function[y]=multiplyB_tilde(Mesh, Data, x, out)

% multiplyB_tilde: Implements the mutiplication between B_tilde and a
% vector x and stores the result in y
% INPUT:
% Mesh:     Structure containing all the mesh parameters
% Data:     Structure containing all the data parameters
% x:        Vector
% out:      Parameter specifying whether the output should be a vector or
%           a matrix
% OUTPUT:
% y:        Result of the multiplication of B_tilde with x

% Retrive mesh parameters
% Number of local degrees of freedom
d=Mesh.nb_dof_local;

w1=reshape(x(Data.body{1}.indx_local,:), d, []);
w2=reshape(x(Data.body{2}.indx_local,:), d, []);
w2=multiplyR12(Data, w2')';

y=w1-w2;

if strcmp(out, 'vec')
    y=y(:);
end
end