First and foremost, any transmittal submitted to Model Viewer as input must pass both Syntax Checker and Rules Checker. If an invalid transmittal is submitted as input, Model Viewer will abort the loading process with an appropriate error message if the errors are fatal in regard to the <Image Library> or <Model Library>.

In a similar vein, if inter-transmittal referencing (ITR) is used in the input transmittal's <Image Library> or <Model Library>, then all transmittals referenced in the context of the <Image Library> or <Model Library> must be provided. For example, if the user specified that <Model> instances were to be loaded and processed, but some of the <Model> instances in the <Model Library> were unavailable due to missing transmittals, the application would abort during the loading process.

If a <Model Library> ML is provided and the user specifies that <Model> instances are to be viewed, then ML must satisfy the following requirements.

1. ML shall contain at least one "root" <Model>, that is, a <Model> instance whose model_reference_type is either SE_MODREFTYP_ROOT or SE_MODREFTYP_ROOT_AND_COMPONENT.

   RATIONALE: Only root <Model> instances can be instanced as free-standing objects, so only root <Model> instances are guaranteed to make some kind of visual sense. Only root <Model> instances, consequently, are displayed by Model Viewer; component <Model> instances appear only in the context of their usage within other <Model> instances.

2. Every <Model> component within ML shall be defined within a 3D LSR spatial reference frame.

3. Every <Model> within ML shall contain a non-empty <Geometry Model> component. (<Feature Model> component are currently ignored, if present; only renderable geometry is displayed.)

4. All <Primitive Colour> instances in use shall have either an <Ambient Colour> component or a <Diffuse Colour> component. <Specular Colour> instances and <Emissive Colour> instances are currently ignored.

Only the following image signatures are supported: LUMINANCE, ALPHA, LUMINANCE_AND_ALPHA, 123COLOUR, COLOUR_COORDINATE_1, COLOUR_COORDINATE_2, and COLOUR_COORDINATE_3.

Mipmapped images, to be used for mipmapping, must comply with OpenGL's restrictions on mipmapped image dimensions. Noncompliant images will be treated as though they had only a single MIP level.

Currently, Model Viewer's texture mapping supports only <Image Mapping Function> instances defined in conjunction with <Texture Coordinate> instances, and consequently only planar projection. Expanded support will be phased in in future; currently such support has a low priority since other varieties of texture mapping have not been in demand by SEDRIS' user base.

• Only polygonal geometry is currently supported. <Linear Geometry>, <Volume Geometry>, and <Finite Element Mesh> instances are ignored when <Model> instances are loaded by Model Viewer, as are <Feature Representation> instances and <Property Grid Hook Point> instances.

• Currently, only <Rotation Control Link> instances can be controlled by the user; other <Control Link> instances in the <Model> instances being viewed are treated as static (i.e., the default values provided in the transmittal are used and cannot be manipulated by the user).

• For Level of Detail, only the distance organizing principle can be controlled by the user.

• Only Level of Detail and State controls are provided for the user.