The Umbra 3 Tomes contain a greatly simplified representations of the original scene geometry. Even though the original geometry may consumes hundreds of megabytes of memory, the Umbra Tome should never be larger than a fraction of the original representation. If you are having difficulties with too large Tomes, the following things - listed in order of importance - should help you:
- Set the view volume to be as tight as possible. The view volume should be no larger than the area where the in-game camera is allowed to go so you can safely leave the back-drop geometry, sky domes etc. completely outside the view volume. You can have multiple view volumes in the scene and you can even override the computation parameters on a per view volume basis to further optimize the memory usage.
- Increase the Smallest Occluder computation parameter. This will reduce the memory usage quite a bit. Remember that this is a trade-off with culling quality.
- Reduce the Backface Limit computation parameter. Note that you should be careful not to have any occluder object back-faces visible when you decrease the value.
- Increase the smallest hole computation parameter.