Advanced Transform-Limited Tuning Techniques

• Use texture to replace complex geometry.

  Textured polygons can be significantly slower than their non-textured counterparts. However, texture can be used instead of extra polygons to add detail to a geometric object. This can greatly simplify geometry, resulting in a net speed increase and an improved picture, as long as it does not cause the program to become fill-limited. Texturing performance varies across the product line, so this technique might not be equally effective on all systems. Experimentation is usually necessary.

• Use glAlphaFunc() in conjunction with one or more textures to give the effect of rather complex geometry on a single polygon.

  Consider drawing an image of a complex object by texturing it onto a single polygon. Set alpha values to zero in the texture outside the image of the object. (The edges of the object can be antialiased by using alpha values between zero and one.) Orient the polygon to face the viewer. To prevent pixels with zero alpha values in the textured polygon from being drawn, call:

  glAlphaFunc(GL_NOTEQUAL, 0.0)

  This effect is often used to create objects like trees that have complex edges or many holes through which the background should be visible (or both).

• Use culling on a separate processor to eliminate objects or polygons that will be out of sight or too small to see.