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The SEDRIS Data Representation Model
APPENDIX A - Classes Aggregate Geometry |
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An instance of a concrete class derived from this DRM class specifies a collection of <Primitive Geometry> or <Geometry Hierarchy> instances, organized according to some organizing principle specific to the particular subclass of <Aggregate Geometry> being considered.
For most subclasses of <Aggregate Geometry>, each individual branch of the aggregation is explicitly identified by a distinct link object. The various subclasses provide different mechanisms for organizing <Geometry Representation> instances, which include:
Each branch is a <Geometry Hierarchy> instance with a different <Hierarchy Data> instance, representing a different way of organizing the same underlying collection of <Geometry Representation> instances.
Each branch is a <Geometry Hierarchy> instance representing a different frame in an animation sequence. This organizing principle has no counterpart on the <Feature Representation> side.
Each branch is a <Geometry Hierarchy> instance with a different <Classification Data> instance, representing either a different thematic layer, or a different classification of <Geometry Representation> instances (for example, roads, railroads) within a single thematic layer.
Each branch is either a <Union Of Primitive Geometry> instance (usually a collection of <Polygon> instances), or a set of fragmented <Polygon> instances that represent the terrain at a finer level of detail at close range (or alternatively, with a coarser level of detail at long range). This mechanism is used to represent continuous terrain or continuous adaptive terrain. This organizing principle has no counterpart on the <Feature Representation> side.
Each branch is a <Geometry Hierarchy> instance with a different (but possibly overlapping) <Base LOD Data> instance, representing an alternative that shall be used at a specified level of detail.
The <Aggregate Geometry> represents an octant, in which each branch is a <Geometry Hierarchy> representing an octant as identified by its <Octant Data> instance.
Each branch is a <Geometry Hierarchy> that is located within a different cell of an irregular spatial index, as defined by its <Perimeter Data> instance.
The <Aggregate Geometry> represents a quadrant, in which each branch is a <Geometry Hierarchy> representing a quadrant, as identified by its <Quadrant Data> instance.
Each branch is a <Separating Plane Relations>, each branch of which is a <Geometry Hierarchy> instance that is on either the positive or negative side of the associated <Separating Plane>, as indicated by the <Separating Plane Data> instance for that branch. This organizing principle has no counterpart on the <Feature Representation> side.
The <Aggregate Geometry> represents a regularly spaced spatial index grid, in which each branch is a <Geometry Hierarchy> instance representing a different cell within the spatial index.
The <Aggregate Geometry> represents something that can take on different state values for a specified EDCS Attribute Code (the state_tag, which shall have the "state-related" property). Each branch is a <Geometry Hierarchy> instance with a different <State Data> instance, representing an alternative that shall be used to represent the <Aggregate Geometry> when it takes on the specified state value.
The <Aggregate Geometry> represents something that has different representations for different time periods, so that each branch is a <Geometry Hierarchy> instance with a distinct (though possibly overlapping) <Time Constraints Data> instance, representing an alternative that shall be used for the time period specified by its <Time Constraints Data>.
Each branch is a <Geometry Hierarchy> instance. The reason for organizing them into separate components is only minimally specified. This mechanism's counterpart on the <Feature Representation> side is <Union Of Features>.
Each branch is a <Primitive Geometry> instance; <Primitive Geometry> can be included in a transmittal only by means of <Union Of Primitive Geometry> instances. This mechanism's counterpart on the <Feature Representation> side is <Union Of Features>.
See concrete subclasses for examples.
| SE_Boolean | unique_descendants; | (notes) |
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| SE_Boolean | strict_organizing_principle; | (notes) |
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> instance on the association relationship.
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> instance on the association relationship.
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> instance on the association relationship.
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> instance on the association relationship.
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 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 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 Representation> instance appears in more than one branch of the aggregation.
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.
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