function [headmodel, sens] = ft_prepare_vol_sens(headmodel, sens, varargin)

% FT_PREPARE_VOL_SENS does some bookkeeping to ensure that the volume conductor model
% and the sensor array are ready for subsequent forward leadfield computations and
% takes care of some pre-computations to make the calculations more efficient.
%
% Use as
%   [headmodel, sens] = ft_prepare_vol_sens(headmodel, sens, ...)
% with input arguments
%   headmodel = structure with volume conductor definition
%   sens      = structure with gradiometer or electrode definition
%
% The headmodel structure represents a volume conductor model of the head,
% its contents depend on the type of model. It is described in more detail
% in FT_DATATYPE_HEADMODEL. The sens structure represents a electrode or
% gradiometer array. It is described in more detail in FT_DATATYPE_SENS.
%
% Additional options should be specified in key-value pairs and can be
%   'channel'  = cell-array with strings (default = 'all')
%
% The detailed behavior of this function depends on whether the input
% consists of EEG or MEG and furthermoree depends on the type of volume
% conductor model:
% - in case of EEG single and concentric sphere models, the electrodes are
%   projected onto the skin surface.
% - in case of EEG boundary element models, the electrodes are projected on
%   the surface and a blilinear interpoaltion matrix from vertices to
%   electrodes is computed.
% - in case of MEG and a localspheres model, a local sphere is determined
%   for each coil in the gradiometer definition.
%  - in case of MEG with a singleshell Nolte model, the volume conduction
%    model is initialized
% In any case channel selection and reordering will be done. The channel
% order returned by this function corresponds to the order in the 'channel'
% option, or if not specified, to the order in the input sensor array.
%
% See also FT_COMPUTE_LEADFIELD, FT_READ_HEADMODEL, FT_READ_SENS

% Copyright (C) 2004-2020, Robert Oostenveld
%
% This file is part of FieldTrip, see http://www.fieldtriptoolbox.org
% for the documentation and details.
%
%    FieldTrip is free software: you can redistribute it and/or modify
%    it under the terms of the GNU General Public License as published by
%    the Free Software Foundation, either version 3 of the License, or
%    (at your option) any later version.
%
%    FieldTrip is distributed in the hope that it will be useful,
%    but WITHOUT ANY WARRANTY; without even the implied warranty of
%    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
%    GNU General Public License for more details.
%
%    You should have received a copy of the GNU General Public License
%    along with FieldTrip. If not, see <http://www.gnu.org/licenses/>.
%
% $Id$

if iscell(headmodel) && iscell(sens)
  % this represents combined EEG, ECoG and/or MEG
  for i=1:numel(headmodel)
    [headmodel{i}, sens{i}] = ft_prepare_vol_sens(headmodel{i}, sens{i}, varargin{:});
  end
  return
end

% get the optional input arguments
% fileformat = ft_getopt(varargin, 'fileformat');
channel = ft_getopt(varargin, 'channel', sens.label);   % cell-array with channel labels, default is all

% ensure that the sensor description is up-to-date (Aug 2011)
sens = ft_datatype_sens(sens);

% this is to support volumes saved in mat-files, particularly interpolated
if ischar(headmodel)
  vpath     = fileparts(headmodel);   % remember the path to the file
  headmodel = ft_read_headmodel(headmodel); % replace the filename with the content of the file
end

% ensure that the volume conduction description is up-to-date (Jul 2012)
headmodel = ft_datatype_headmodel(headmodel);

% determine whether the input contains EEG or MEG sensors
iseeg = ft_senstype(sens, 'eeg');
ismeg = ft_senstype(sens, 'meg');

% determine the skin compartment
if ~isfield(headmodel, 'skin_surface')
  if isfield(headmodel, 'bnd')
    headmodel.skin_surface   = find_outermost_boundary(headmodel.bnd);
  elseif isfield(headmodel, 'r') && length(headmodel.r)<=4
    [dum, headmodel.skin_surface] = max(headmodel.r);
  end
end

