CPU Tuning: Display Lists

CPU Tuning: Display Lists

In display-list mode, pieces of the database are compiled into static chunks that can then be sent to the graphics pipeline. In this case, the display list is a separate copy of the database that can be stored in main memory in a form optimized for feeding the rest of the pipeline. The database traversal task is to hand the correct chunks to the graphics pipeline. Display lists can be recreated easily with some additional performance cost.

Tuning for display lists focuses mainly on reducing storage requirements. Performance improves if the data fit in the cache because this avoids cache misses when the data is retraversed.

Follow these rules to optimize display lists:

If possible, compile and execute a display list in two steps instead of using GL_COMPILE_AND_EXECUTE.
Call glDeleteLists() to delete display lists that are no longer needed.
This frees storage space used by the deleted display lists and expedites the creation of new display lists.
Avoid duplication of display lists.
For example, if you have a scene with 100 spheres of different sizes and materials, generate one display list that is a unit sphere centered about the origin. Then for each sphere in the scene, follow these steps:
1. Set the material for the current sphere.
2. Issue the necessary scaling and translation commands for sizing and positioning the sphere.
3. Invoke glCallList() to draw the unit sphere display list.
  In this way, a reference to the unit sphere display list is stored instead of all of the sphere vertices for each instance of the sphere.
Make the display list as flat as possible, but be sure not to exceed the cache size.
Avoid using an excessive hierarchy with many invocations of glCallList(). Each glCallList() invocation causes a table lookup to find the designated display list. A flat display list requires less memory and yields simpler and faster traversal. It also improves cache coherency.

On the other hand, excessive flattening increases the size. For example, if you're drawing a car with four wheels, having a hierarchy with four pointers from the body to one wheel is preferable to a flat structure with one body and four wheels.

Display lists are best used for static objects. Do not put dynamic data or operations in display lists. Instead, use a mixture of display lists for static objects and immediate mode for dynamic operations.

Note: See Chapter 14, "System-Specific Tuning," for potential display list optimizations on the system you are using.