Optimizing the Data Organization

Optimizing the Data Organization

It is common for scenes to have hierarchical definitions. Scene management techniques may rely on specific hierarchical information. However, a hierarchical organization of the data raises several performance concerns:

The time spent traversing pointers to different sections of a hierarchy can create a CPU bottleneck.
This is partly because of the number of extra instructions executed, but it is also a result of the inefficient use of cache and memory. Overhead data not needed for rendering is brought through the cache and can push out needed data, causing subsequent cache misses.
Traversing hierarchical structures can cause excessive memory paging.
Hierarchical structures can be distributed throughout memory. It is difficult to be sure of the exact amount of data you are accessing and where it is located; traversing hierarchical structures can therefore access a costly number of pages.
Complex operations may need access to both the geometric data and other scene information, complicating the data structure.
Caching behavior is often difficult to predict for dynamic hierarchical data structures.

For these reasons, hierarchy should be used with care. In general, store the geometry data used for rendering in static, contiguous buffers, rather than in the hierarchical data structures.

Flatten your rendering data (minimize the number of levels in the hierarchy) as much as cache and memory considerations and your application constraints permit.
This is an example of a decision that should be guided by the choice of system on which your application will run. For system-specific tuning information, see Chapter 14, "System-Specific Tuning."
Balance the data hierarchy. This makes application culling (the process of eliminating objects that don't fall within the viewing frustum) more efficient and effective.