% determine the inner_skull_surface compartment
if ~isfield(headmodel, 'inner_skull_surface')
  if isfield(headmodel, 'bnd')
    headmodel.inner_skull_surface  = find_innermost_boundary(headmodel.bnd);
  elseif isfield(headmodel, 'r') && length(headmodel.r)<=4
    [dum, headmodel.inner_skull_surface] = min(headmodel.r);
  end
end

% FT_HEADMODELTYPE to an empty struct won't work further down
if isempty(headmodel)
  headmodel = [];
end

% this makes them easier to recognise
sens.type       = ft_senstype(sens);
headmodel.type  = ft_headmodeltype(headmodel);

if isfield(headmodel, 'unit') && isfield(sens, 'unit') && ~strcmp(headmodel.unit, sens.unit)
  ft_error('inconsistency in the units of the volume conductor and the sensor array');
end

if ismeg && iseeg
  % this is something that could be implemented relatively easily
  ft_error('simultaneous EEG and MEG not yet supported');

elseif ~ismeg && ~iseeg
  ft_error('the input does not look like EEG, nor like MEG');

elseif ismeg

  % always ensure that there is a linear transfer matrix for combining the coils into gradiometers
  if ~isfield(sens, 'tra');
    Nchans = length(sens.label);
    Ncoils = size(sens.coilpos,1);
    if Nchans~=Ncoils
      ft_error('inconsistent number of channels and coils');
    end
    sens.tra = eye(Nchans, Ncoils);
  end

  if ~ft_headmodeltype(headmodel, 'localspheres')
    % select the desired channels from the gradiometer array
    [selchan, selsens] = match_str(channel, sens.label);
    % only keep the desired channels, order them according to the users specification
    try, sens.chantype = sens.chantype(selsens,:); end
    try, sens.chanunit = sens.chanunit(selsens,:); end
    try, sens.chanpos  = sens.chanpos (selsens,:); end
    try, sens.chanori  = sens.chanori (selsens,:); end
    sens.label    = sens.label(selsens);
    sens.tra      = sens.tra(selsens,:);
  else
    % for the localspheres model it is done further down
  end

  % remove the coils that do not contribute to any channel output
  selcoil      = any(sens.tra~=0,1);
  sens.coilpos = sens.coilpos(selcoil,:);
  sens.coilori = sens.coilori(selcoil,:);
  sens.tra     = sens.tra(:,selcoil);

  switch ft_headmodeltype(headmodel)
    case {'infinite' 'infinite_monopole' 'infinite_currentdipole' 'infinite_magneticdipole'}
      % nothing to do

    case 'singlesphere'
      % nothing to do

    case 'concentricspheres'
      % nothing to do

    case 'neuromag'
      %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      % if the forward model is computed using the external Neuromag toolbox,
      % we have to add a selection of the channels so that the channels
      % in the forward model correspond with those in the data.
      %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      [selchan, selsens] = match_str(channel, sens.label);
      headmodel.chansel = selsens;

    case 'localspheres'
      %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      % If the volume conduction model consists of multiple spheres then we
      % have to match the channels in the gradiometer array and the volume
      % conduction model.
      %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

      % the initial localspheres volume conductor has a local sphere per
      % channel, whereas it should have a local sphere for each coil
      if size(headmodel.r,1)==size(sens.coilpos,1) && ~isfield(headmodel, 'label')
        % it appears that each coil already has a sphere, which suggests
        % that the volume conductor already has been prepared to match the
        % sensor array
        return
      elseif size(headmodel.r,1)==size(sens.coilpos,1) && isfield(headmodel, 'label')
        if ~isequal(headmodel.label(:), sens.label(:))
          % if only the order is different, it would be possible to reorder them
          ft_error('the coils in the volume conduction model do not correspond to the sensor array');
        else
          % the coil-specific spheres in the volume conductor should not have a label
          % because the label is already specified for the coils in the
          % sensor array
          headmodel = rmfield(headmodel, 'label');
        end
        return
      end

