General info for all markers

What a marker is

A marker is the interface between the items that have coordinates - nodes and bodies - and the items that act on them - connectors, constraints, joints and loads. A joint does not need to know whether it is attached to a node or to a point of a body: it asks its markers for what it needs. The Interface of every connector and load names the marker types it requests, and the Interface of every marker the items that can use it; the table below lists all markers.

What a marker provides

A marker computes, in the current configuration, the marker quantities its types promise:

type	quantities
Position	the global position \(\LU{0}{\pv}_m\) and velocity \(\LU{0}{\vv}_m\)
Orientation	the rotation matrix \(\LU{0m}{\Rot}\) and the local angular velocity \(\LU{m}{\tomega}\)
Coordinate, Coordinates	one or several coordinates and their time derivatives
BodyMass	only a Jacobian, for a load proportional to the mass

and the Jacobians through which a force acts: the position Jacobian \(\LU{0}{\Jm_{pos}} = \partial \LU{0}{\vv}_m / \partial \dot\qv\), the rotation Jacobian \(\LU{0}{\Jm_{rot}} = \partial \LU{0}{\tomega}_m / \partial \dot\qv\), or for a coordinate a row of the unit matrix. A marker on a body takes position, orientation, velocity and angular velocity as the output variables of the body define them, at its local position, and the Jacobians from the body; a marker on a node takes them from the node.

How a force reaches the coordinates

The virtual work of a force \(\LU{0}{\fv}\) at the marker, \(\delta W = \delta \LU{0}{\pv}_m\tp \LU{0}{\fv}\), gives the generalized forces

\[ \Qm = \LU{0}{\Jm_{pos}}\tp\, \LU{0}{\fv} , \]

and a torque \(\LU{0}{\ttau}\) gives \(\Qm = \LU{0}{\Jm_{rot}}\tp\, \LU{0}{\ttau}\); a force \(f\) on a coordinate marker gives \(\Qm = \Jm\tp f\). A marker on the ground - a NodePointGround, an ObjectGround - has an empty Jacobian, and nothing acts there.

With or without reference values

MarkerNodeCoordinate and MarkerNodeODE1Coordinate give the current coordinate, without its reference value; MarkerNodeCoordinates, MarkerObjectODE2Coordinates and the cable and beam markers give the coordinates including the reference values. A coordinate constraint between two markers therefore means something different with the one and with the other.

Markers in sensors, and markers that are created

SensorMarker measures the marker quantities, in the current configuration. Most markers of a model are added by the Create functions: CreateRevoluteJoint adds two MarkerBodyRigid, CreateForce a MarkerBodyPosition or MarkerBodyRigid, CreateRigidBody with gravity a MarkerBodyMass.

Drawing

A marker with a position is drawn if markers.show and its own show are True: as a symbol of size markers.defaultSize - a fraction of openGL.advanced.initialMaxSceneSize if -1 -, three crossed lines with markers.drawSimplified, a cube otherwise, in markers.defaultColor; with markers.showBasis the frame of a marker that has an orientation, of length markers.basisSize (general.axesTiling). markers.showNumbers writes its number. A marker on coordinates has no position and draws nothing.

The settings: markers.show, markers.defaultSize, markers.defaultColor, markers.drawSimplified, markers.showNumbers, markers.showBasis, markers.basisSize, general.axesTiling, openGL.advanced.initialMaxSceneSize.

All markers

marker	attached to	provides	usable by
MarkerBodyMass	a body	BodyMass	1 item
MarkerBodyPosition	a body	Position	10 items
MarkerBodyRigid	a body	Position, Orientation	23 items
MarkerNodePosition	a node with Position or Position2D	Position	10 items
MarkerNodeRigid	a node with Position or Position2D and Orientation or Orientation2D	Position, Orientation	23 items
MarkerNodeCoordinate	a node	Coordinate	5 items
MarkerNodeCoordinates	a node	Coordinates	1 item
MarkerNodeODE1Coordinate	a node	Coordinate	5 items
MarkerNodeRotationCoordinate	a node with Orientation	Coordinate	5 items
MarkerBodiesRelativeTranslationCoordinate	two bodies	Coordinate	5 items
MarkerBodiesRelativeRotationCoordinate	two bodies	Coordinate	5 items
MarkerSuperElementPosition	mesh nodes of a super element	Position	10 items
MarkerSuperElementRigid	mesh nodes of a super element	Position, Orientation	23 items
MarkerKinematicTreeRigid	a link of a kinematic tree	Position, Orientation	23 items
MarkerObjectODE2Coordinates	a body	Coordinates	1 item
MarkerBodyCable2DShape	a body	Coordinate	5 items
MarkerBodyCable2DCoordinates	a body	Coordinate	5 items
MarkerBodyBeamShape	a body	Beam3DShape	the items that name it