The SEDRIS Data Representation Model
APPENDIX A - Classes
Union Of Geometry

Abstract Class Name: Union Of Geometry

Superclass - <Aggregate Geometry>

Subclasses

Definition

An instance of this DRM class is an aggregation of <Geometry Representation> instances specifying a standardized mechanism by which to organize the members that compose the union.

Primary Page in DRM Diagram:

Secondary Pages in DRM Diagram:

Example

See concrete subclasses for examples.

FAQs

Consider a data provider with a polygonal representation of the outside of a house, which has four walls and four windows. This particular polygonal representation was developed for a non-Zbuffered system, such that the representation groups the polygons in a fixed order to indicate rendering order. Assuming that the polygons are all grouped together, and that the data provider wishes to represent this house with a <Model> instance, how is the house represented?

The house is represented by a <Model> instance M, such that M has a <Geometry Model> component G and a <Classification Data> component C, where the tag field of C has the value ECC_HOUSE. G has a <Union Of Primitive Geometry> component U.

The ordering_reason field of U is set to SE_ORDREAS_FIXED_LISTED. The lowest priority <Polygon> instances (the walls) are specified first and the higher priority polygons (the windows) are specified last.

What if the house in the previous example were developed for a Zbuffered system?

The polygons would typically be grouped in layers. There are two approaches that could be employed by the modeller depending on how the polygons are grouped.

  1. If all the polygons are one-sided front-facing polygons, the house could be represented by a <Union Of Geometry Hierarchy> instance with ordering_reason = SE_ORDREAS_LAYERED_HIGH_QUALITY_RENDERING with two components. The first component is the base layer (all the walls) and the second component (the first decal layer) contains all the windows. In this case, the polygons under a SE_ORDREAS_LAYERED_HIGH_QUALITY_RENDERING <Union Of Geometry> instance are not coplanar, but the rendering priority can still be resolved.

  2. If the modeller grouped the layers into coplanar unions of <Polygon> instances, the DRM structure might be represented as <Union Of Geometry> instances with four layered <Union Of Geometry> (wall and window) lists the wall first and the window next.

When would a <Union Of Geometry> instance have a <Reference Vector> component?

A <Union Of Geometry> instance U has a <Reference Vector> component only when U represents an environmental object, and then only when the environmental object's representation supplies information appropriate to such representation. An example of this is a building represented by a <Union Of Geometry> instance, which has a <Property Table> component of radar cross-sections, for which a <Reference Vector> instance is needed to establish the zero azimuth direction. (See Example 1).

Constraints

Associated to (one-way) (inherited)

Associated by (one-way) (inherited)

Composed of (two-way) (inherited)

Composed of (two-way)

Composed of (two-way metadata) (inherited)

Component of (two-way) (inherited)

Inherited Field Elements

SE_Boolean unique_descendants; 9
SE_Boolean strict_organizing_principle; 10

Field Elements

SE_Union_Reason union_reason; 11
SE_Ordering_Reason ordering_reason; 12

Notes

Associated to Notes


Feature_Representation

An association between a <Feature Representation> instance and a <Geometry Hierarchy> instance indicates that the environmental object(s) that they represent have the semantic relationship indicated by the <Base Association Data> link object on the association relationship.


Geometry_Hierarchy

An association between two <Geometry Hierarchy> instances indicates that the environmental object(s) that they represent have the semantic relationship indicated by the <Base Association Data> link object on the association relationship.

Associated from Notes


Feature_Representation

An association between a <Feature Representation> instance and a <Geometry Hierarchy> instance indicates that the environmental object(s) that they represent have the semantic relationship indicated by the <Base Association Data> link object on the association relationship.


Geometry_Hierarchy

An association between two <Geometry Hierarchy> instances indicates that the environmental object(s) that they represent have the semantic relationship indicated by the <Base Association Data> link object on the association relationship.


Hierarchy_Summary_Item

An association from a <Hierarchy Summary Item> instance to a <Geometry Hierarchy> instance indicates that the <Hierarchy Summary Item> instance summarizes that <Geometry Hierarchy> instance.


Reference_Surface

An association from a <Reference Surface> instance to a <Geometry Hierarchy> instance indicates that the <Geometry Hierarchy> instance organizes the geometric objects that specify the reference surface geometry of the <Reference Surface> instance.

Composed of Notes


Collision_Volume

In the case where multiple <Collision Volume> components are specified for the <Aggregate Geometry> instance, the union of the volumes thus specified is used in collision detection.


LSR_Transformation

An <Aggregate Geometry> instance can have an <LSR Transformation> component only when the <Aggregate Geometry> instance is within the scope of an LSR 3D SRF.

Fields Notes

unique_descendants

If the value of the unique_descendants field is SE_TRUE, each descendant of this aggregation, that is, each <Geometry Representation> instance that exists in the component tree rooted at the <Aggregate Geometry> instance 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 Representation> instance appears in more than one branch of the aggregation.

strict_organizing_principle

If the value of the strict_organizing_principle field 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.

union_reason

The union_reason field specifies the reason why the <Union Of Geometry> instance was created to organize its components.

ordering_reason

The ordering_reason field specifies whether the ordering of the <Geometry Representation> components of the <Union Of Geometry> instance is semantically significant and, if so, what that significance is.


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Last updated: July 26, 2006 Copyright © 2006 SEDRIS