      % the CTF way of representing the headmodel is one-sphere-per-channel
      % whereas the FieldTrip way of doing the forward computation is one-sphere-per-coil
      Nchans   = size(sens.tra,1);
      Ncoils   = size(sens.tra,2);
      Nspheres = size(headmodel.label);

      if isfield(headmodel, 'orig')
        % these are present in a CTF *.hdm file
        singlesphere.o(1,1) = headmodel.orig.MEG_Sphere.ORIGIN_X;
        singlesphere.o(1,2) = headmodel.orig.MEG_Sphere.ORIGIN_Y;
        singlesphere.o(1,3) = headmodel.orig.MEG_Sphere.ORIGIN_Z;
        singlesphere.r      = headmodel.orig.MEG_Sphere.RADIUS;
        % ensure consistent units
        singlesphere = ft_convert_units(singlesphere, headmodel.unit);
        % determine the channels that do not have a corresponding sphere
        % and use the globally fitted single sphere for those
        missing = setdiff(sens.label, headmodel.label);
        if ~isempty(missing)
          ft_warning('using the global fitted single sphere for %d channels that do not have a local sphere', length(missing));
        end
        for i=1:length(missing)
          headmodel.label(end+1) = missing(i);
          headmodel.r(end+1,:)   = singlesphere.r;
          headmodel.o(end+1,:)   = singlesphere.o;
        end
      end

      % make a new structure that only holds the local spheres, one per coil
      localspheres = [];
      localspheres.type = headmodel.type;
      localspheres.unit = headmodel.unit;

      % for each coil in the MEG helmet, determine the corresponding channel and from that the corresponding local sphere
      for i=1:Ncoils
        coilindex = find(sens.tra(:,i)~=0); % to which channel does this coil belong
        if length(coilindex)>1
          % this indicates that there are multiple channels to which this coil contributes,
          % which happens if the sensor array represents a synthetic higher-order gradient.
          [dum, coilindex] = max(abs(sens.tra(:,i)));
        end

        coillabel = sens.label{coilindex};               % what is the label of this channel
        chanindex = find(strcmp(coillabel, headmodel.label));  % what is the index of this channel in the list of local spheres
        localspheres.r(i,:) = headmodel.r(chanindex);
        localspheres.o(i,:) = headmodel.o(chanindex,:);
      end
      headmodel = localspheres;

      % finally do the selection of channels and coils
      % order them according to the users specification
      [selchan, selsens] = match_str(channel, sens.label);

      % first only modify the linear combination of coils into channels
      try, sens.chantype = sens.chantype(selsens,:); end
      try, sens.chanunit = sens.chanunit(selsens,:); end
      try, sens.chanpos  = sens.chanpos (selsens,:); end
      try, sens.chanori  = sens.chanori (selsens,:); end
      sens.label   = sens.label(selsens);
      sens.tra     = sens.tra(selsens,:);
      % subsequently remove the coils that do not contribute to any sensor output
      selcoil      = find(sum(sens.tra,1)~=0);
      sens.coilpos = sens.coilpos(selcoil,:);
      sens.coilori = sens.coilori(selcoil,:);
      sens.tra     = sens.tra(:,selcoil);
      % make the same selection of coils in the localspheres model
      headmodel.r = headmodel.r(selcoil);
      headmodel.o = headmodel.o(selcoil,:);

    case 'singleshell'
      %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      % if the forward model is computed using the code from Guido Nolte, we
      % have to initialize the volume model using the gradiometer coil
      % locations
      %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

      % compute the surface normals for each vertex point
      if ~isfield(headmodel.bnd, 'nrm')
        fprintf('computing surface normals\n');
        headmodel.bnd.nrm = surface_normals(headmodel.bnd.pos, headmodel.bnd.tri);
      end

      % estimate center and radius
      [center, radius] = fitsphere(headmodel.bnd.pos);

      % order of spherical spherical harmonics, for 'real' realistic volume conductors order=10 is o.k
      if isfield(headmodel, 'order')
        order = headmodel.order;
      else
        order = 10;
      end

