Unclear relationships between the Bounding Box's center and the origin of the local coordinate system

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• Branch: master

Problem description

There is a contradiction between documents regarding the definitions and relations between the Bounding Box center and origin of the local coordinate system (Vehicle, Pedestrian, and MiscObject). According to the UML Model, the Center of the Bounding Box is defined as a derivation calculated in the local coordinates (e.g., in the vehicle coordinate system). It is implied to be valid for all kinds of Entities (Vehicle, Pedestrian, and MiscObject).

"Center - Represents the geometrical center of the bounding box expressed in coordinates that refer to the coordinate system of the entity (e.g. the vehicle coordinate system)."

According to the OpenSCENARIO User Guide (chapters 6.3.4, 6.3.5),

a) for Vehicles: The origin point of the local (vehicle) coordinate system is defined with respect to the sprung mass point of an abstract physical body (vehicle). It does not define any explicit relation to the vehicle's Bounding Box (defined in the UML Model). In fact, there is no possibility to find a vehicle's rear axis axle (it is undefined). This causes an uncertainty about what is an origin and what is a derivation.

"In ASAM OpenSCENARIO, the origin of this coordinate system is derived by projecting the center of the vehicle’s rear axis to the ground plane at neutral load conditions. The origin remains fixed to the vehicle sprung mass"

b) for Pedestrian and MiscObject: On the contrary, the origin point of the local coordinate system is defined with respect to the Bounding Box. It means, a Pedestrian/MiscObject is fully represented in a scenario by its Bounding Box.

"The origin of this coordinate system is derived from the geometrical center of the object’s bounding box under neutral load conditions (if applicable) projected onto the ground plane."

The sequence of the aforementioned uncertainties is that it is unclear with respect to what point a rotation (orientation) of Entities should be assumed. Should the center of rotations be the origin of the local coordinate system or the center of the Bounding Box?
Although in the OpenSCENARIO, an orientation of Entities (Bounding Boxes) themselves is not defined (only an orientation of Position points is defined), it is implied anyway since the orientation of a Position implies all in all the orientation of the Entity which is at that position. The standard should clarify this point anyhow.

Also it is obvious that a mass has no impact on the geometrical Bounding Box characteristics within the OpenSCENARIO.

Proposal for improvements

To deprecate the present definitions in chapters 6.3.4 and 6.3.5 and replace them all with the following single chapter (valid for all Entities -- Vehicle, Pedestrian, and MiscObject):

New content:

6.3.4 Local coordinate system (X, Y, Z)

In the ASAM OpenSCENARIO, Entities (Vehicle, Pedestrian, and MiscObject) are represented by their abstract geometrical 3D model described by the Bounding Box. The local cartesian (X/Y/Z) coordinate system is defined with regard to the geometrical center of the Bounding Box.
By default, the origin of this coordinate system is assumed to be a projection of the geometrical center of the Bounding Box on its bottom face (i.e., the center of the bottom rectangle). With respect to this point, the center of the Bounding Box can also have a non-zero offset (x/y/z)-coordinates within the local coordinate system in particular cases.

The Bounding Box is defined as an undirected spatial body and therefore directions of axes of the local coordinate system are defined by a convention between the scenario modeling tool and a simulation environment. For example, if the Bounding Box represents a Vehicle, a positive offset of the center from the origin point along a single horizontal dimension might be deemed as the positive direction of the local X-axis headed along this dimension.

Positioning of Entities is defined by coordinates of the origin point of the local coordinate system in the respective external coordinate system.

Backward compatibility

Compatible as no changes in the XSD schema.