Q: How do I make a new type of layer?

First one must understand what a layer is before they can understand how to make one:

A layer in uDig is (very generally) a set of IGeoResources which are handles for the same "real" resource. An example is a WFS and a WMS may be backed onto the same data. As a result both IGeoResources would be members of the layer's set of IGeoResources.

So, to define a new type of layer one must create a new IGeoResource (and IService since IServices contain the IGeoResources). For example a Web Terrain layer would require a Web Terrain IService and IGeoResource.

For more information on the Catalog API see the Catalog Overview in the Programmer's Guide.

However once a new layer type is created it does not automatically get rendered unless one of its IGeoResources resolves to a DataStore, GridCoverage or a WebMapServer. If one of the IGeoResources does resolve to one of those objects then you are done and the layer will render. If not then a new Renderer must be developed. For more information on the Renderer API please look at the
Renderers page.