      % initialize the forward calculation (only if  coils are available)
      if size(sens.coilpos,1)>0 && ~isfield(headmodel, 'forwpar')
        s = ft_scalingfactor(headmodel.unit, 'cm');
        headmodel.forwpar = meg_ini([s*headmodel.bnd.pos headmodel.bnd.nrm], s*center', order, [s*sens.coilpos sens.coilori]);
        headmodel.forwpar.scale = s;
      end

    case  'openmeeg'
      % don't do anything, h2em or h2mm generated later in ft_prepare_leadfield

    case 'duneuro'
           
      %compute transfer matrix
      if(~isfield(headmodel,'meg_transfer'))

        % set coils and projections
        coils = sens.coilpos;
        projections = sens.coilori;
        headmodel.driver.set_coils_and_projections(coils', projections');

        % compute transfer matrix
        cfg = [];
        cfg.solver.reduction   = headmodel.reduction;
        cfg.solver.intorderadd = headmodel.intorderadd;
        headmodel.meg_transfer = headmodel.driver.compute_meg_transfer_matrix(cfg);
      end
    
    case 'interpolate'
      % this is to allow moving leadfield files
      if ~exist(headmodel.filename{1}, 'file')
        for i = 1:length(headmodel.filename)
          [p, f, x] = fileparts(headmodel.filename{i});
          headmodel.filename{i} = fullfile(vpath, [f x]);
        end
      end

      matchlab = isequal(sens.label, headmodel.sens.label);
      matchpos = isequal(sens.coilpos, headmodel.sens.coilpos);
      matchtra = (~isfield(sens, 'tra') && ~isfield(headmodel.sens, 'tra')) || (isfield(sens, 'tra') && isfield(headmodel.sens, 'tra') && isequal(sens.tra, headmodel.sens.tra));

      if matchlab && matchpos && matchtra
        % the input sensor array matches precisely with the forward model
        % no further interpolation is needed
      else
        % interpolate the channels in the forward model to the desired channels
        filename = tempname;
        headmodel  = ft_headmodel_interpolate(filename, sens, headmodel);
        % update the sensor array with the one from the volume conductor
        sens = headmodel.sens;
      end % if recomputing interpolation

      % for the leadfield computations the @nifti object is used to map the image data into memory
      ft_hastoolbox('spm8up', 1);
      for i=1:length(headmodel.sens.label)
        % map each of the leadfield files into memory
        headmodel.chan{i} = nifti(headmodel.filename{i});
      end

    case 'simbio'
      ft_error('MEG not yet supported with simbio');

    otherwise
      ft_error('unsupported volume conductor model for MEG');
  end

elseif iseeg

  % the electrodes are used, the channel positions are not relevant any more
  % channel positinos need to be recomputed after projecting the electrodes on the skin
  if isfield(sens, 'chanpos'); sens = rmfield(sens, 'chanpos'); end

  % select the desired channels from the electrode array
  % order them according to the users specification
  [selchan, selsens] = match_str(channel, sens.label);
  Nchans = length(sens.label);

  sens.label     = sens.label(selsens);
  try, sens.chantype  = sens.chantype(selsens); end
  try, sens.chanunit  = sens.chanunit(selsens); end

  if isfield(sens, 'tra')
    % first only modify the linear combination of electrodes into channels
    sens.tra     = sens.tra(selsens,:);
    % subsequently remove the electrodes that do not contribute to any channel output
    selelec      = any(sens.tra~=0,1);
    sens.elecpos = sens.elecpos(selelec,:);
    sens.tra     = sens.tra(:,selelec);
  else
    % the electrodes and channels are identical
    sens.elecpos = sens.elecpos(selsens,:);
  end

  switch ft_headmodeltype(headmodel)
    case {'infinite' 'infinite_monopole' 'infinite_currentdipole'}
      % nothing to do

