16.3.1 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 even when the polygons are too close to show fog, and even when the fog density is set to zero. For these conditions, 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, use flat shading whenever possible. 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. This is particularly important for high-performance line drawing.