Using Expensive Modes Efficiently

Using 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

glDisable(GL_FOG)
Minimize expensive mode changes and sort operations by the most expensive mode. Specifically, consider these tips:
- Use small numbers of texture maps to avoid the cost of switching between textures. If you have many small textures, consider combining them into a single larger, tiled texture. Rather than switching to a new texture before drawing a textured polygon, choose texture coordinates that select the appropriate small texture tile within the large texture.
- Avoid changing the projection matrix or changing glDepthRange() parameters.
- When fog is enabled, avoid changing fog parameters.
- Turn fog off for rendering with a different projection (for example, orthographic) and turn it back on when returning to the normal projection.
Beware of excessive mode changes, even mode changes considered cheap, such as changes to shade model, depth buffering, and blending function.