Hello. There is mention in the specification of additive vs. exchange behavior for individual OpenFlight models but we haven't seen anything that mentions how CDB rendering, in general, should behave. Should a client use an additive or exchange approach when rendering tiled datasets by LOD? We have seen CDB samples where it could make sense either way. Maybe there is something in a metadata field that indicates the desired behavior?

Thanks much.

Hi,

Regarding OpenFlight 3D models, the CDB specification supports both additive and exchange LODs. The choice to use one or the other or a combination of both is up to the database builder and modeler to make. Client application must support both methods of rendering OpenFlight models and leverage the Significant Size information to manage transitions between LODs. CDB spec 3.2 section 6.8.1 and 6.8.2 define the use of exchange and additive LODs in OpenFlight models.

Note that the vertex count constraint per LOD imposed on models (128 vertex on first LOD, 512 on second etc…) will limit your ability to have exchange/additive LODs on coarser model representation. However, in finer versions of the models, those LOD nodes in the OpenFlight file allow you to have a finer control (via SigSize) over the LOD geometry selection at rendering time.

Now, about tiled datasets, you should consider to exchange tiles at LOD n with tiles at LOD n+1. This is a general principle that applies to all CDB tiled datasets. You always exchange coarse tiles with finer tiles.

So, to recap, the GSFeature dataset is tiled and is “exchanged” between coarser and finer representation. Within a GSFeature tile, at a given LOD, the client uses the reference to the OpenFlight Model and the MLOD field to load the right OpenFlight mode. The client devise can then use the SigSize value on the LOD nodes to trigger additive/exchange LODs in the geometry.

If this does not answer your question, please be more specific on the type of information you would expect from the spec.

Regards,

Herm

This does indeed answer my question. Many thanks Herm.