Optimizing Per-Fragment Operations on Mid-Range Systems

• Alpha Blending. Mid-range graphics systems support alpha blending in hardware. All primitives can be blended, with the exception of antialiased lines and points, which use the blending hardware to determine pixel coverage. The alpha value is ignored for these primitives. Pixel blends work best in 24-bit, single-buffered RGB mode. In double-buffered RGB mode, the blend quality degrades.

• Dithering. Dithering is used to expand the range of colors that can be created from a group of color components and to provide smooth color transitions. Disabling dither can improve the performance of glClear(). Dithering is enabled by default. To change that, call

  glDisable(GL_DITHER)

• Fog. Mid-range graphics systems do not accelerate per-fragment fog modes. To select a hardware-accelerated fog mode, call

  glHint (GL_FOG_HINT, GL_FASTEST)

• Lighting. Mid-range graphics systems accelerate all lighting features.

• Pixel formats. The GL_ABGR_EXT pixel format is much faster than the GL_RGBA pixel format. For details, see "The ABGR Extension".

  The combinations of types and formats shown in Table 14-2 are the fastest.

  Pixel Formats and Types That Are Fast on Mid-Range Systems

  Format	Type
  GL_RGBA	GL_UNSIGNED_BYTE
  GL_ABGR_EXT	GL_UNSIGNED_BYTE
  GL_COLOR_INDEX	GL_UNSIGNED_SHORT
  GL_COLOR_INDEX	GL_UNSIGNED_BYTE

• Texture Mapping. All texture mapping is performed in software. As a result, textured primitives run with reduced performance.

• Elan Graphics(TM) accelerates depth buffer operations on systems that have depth buffer hardware installed (default on Elan, optional on XS and XS24, not available on Entry systems).

• Fast Clear Operations. The hardware performs combined color and depth clear under the following conditions:

  • depth buffer is cleared to 1 and the depth test is GL_LEQUAL

  • depth buffer is cleared to 0 and the depth test is GL_GEQUAL