Colour Table Group

The SEDRIS Data Representation Model APPENDIX A - Classes Colour Table Group

Class Name: Colour Table Group

This DRM class is concrete and has no subclasses.

An instance of this DRM class specifies an interchangeable group of one or more <Colour Table> instances. The primary <Colour Table> instance in the group is indicated by the primary_table_index. When a reference is made to a <Colour Table> instance (for example, from a <Colour Index> component of a <Polygon> instance), the reference identifies the <Colour Table Group> instance and the index within the <Colour Table> instance, but not the specific <Colour Table> instance within the <Colour Table Group> instance. By definition, a <Colour Index> instance refers to a <Primitive Colour> instance from the primary <Colour Table> instance of the indicated <Colour Table Group> instance. An alternate <Colour Table> instance from within the <Colour Table Group> instance can be chosen at the discretion of the data consumer.

Consider a <Colour Table Group> instance with two <Colour Table> components, where one <Colour Table> component is for normal, OTW viewing, and the other <Colour Table> component exists to change the appearance of the view to represent a view as seen through night vision goggles.
Consider a <Colour Table Group> instance with three <Colour Table> components with the same usage of OTW. One <Colour Table> instance defines the colours as originally created by the data modellers. The second <Colour Table> instance has different shades of blue for the lakes and skies because a company executive came through and did not like the blues that were there. The third <Colour Table> instance contains yet another set of blues for the lakes, and different shades of green for the trees and tanks, because the customer in charge of the program came through and did not think the colours were realistic. In this particular example, it is left as an exercise to the data provider to determine which <Colour Table> component should be the primary <Colour Table> component for the group.

Can a transmittal in any way refer to a <Colour Table> instance component of a <Colour Table Group> instance other than the primary <Colour Table> component?: No. The only <Colour Table> component that can be referenced of any <Colour Table Group> instance is the primary <Colour Table> instance.
Since they cannot be referenced, why bother to have alternate <Colour Table> instances within a <Colour Table Group> instance?: Because in real life, many run-time systems do have multiple colour tables they can switch between for various reasons, and these tables should be shared to promote interoperability. See the example section, below.