FixedProjectiveConstraint

This component belongs to the category of Projective Constraint. The FixedProjectiveConstraint projects a constant velocity. If the fixed points have a zero velocity at the simulation start, they will keep a zero velocity i.e. be fixed.

As introduced in the page about the Projective Constraint, the FixedProjectiveConstraint corresponds to a projection matrix noted which will multiply the system matrix so that: . This projection matrix is the identity matrix in which the diagonal value corresponding to the indices of the fixed points equals zero. These lines and columns equals 0. As a consequence, when the integration scheme (ODESolver) will call the projectResponse() or projectVelocity() the constraint will be applied, ensuring that the desired degrees of freedom remain fixed.

Example of a system of size 6, with a fixed constraint at the indice 5:

By projecting this matrix on the right hand side vector we have . This ensures to have the projection , thus preventing any time evolution of the fifth degree of freedom. In such case, we function projectResponse():

template <class DataTypes>
void FixedProjectiveConstraint<DataTypes>::projectResponse(const core::MechanicalParams* mparams, DataVecDeriv& resData)
{
    SOFA_UNUSED(mparams);

    helper::WriteAccessor<DataVecDeriv> res (resData );
    const SetIndexArray & indices = d_indices.getValue();

    if( d_fixAll.getValue() )
    {
        // fix everything
        typename VecDeriv::iterator it;
        for( it = res.begin(); it != res.end(); ++it )
        {
            *it = Deriv();
        }
    }
    else
    {
        for (SetIndexArray::const_iterator it = indices.begin(); it != indices.end(); ++it)
        {
            res[*it] = Deriv();
        }
    }
}

Data

The FixedProjectiveConstraint can be initialized using three input data:

• indices: corresponding to the indices of the fixed points
• fixAll: filters all the DOF to implement a fixed object
• activate_projectVelocity: if true, projects not only a constant but a zero velocity

Usage

The FixedProjectiveConstraint requires a MechanicalObject to store the degrees of freedom associated to the nodes, as well as a Mass so that the system matrix is not null. An integration scheme and a solver are also necessary to solve the linear system at each time step.

Note that if only a part of the degrees of freedom must be constraint, you can use the PartialFixedProjectiveConstraint working in the same way as the FixedProjectiveConstraint.

Example

This component is used as follows in XML format:

<FixedProjectiveConstraint name="FixedProjectiveConstraint" indices="3 39 64" />

or using SofaPython3:

node.addObject('FixedProjectiveConstraint', indices='3 39 64')

An example scene involving a FixedProjectiveConstraint is available in examples/Component/Constraint/Projective/FixedProjectiveConstraint.scn

Target: Sofa.Component.Constraint.Projective

namespace: sofa::component::constraint::projective

parents:

• ProjectiveConstraintSet

Data:

Name	Description	Default value
name	object name	unnamed
printLog	if true, emits extra messages at runtime.	0
tags	list of the subsets the objet belongs to
bbox	this object bounding box
componentState	The state of the component among (Dirty, Valid, Undefined, Loading, Invalid).	Undefined
listening	if true, handle the events, otherwise ignore the events	0
group	ID of the group containing this constraint. This ID is used to specify which constraints are solved by which solver, by specifying in each solver which groups of constraints it should handle.	0
endTime	The constraint stops acting after the given value. Use a negative value for infinite constraints	-1
indices	Indices of the fixed points
fixAll	filter all the DOF to implement a fixed object	0
activate_projectVelocity	activate project velocity to set velocity	0
Visualization
showObject	draw or not the fixed constraints	1
drawSize	0 -> point based rendering, >0 -> radius of spheres	0

Links:

Name	Description
context	Graph Node containing this object (or BaseContext::getDefault() if no graph is used)
slaves	Sub-objects used internally by this object
master	nullptr for regular objects, or master object for which this object is one sub-objects
mechanicalStates	List of mechanical states to which this component is associated
mstate	MechanicalState used by this component
topology	link to the topology container