;
    
    
     for i = 1:length(Efield_lines)              % updating electric field lines
        if i <= length(Efield_lines)/2 
            tmp_num = i;
        else
            tmp_num = i+1;
        end
        name_line = strcat('E', num2str(tmp_num), '_spine');
        line = vrnode(w, name_line);
        if length(Efield_lines(i).spline) > 0
            %temp1 = field_lines(i).spline;
            line.spine = Efield_lines(i).spline;
        else
            line.spine = [];
        end;
        line.crossSection = cross;                        
        % TODO: crossSection of a electric field line could represent the
        % strength of the electric field in the particular area
        line.solid = true;            
    end;
    
    w.trajectory_spine.spine = [0 0 0; 0 0 0];
    w.trajectory_spine.crossSection = cross;
    
    w.guide_spine.spine = [0 0 0; 0 0 0];
    w.guide_spine.crossSection = cross;
    
    save(w, get(w, 'FileName'));

    vrdrawnow;

end