# DiagonalMass

This component belongs to the category of Masses. In the dynamic equation (see Physics integration page), the mass density results from the first derivative in time of the momentum term. Like the MeshMatrixMass, the DiagonalMass computes the integral of this mass density over the volume of the object geometry. To do so and for any given topology (edges, triangles, quads, tetrahedra or hexahedra), the DiagonalMass integrates the mass density inside each elements and sums the mass matrix in the system matrix .

However, the DiagonalMass makes a strong simplification: it considers the mass matrix as being diagonal. To build this diagonal mass matrix, the DiagonalMass relies on a numerical method called the mass lumping. It consists in summing all mass values of a line on the diagonal. This approach is already implemented in the MeshMatrixMass but the DiagonalMass proposes an optimized version of the mass lumping and extend it to edge topology.

For details on the volume integration, please report to the MeshMatrixMass page. As demonstrated in the MeshMatrixMass page, in case of a topology using linear tetrahedra, the diagonal mass matrix corresponds to:

By making the matrix diagonal (i.e. removing extra-diagonal terms), the lumping method removes the connectivity (neighborhood) information from the matrix. Due to this numerical approximation, the accuracy of the integration is decreased compared to the MeshMatrixMass integration. It is therefore advised to use the DiagonalMass carefully.

### API

Depending on the type of LinearSolver used:

- for iterative solvers, the result of the multiplication between the mass matrix and an approximated solution is computed by the function:

```
template <class DataTypes, class MassType>
void DiagonalMass<DataTypes, MassType>::addMDx(const core::MechanicalParams* /*mparams*/, DataVecDeriv& res, const DataVecDeriv& dx, SReal factor)
{
const MassVector &masses= d_vertexMass.getValue();
helper::WriteAccessor< DataVecDeriv > _res = res;
helper::ReadAccessor< DataVecDeriv > _dx = dx;
size_t n = masses.size();
for (size_t i=0; i<n; i++)
{
_res[i] += (_dx[i] * masses[i]) * (Real)factor;
}
}
```

- for direct solvers, the mass matrix is built by the function:

```
template <class DataTypes, class MassType>
void DiagonalMass<DataTypes, MassType>::addMToMatrix(const core::MechanicalParams *mparams, const sofa::core::behavior::MultiMatrixAccessor* matrix)
{
const MassVector &masses= d_vertexMass.getValue();
const int N = defaulttype::DataTypeInfo<Deriv>::size();
AddMToMatrixFunctor<Deriv,MassType> calc;
sofa::core::behavior::MultiMatrixAccessor::MatrixRef r = matrix->getMatrix(this->mstate);
Real mFactor = (Real)mparams->mFactorIncludingRayleighDamping(this->rayleighMass.getValue());
for (unsigned int i=0; i<masses.size(); i++)
calc(r.matrix, masses[i], r.offset + N*i, mFactor);
}
```

## Data

The DiagonalMass can be initialized using two different input data:

**totalMass**: corresponding to the total mass of the object, which will be distributed over its volume taking into account the geometry**massDensity**: corresponding to the mass density used for the integration detailed above

## Usage

The DiagonalMass **requires** a MechanicalObject to store the degrees of freedom associated to the nodes, as well as a Topology. An integration scheme and a solver are also necessary to solve the linear system at each time step.

All topologies are handled by the DiagonalMass, namely: edges, triangles, quads, tetrahedra or hexahedra.

## Example

This component is used as follows in XML format:

or using Python:

An example scene involving a DiagonalMass is available in *examples/Components/mass/DiagonalMass.scn*

Last modified: 14 May 2020