    case {'halfspace', 'halfspace_monopole'}
      % electrodes' all-to-all distances
      numelec = size(sens.elecpos,1);
      ref_el = sens.elecpos(1,:);
      md = dist( (sens.elecpos-repmat(ref_el,[numelec 1]))' );
      % take the min distance as reference
      md = min(md(1,2:end));
      pos = sens.elecpos;
      % scan the electrodes and reposition the ones which are in the
      % wrong halfspace (projected on the plane)... if not too far away!
      for i=1:size(pos,1)
        P = pos(i,:);
        is_in_empty = acos(dot(headmodel.ori,(P-headmodel.pos)./norm(P-headmodel.pos))) < pi/2;
        if is_in_empty
          dPplane = abs(dot(headmodel.ori, headmodel.pos-P, 2));
          if dPplane>md
            ft_error('Some electrodes are too distant from the plane: consider repositioning them')
          else
            % project point on plane
            Ppr = pointproj(P,[headmodel.pos headmodel.ori]);
            pos(i,:) = Ppr;
          end
        end
      end
      sens.elecpos = pos;

    case {'slab_monopole'}
      % electrodes' all-to-all distances
      numelc  = size(sens.elecpos,1);
      ref_elc = sens.elecpos(1,:);
      md  = dist( (sens.elecpos-repmat(ref_elc,[numelc 1]))' );
      % choose min distance between electrodes
      md  = min(md(1,2:end));
      pos = sens.elecpos;
      % looks for contacts outside the strip which are not too far away
      % and projects them on the nearest plane
      for i=1:size(pos,1)
        P = pos(i,:);
        instrip1 = acos(dot(headmodel.ori1,(P-headmodel.pos1)./norm(P-headmodel.pos1))) > pi/2;
        instrip2 = acos(dot(headmodel.ori2,(P-headmodel.pos2)./norm(P-headmodel.pos2))) > pi/2;
        is_in_empty = ~(instrip1&instrip2);
        if is_in_empty
          dPplane1 = abs(dot(headmodel.ori1, headmodel.pos1-P, 2));
          dPplane2 = abs(dot(headmodel.ori2, headmodel.pos2-P, 2));
          if dPplane1>md && dPplane2>md
            ft_error('Some electrodes are too distant from the planes: consider repositioning them')
          elseif dPplane2>dPplane1
            % project point on nearest plane
            Ppr = pointproj(P,[headmodel.pos1 headmodel.ori1]);
            pos(i,:) = Ppr;
          else
            % project point on nearest plane
            Ppr = pointproj(P,[headmodel.pos2 headmodel.ori2]);
            pos(i,:) = Ppr;
          end
        end
      end
      sens.elecpos = pos;

    case {'singlesphere', 'concentricspheres'}
      % ensure that the electrodes ly on the skin surface
      radius = max(headmodel.r);
      pos    = sens.elecpos;
      if isfield(headmodel, 'o')
        % shift the the centre of the sphere to the origin
        pos(:,1) = pos(:,1) - headmodel.o(1);
        pos(:,2) = pos(:,2) - headmodel.o(2);
        pos(:,3) = pos(:,3) - headmodel.o(3);
      end
      distance = sqrt(sum(pos.^2,2)); % to the center of the sphere
      if any((abs(distance-radius)/radius)>0.005)
        ft_warning('electrodes do not lie on skin surface -> using radial projection')
      end
      pos = pos * radius ./ [distance distance distance];
      if isfield(headmodel, 'o')
        % shift the center back to the original location
        pos(:,1) = pos(:,1) + headmodel.o(1);
        pos(:,2) = pos(:,2) + headmodel.o(2);
        pos(:,3) = pos(:,3) + headmodel.o(3);
      end
      sens.elecpos = pos;

    case {'bem', 'dipoli', 'asa', 'bemcp'}
      %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      % do postprocessing of volume and electrodes in case of BEM model
      %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

