External references are what allow a project to make reference to files. At their most basic, they are simple a URL to a file, but it certain situations they can perform more advanced functions.

External references have (at least) the following attributes:

• URL – The location of the file being referenced. This value is always made relative to the location of the relevant resource directory. For example: if the file ‘preview.jpg’ was at the location C:\Projects\resources\previews\, and the resource directory was set to C:\Projects\resources\, the URL for the external reference would be ‘previews\preview.jpg’.
• Preview URL – This value is largely obsolete, but was originally intended to be used when specifying a preview image for a product or option. Now, it is preferred to use a separate external reference, and use ‘NavigationImage’ as it’s usage.
• Usage – Indicates how the file should be used. Includes a type and a quality. Quality can be ‘Low’, ‘Medium’, and ‘High’. In the case that multiple quality files are referenced, CET will attempt to use the most appropriate quality. Accepted reference types are:
  • 3DView – When the file represents a 3D model.
  • PlanView – When the file represents a 2D model.
  • 3DAndPlanView – When the file represents a 3D and 2D model.
  • SwatchImage – Used by materials to reference a swatch file.
  • NavigationImage – Used by products and options to represent a preview image.

A more advanced feature of external references becomes available when they are placed on products and options. A model referenced in one of these ways may also specify a placement for the model.

The placement can describe the model’s location, relative to the symbol’s zero-point, the scale of model in all three dimensions, and the orientation of the model. The orientation is described in both a rotation vector and a magnitude of rotation.

Additionally, an external reference on an option may defer placement of its model to a location specified by the product. This is accomplished by use of insertion IDs and named points. First, external references placed on a product may specify both an insertion ID and any number of named points. An insertion ID is just text, and named points are just a combination of a name and a set of coordinates. An external reference placed on an option may choose, instead of a normal placement, to make reference to a location specified by a named point on the product. This is done by specifying the matching insertion ID and named point name on the external reference.