18.5.3 Using Display Lists

You can often improve performance by storing frequently used commands in a display list. If you plan to redraw the same geometry multiple times, or if you have a set of state changes that need to be applied multiple times, consider using display lists. Display lists allow you to define the geometry and/or state changes once and execute them multiple times. Some graphics hardware may store display lists in dedicated memory or may store the data in an optimized form for rendering.

The biggest drawback of using display lists is data expansion. The display list contains an entire copy of all your data plus additional data for each command and for each list. As a result, tuning for display lists focuses mainly on reducing storage requirements. Performance improves if the data that is being traversed fits in the cache. Follow these rules to optimize display lists:

• 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 reference the sphere many times, setting the appropriate material properties and transforms each time.

• Make the display lists as flat as possible, but be sure not to exceed the cache size. Avoid using an excessive hierarchy with many invocations to glCallList(). Each glCallList() invocation requires the OpenGL implementation to do some work (e.g., 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.

• Avoid creating very small display lists. Very small lists may not perform well since there is some overhead when executing a list. Also, it is often inefficient to split primitive definitions across display lists.

• If appropriate, store state settings with geometry; it may improve performance.

  For example, suppose you want to apply a transformation to some geometric objects and then draw the result. If the geometric objects are to be transformed in the same way each time, it is better to store the matrix in the display list.