      % project the electrodes on the skin and determine the bilinear interpolation matrix
      if ~isfield(headmodel, 'tra') && (isfield(headmodel, 'mat') && ~isempty(headmodel.mat))
        % determine boundary corresponding with skin and inner_skull_surface
        if ~isfield(headmodel, 'skin_surface')
          headmodel.skin_surface = find_outermost_boundary(headmodel.bnd);
          fprintf('determining skin compartment (%d)\n', headmodel.skin_surface);
        end
        if ~isfield(headmodel, 'source')
          headmodel.source = find_innermost_boundary(headmodel.bnd);
          fprintf('determining source compartment (%d)\n', headmodel.source);
        end
        if size(headmodel.mat,1)~=size(headmodel.mat,2) && size(headmodel.mat,1)==length(sens.elecpos)
          fprintf('electrode transfer and system matrix were already combined\n');
        else
          fprintf('projecting electrodes on triangulated skin surface\n');
          % compute linear interpolation from triangle vertices towards electrodes
          [el, prj] = project_elec(sens.elecpos, headmodel.bnd(headmodel.skin_surface).pos, headmodel.bnd(headmodel.skin_surface).tri);
          tra       = transfer_elec(headmodel.bnd(headmodel.skin_surface).pos, headmodel.bnd(headmodel.skin_surface).tri, el);

          % replace the original electrode positions by the projected positions
          sens.elecpos = prj;

          if size(headmodel.mat,1)==size(headmodel.bnd(headmodel.skin_surface).pos,1)
            % construct the transfer from only the skin vertices towards electrodes
            interp = tra;
          else
            % construct the transfer from all vertices (also inner_skull_surface/outer_skull_surface) towards electrodes
            interp = [];
            for i=1:length(headmodel.bnd)
              if i==headmodel.skin_surface
                interp = [interp, tra];
              else
                interp = [interp, zeros(size(el,1), size(headmodel.bnd(i).pos,1))];
              end
            end
          end

          % incorporate the linear interpolation matrix and the system matrix into one matrix
          % this speeds up the subsequent repeated leadfield computations
          fprintf('combining electrode transfer and system matrix\n');

          % convert to sparse matrix to speed up the subsequent multiplication
          interp  = sparse(interp);
          headmodel.mat = interp * headmodel.mat;
          % ensure that the model potential will be average referenced
          avg = mean(headmodel.mat, 1);
          headmodel.mat = headmodel.mat - repmat(avg, size(headmodel.mat,1), 1);
        end
      end
    case  'openmeeg'
      % don't do anything, h2em or h2mm generated later in ft_prepare_leadfield

    case 'fns'
      if isfield(headmodel,'bnd')
        [el, prj] = project_elec(sens.elecpos, headmodel.bnd.pos, headmodel.bnd.tri);
        sens.tra = transfer_elec(headmodel.bnd.pos, headmodel.bnd.tri, el);
        % replace the original electrode positions by the projected positions
        sens.elecpos = prj;
      end

    case 'simbio'
      % check that the external toolbox is present
      ft_hastoolbox('simbio', 1);

      % extract the outer surface
      bnd = mesh2edge(headmodel);
      for j=1:length(sens.label)
        d = bsxfun(@minus, bnd.pos, sens.elecpos(j,:));
        [d, i] = min(sum(d.^2, 2));
        % replace the position of each electrode by the closest vertex
        sens.elecpos(j,:) = bnd.pos(i,:);
      end

      if (isfield(headmodel,'transfer') && isfield(headmodel,'elec'))
        if all(ismember(sens.label,headmodel.elec.label))
          [sensmember, senslocation] = ismember(sens.label,headmodel.elec.label);
          if (norm(sens.elecpos - headmodel.elec.elecpos(senslocation,:))<1e-8)
            headmodel.transfer = headmodel.transfer(senslocation,:);
            headmodel.elec = sens;
          else
            ft_error('Electrode positions do not fit to the given transfer matrix!');
          end
        else
          ft_error('Transfer matrix does not fit the given set of electrodes!');
        end
      else
        headmodel.transfer = sb_transfer(headmodel,sens);
        headmodel.elec = sens;
      end

    case 'duneuro'
      ft_hastoolbox('duneuro', 1);

      if(~isfield(headmodel,'eeg_transfer'))
        % set electrodes
        cfg = [];
        cfg.type = headmodel.electrodes;
        cfg.codims = headmodel.subentities;
        headmodel.driver.set_electrodes(sens.elecpos', cfg);
        
