18.3.1 Use Expensive Modes Efficiently

OpenGL offers many features that create sophisticated effects with excellent performance. However, these features have some performance cost, compared to drawing the same scene without them. Use these features only where their effects, performance, and quality are justified.

• Turn off features when they are not required. Once a feature has been turned on, it can slow the transform rate even when it has no visible effect.

  For example, the use of fog can slow the transform rate of polygons. When the polygons are too close to show fog, or when the fog density is set to zero, turn off fog explicitly with glDisableGL_ FOG(GL_ FOG).

• Minimize mode changes. Be especially careful about expensive mode changes such as changing glDepthRange() parameters and changing fog parameters when fog is enabled.

• For optimum performance of most software renderers and many hardware renderers as well, use flat shading. This reduces the number of lighting computations from one per-vertex to one per-primitive, and also reduces the amount of data that must be processed for each primitive. Keep in mind that long triangle strips approach one vertex per primitive and may show little benefit from flat shading.