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The SEDRIS Data Representation Model
APPENDIX A - Classes Variable |
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An instance of this DRM class is used to connect an <Interface Template> instance to a location within an <Expression> tree where outside control may be exerted.
For a <Variable> instance contained within a <Model>, evaluation is valid only for a specific instance of that <Model>. The value is determined by an <Expression> that is aggregated by the specific <Geometry Model Instance> or <Feature Model Instance>. This <Expression> shall be associated to the same <Interface Template> instance that is associated with the <Variable>.
For a <Variable> instance contained within an <Environment Root>, the evaluation can only be performed within the context of values that shall be supplied by the consuming system.
Consider a source database containing a <Model> with some polygons that reference "table driven texture". Such a polygon is defined as a <Polygon> that has one reference to an <Image> with multiple references to s and t values, which are stored in a <Data Table>. The index into this table is decided at run-time. Each <Polygon> containing a "table-driven texture" has an identifier that is used, along with the index, at run-time.
In SEDRIS, the <Polygon> uses a <Property Table Reference> to index into the <Data Table> containing the different s and t values. Attached to the <Property Table Reference> is a <Property Table Reference Control Link>. The <Variable> attached to this <Property Table Reference Control Link> is ultimately be associated to the <Interface Template> on the <Model>. The original identifier on the <Polygon> shall reside on the <Variable> rather than the <Polygon>, since the <Variable> controls the s,t values.
The following is another example of how a
runtime_label on a
<Variable> would be used.
On moving models there might be polygons that have IR values attached
to them. Consider a tank <Model> in such a database.
In SEDRIS, the IR values are stored in <Property Table> instances. The <Polygon>s of the tank reference these elements through a <Property Table Reference>. A <Property Table Reference Control Link> is used for those <Polygon> instances that have "heat producing" capability (for instance, the gun barrel), so that the index into the IR table can be changed to other values as the gun is used and heats up. More than one <Polygon> will contain the same ID. The consumer shall be able to identify the <Polygon> instances (through the <Control Link> and <Interface Template> instances) by this ID so that he can switch these <Polygon> instances to be "heat producing".
So that consumers can control what value is being plugged in, depending on how it is identified. See examples 1, 2 for how this is used and why it is needed.
The meaning field specifies the quantity represented by the given <Variable> instance.
The value_unit field specifies the unit of measurement of the quantity represented by the given <Variable> instance.
The value_scale field specifies the scale applicable to value_unit.
The value_type field specifies the value type of the given <Variable> instance.
The description field specifies a meaningful explanation of the purpose of the given <Variable> instance.
The runtime_label field is used for <Variable> instances that consumers need to identify which represent run-time flags, provided so that appropriate values can be "plugged in", which then affect any <Control Link> instances driven by such a <Variable> instance. If a <Variable> instance does not need a runtime_label, the field is set to the empty string.
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