        % compute transfer matrix
        cfg = [];
        cfg.solver.reduction = headmodel.reduction;
        headmodel.eeg_transfer = headmodel.driver.compute_eeg_transfer_matrix(cfg);
      end

    case 'interpolate'
      % this is to allow moving leadfield files
      if ~exist(headmodel.filename{1}, 'file')
        for i = 1:length(headmodel.filename)
          [p, f, x] = fileparts(headmodel.filename{i});
          headmodel.filename{i} = fullfile(vpath, [f x]);
        end
      end

      matchlab = isequal(sens.label, headmodel.sens.label);
      matchpos = isequal(sens.elecpos, headmodel.sens.elecpos);
      matchtra = (~isfield(sens, 'tra') && ~isfield(headmodel.sens, 'tra')) || (isfield(sens, 'tra') && isfield(headmodel.sens, 'tra') && isequal(sens.tra, headmodel.sens.tra));

      if matchlab && matchpos && matchtra
        % the input sensor array matches precisely with the forward model
        % no further interpolation is needed
      else
        % interpolate the channels in the forward model to the desired channels
        filename = tempname;
        headmodel  = ft_headmodel_interpolate(filename, sens, headmodel);
        % update the sensor array with the one from the volume conductor
        sens = headmodel.sens;
      end % if recomputing interpolation

      % for the leadfield computations the @nifti object is used to map the image data into memory
      ft_hastoolbox('spm8up', 1);
      for i=1:length(headmodel.sens.label)
        % map each of the leadfield files into memory
        headmodel.chan{i} = nifti(headmodel.filename{i});
      end

    otherwise
      ft_error('unsupported volume conductor model for EEG');
  end

  % FIXME this needs careful thought to ensure that the average referencing which is
  % now done here and there, and that the linear interpolation in case of BEM are all
  % dealt with consistently always ensure that there is a linear transfer matrix for
  % rereferencing the EEG potential
  %
  % if ~isfield(sens, 'tra');
  %   sens.tra = eye(length(sens.label)) - 1/length(sens.label);
  % end

  % update the channel positions as the electrodes were projected to the skin surface
  [pos, ori, lab] = channelposition(sens);
  [selsens, selpos] = match_str(sens.label, lab);
  sens.chanpos = nan(length(sens.label),3);
  sens.chanpos(selsens,:) = pos(selpos,:);

end % if iseeg or ismeg

if isfield(sens, 'tra')
  if issparse(sens.tra) && size(sens.tra, 1)==1
    % this multiplication would result in a sparse leadfield, which is not what we want
    % the effect can be demonstrated as sparse(1)*rand(1,10), see also http://bugzilla.fieldtriptoolbox.org/show_bug.cgi?id=1169#c7
    sens.tra = full(sens.tra);
  elseif ~issparse(sens.tra) && size(sens.tra, 1)>1
    % the multiplication of the "sensor" leadfield (electrode or coil) with the tra matrix to get the "channel" leadfield
    % is faster for most cases if the pre-multiplying weighting matrix is made sparse
    sens.tra = sparse(sens.tra);
  end
end

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% SUBFUNCTION
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function Ppr = pointproj(P,plane)
% projects a point on a plane
% plane(1:3) is a point on the plane
% plane(4:6) is the ori of the plane
Ppr  = [];
ori  = plane(4:6);
line = [P ori];
% get indices of line and plane which are parallel
par = abs(dot(plane(4:6), line(:,4:6), 2))<1e-14;
% difference between origins of plane and line
dp = plane(1:3) - line(:, 1:3);
% Divide only for non parallel vectors (DL)
t = dot(ori(~par,:), dp(~par,:), 2)./dot(ori(~par,:), line(~par,4:6), 2);
% compute coord of intersection point
Ppr(~par, :) = line(~par,1:3) + repmat(t,1,3).*line(~par,4:6);

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% This function serves as a replacement for the dist function in the Neural
% Networks toolbox.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
function [d] = dist(x)
n = size(x,2);
d = zeros(n,n);
for i=1:n
  for j=(i+1):n
    d(i,j) = sqrt(sum((x(:,i)-x(:,j)).^2));
    d(j,i) = d(i,j);
  end
end
