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=Authoring Deterministic Dynamic Objects=
==Behavior==

Deterministic Dynamic Objects (DDOs) are objects with no autonomous behavior. These objects follow a pre-scripted path in the virtual environment. There are two types of DDOs: regular and dependent. Regular the user specifies the velocity, whereas dependent DDOs the velocity is controlled relative to the closure rate of another object toward a target point.

The trajectory of a DDO is described by a series of user-specified control points. During scenario execution, the DDO follows a path that visits all control points in order. The exact path is set using a geometric spline to smooth the motion of the DDO between control points. The DDO always starts at the beginning of the trajectory, which must be placed at a point of the virtual environment that contains underlying terrain. All intersections, roads, and the road shoulder including various SOL objects provide terrain information. Control points other than the first one can be placed on points in the virtual environment where there is no terrain. When the DDO crosses from a point with terrain to a point with no terrain, it uses the most recent value from terrain elevation. If the path crosses into an area that provides elevation again, then the DDO will then use the new elevation.

The execution of DDOs is also controlled by the three standard parameters: creation radius, activation delay, and lifetime. These were described in detail in Section 3.17.

The velocity of regular DDOs is explicitly specified by the user at each control point. When traveling along the path, the DDOs velocity will be at the specified value for the control point it is crossing; when it is between two control points, the velocity will be linearly interpolated.

[[File:tdmap.png|800px|thumb|center|'''Figure 4-42 illustrates two DDOs as they appear on the ISAT's top down map.''']]

The DDO scenario element is rendered using the actual top down representation of the object in the SOL, and the path of the DDO is shown as a series of points connected by a line. The line uses the same spline interpolation technique that is used by the DDO at runtime, resulting in the display of the same path the DDO will traverse at runtime. When selected, a DDO will be drawn with a red outline. In addition, a circle will be drawn to indicate the activation radius, as in the top DDO shown in.
 
[[File:tdmap.png|800px|thumb|center|'''Figure 4-42. If no such circle appears, the DDO has no defined activation radius.''']]


[[File:tdmap.png|800px|thumb|center|'''Figure 4-42 Appearance of DDO’s.''']]

===Dependent DDO Velocity Control===

As described earlier, the key difference between a DDO and a DDDO is the method by which their velocity is determined at runtime. The DDO's velocity is explicitly specified at every control point. On a DDDO, however, the speed is controlled by the relative motion of another object in the simulation. To achieve this, a DDDO has three more parameters associated with it. These are the target control point, the controlling object, and the controlling object target location. The target control point is one of the control points on the DDDO trajectory. The DDDO will continuously adapt its velocity so that it arrives on the target control point at the same time that the controlling object arrives at the target location. For example, consider the illustration in Figure 4-2.

[[File: locationTar.png|800px|thumb|center|'''Figure 4-43 Dependent DDO illustration.''']]

The DDDO is shown traveling south on the road controlled by the STOP sign. The highlighted control point is acting as the target control point. The controlling object is traveling from right to left, and its target location is shown with a red striped circle. In this example, the DDDO will adjust its speed so that no matter how fast or how slow the westbound vehicle is traveling, the DDDO will arrive at the target control point at the same time that the vehicle arrives at the target location.

Once the DDDO reaches the target control point, it will continue on its trajectory using the velocity specified at the control points.

Note that the DDDO will never go backwards, including the case where the target location is defined such that the controlling object is traveling away from it. In such a case, the DDO will stop (i.e., the minimum attained velocity will be zero).