Definition

The number of <Image Mapping Function> instances referenced by a <Geometry> instance shall equal the number of <Texture Coordinate> components for each <Base Vertex> and <Tack Point> within the object tree rooted at that <Geometry>.

<Image Mapping Function> instances referenced by <Feature> instances, on the other hand, shall either have <Image Anchor> components, or reference <Image> instances that have <Image Anchor> components.

EXCEPTION:
If an <Image Mapping Function> is used to specify a non-planar projection (e.g., spherical, cylindrical), it shall use an <Image Anchor> component, and the <Geometry> to which the <Image Mapping Function> is attached cannot have <Texture Coordinate> or <Tack Point> instances within its component tree.

Rationale

The multiple <Image Mapping Function> and multiple <Texture Coordinate> instances are ordered, and are defined to correspond to each other as if they were in parallel arrays.

<Image Mapping Function> instances referenced by a <Feature> instance are attributes for geometry that is to be derived by the consumer for the <Feature>. Since <Texture Coordinate> and <Tack Point> instances are not applicable to <Feature> instances, such <Image Mapping Function> instances shall be specified with <Image Anchor> components.

Example

1. Consider a triangular <Polygon> with one <Image Mapping Function> for the OTW (out-the-window) domain. Each of the <Polygon>'s 3 <Vertex> components has one <Texture Coordinate>, specifying the (s,t) within the image space that will be mapped to that <Vertex>.

   

2. Consider a triangular <Polygon> that has different texture maps, one for OTW and one for thermal. The <Polygon> thus has 2 ordered <Image Mapping Functions>, so each of its <Vertex> components will have 2 ordered <Texture Coordinate> components, one for each <Image Mapping Function>.

   

3. See <Texture Coordinate Table> for examples of how to use <Image Mapping Functions> with <Texture Coordinates> specified in tables.

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