void ComputeMassMat::compute(SXMatrix &M,const TetMesh&mesh, const vector<SX> &density){
assert_eq(density.size(),mesh.tets().size());
this->_density = density;
_M.setZero();
computeCompactM(_M,mesh);
const int n = mesh.nodes().size();
M.resize(n*3,n*3);
M.setZero();
for (int i = 0; i < _M.size1(); ++i){
for (int j = 0; j < _M.size2(); ++j){
if( _M.hasNZ(i,j) ){
M.elem(i*3+0,j*3+0) = _M.elem(i,j);
M.elem(i*3+1,j*3+1) = _M.elem(i,j);
M.elem(i*3+2,j*3+2) = _M.elem(i,j);
}
}
}
}
