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The SEDRIS Data Representation Model
APPENDIX A - Classes Union Of Geometry |
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An instance of one of the concrete subclasses of this abstract DRM class is an aggregation of <Geometry> specifying a standardized mechanism by which to organize the members that compose the union.
An antenna assembly is contained in a weather protection enclosure. Visually, only the opaque enclosure can be seen. But at microwave frequencies, the enclosure is invisible and only the antenna can be "seen". The entire structure is contained in a <Union Of Geometry>. What the Radar "sees" is modeled with a Radar Cross-section (RCS) <Property Table>. The algorithm (or field measurements) that computed the RCS table used axes that do not match the spatial reference frame (world or model as the case may be). Therefore, RCS axes of azimuth and elevation angle are misused unless some REFERENCE DIRECTIONS can be attached to the entire <Union Of Geometry>.
The fixed order of the polygons would be reflected in an <Union Of Primitive Geometry> with the ordering_reason set to SE_ORDRNG_REASON_FIXED_LISTED. The lowest priority <Polygon> instances (the walls) would be specified first and the higher priority polygons (the windows) would be specified last.
The polygons would typically be grouped in layers. There are 2 approaches that could be employed by the modeler depending on how the polygons are grouped.
If all the polygons are one-sided front-facing polygons, then the house could be represented with an <Union Of Geometry Hierarchy> with ordering_reason SE_LAYERED with 2 children. The 1st child would be the base layer (all the walls) and the 2nd child (the first decal layer) would contain all the windows. In this case, the polygons under a LAYERED <Union Of Geometry> are not coplanar, but the rendering priority can still be resolved.
If the modeler grouped the layers into coplanar unions of <Polygon> instances, the SEDRIS structure might be represented as <Union Of Geometry> with 4 layered <Union Of Geometry> (wall and window) would list the wall first and the window next.
A <Reference Vector> would be used when a <Union Of Geometry> represented a "thing" in the environment. An example of this is a building represented by a <Union Of Geometry>, and which has a <Property Table> of radar cross-sections. The table would need a <Reference Vector> to establish the zero azimuth direction. (See Example 1).
| SE_Boolean | unique_descendants; | (notes) |
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| SE_Boolean | strict_organizing_principle; | (notes) |
| SE_Ordering_Reason | ordering_reason; |
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An association between a <Geometry Hierarchy> instance and a <Feature> instance indicates that the <Geometry Hierarchy> and the <Feature> are alternate representations of the same environmental object.
An association between two <Geometry Hierarchy> instances indicates that they are alternate representations of the same environmental object.
An association from a <Hierarchy Summary Item> instance to a <Geometry Hierarchy> indicates that the <Hierarchy Summary Item> summarizes that <Geometry Hierarchy>.
An association from a <Reference Surface> instance to a <Geometry Hierarchy> indicates that the <Geometry Hierarchy> organizes the geometric objects that specify the resolution surface of the <Reference Surface>.
In the case where multiple <Collision Volume> components are specified for a given <Aggregate Geometry>, the union of the volumes thus specified is used in collision detection.
If this value is SE_TRUE, each 'descendant' of this aggregation - that is, each <Geometry> instance that exists in the component tree rooted at the given <Aggregate Geometry> - shall be unique, in the sense that it shall appear in only one 'branch' of this aggregation. If unique_descendants is SE_FALSE, at least one <Geometry> instance appears in more than one 'branch' of the aggregation.
If this value is SE_TRUE, each 'branch' of this aggregation strictly complies with the organizing principle for its particular subclass. If this value is SE_FALSE, at least one 'branch' does not strictly comply with the given organizing principle. See the organizing principle constraint for each specific subclass for details.
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