% generate synthetic left and right hemisphere data
% (this is just example data for illustration)
ds_left=cosmo_synthetic_dataset('type','surface','seed',1);
ds_right=cosmo_synthetic_dataset('type','surface','seed',2);

% Set the number of vertices of the left surface.
% If the surface is sparse (it does not have data for all nodes), it *may*
% be necessary to adjust this value manually. In that case, consider to:
%
%  - compute the number of vertices, if it is a standardized surface from
%    MapIcosahedron. If the ld parameter was set to 64, then the number of
%    vertices is 10*64^2+2=40962.
%  - get the number of vertices using:
%       [v,f]=surfing_read('left_surface.asc');
%       nverts=max(size(v));
%
[unused, index]=cosmo_dim_find(ds_left, 'node_indices');
nverts_left=max(ds_left.a.fdim.values{index});

% get the offset to set the feature attribute index later
offset_left=numel(ds_left.a.fdim.values{index});

% update node indices to support indexing data from two hemispheres
node_indices=[ds_left.a.fdim.values{index}, ...
                nverts_left+ds_right.a.fdim.values{index}];
ds_left.a.fdim.values{index}=node_indices;
ds_right.a.fdim.values{index}=node_indices;

% update node indices for right hemisphere
assert(all(ds_left.fa.node_indices<=offset_left)); % safety check
ds_right.fa.node_indices=ds_right.fa.node_indices+offset_left;

% merge hemisphes
ds_left_right=cosmo_stack({ds_left,ds_right},2);