Dummies are multipurpose objects. They can have very specific functions, or they can just be used as helper objects. Following dummy functions are common:
Used as a point or reference system: This is the case when you want to track the position and/or orientation of a specific point in the scene. The dummy position can be matched to the position of a shape vertex for instance, and will allow to precisely positioning the shape: in the vertex edit mode, create a dummy from a vertex, then attach the shape to the dummy (make the dummy parent). Now you can manipulate/position the shape through the dummy which has the same position as the selected vertex.
Used as proxy or supporting object for models: Models are defined by marking an object with the object is model base flag. Any object can be the base of a model, however dummies are the prefered objects for that.
Used as marking points: When evaluating the work space of a robot for instance, a dummy could be positioned at the same coordinates as the robot's end-effector, at various time intervals, and finally the convex hull of all added dummies can be extracted and displayed. Refer to the section on convex hull calculation for more details.
Used to specify end-effector, and end-effector target positions/orientations in inverse kinematics calculations: The inverse kinematics calculation module handles kinematic chains. Each chain is specified with a base object and a tip object. The tip object has to be a dummy, and often the user positions and orients the tip dummy to coincide with a robot's end-effector. At the same time, in order to move the kinematic chain in inverse kinematics mode, a target position/orientation for the robot's end-effector is required. There again, a dummy is used as the target object.
Used to create loop closure constraints for mechanisms dynamically simulated: Refer to the section on designing dynamic simulations for more details.
Used as path following objects: Dummy objects are the only objects that can be assigned to stay on a path (free on path, freely sliding) or to follow the path position (fixed on path, fixed along the path at its current intrinsic position). Following figure shows a dummy assigned to stay on a path:
[Dummy assigned to stay on a path. The dummy's position and orientation is interpolated from the path's Bezier points]