Using Images

On many systems, a program encounters a noticeable performance cliff when a certain specific feature (for example depth-buffering) is turned on, or when the number of modes or components exceeds a certain limit.

On Indigo2 IMPACT systems, performance scales with the number of components. For example, on some systems, a switch from RGBA to RGB may not result in a change in performance, while on Indigo2 IMPACT systems, you should expect a performance improvement of 25%. (Note that while this applies to loading textures, it does not apply to using loaded textures.)

Here are some additional hints for optimizing image processing:

• Instead of glPixelMaps(), use the Silicon Graphics color table extension, discussed in "The Color Table Extension", especially when working with GL_LUMINANCE or GL_LUMINANCE_ALPHA images.

  OpenGL requires expansion of pixels using formats other than GL_RGBA to GL_RGBA. Conceptually, this expansion takes place before any pixel operation is applied. Indigo2 IMPACT systems attempt to postpone expansion as long as possible: this improves performance (operations must be performed on all components present in an image--a non-expanded image has fewer components and therefore requires less computation). Because pixel maps are inherently four components, GL_LUMINANCE and GL_LUMINANCE_ALPHA images must be expanded (a different lookup table is applied to the red, green, and blue components derived from the luminance value). However, if the internal format of an image matches the internal format of the color table, Indigo2 IMPACT hardware postpones the expansion, which speeds up processing.

• The convolution extension, discussed in "The Convolution Extension" has been optimized. If possible, use the extension with separable convolution filters.

  Indigo2 IMPACT systems are tuned for 3 x 3, 5 x 5, and 7 x 7 convolution kernels. If you choose a kernel size not in that set, performance is comparable to that of the closest member of the set. For example, if you specify 2 x 7, performance is similar to using 7 x 7.

• Use texture-based zooming instead of glPixelZoom().

  Texture loading and interpolation is fast on Indigo2 IMPACT, and texture-based zooming therefore results in a speed increase and higher-quality, more controllable results.

• Where possible, minimize color table and histogram sizes and the number of color tables activated. If you don't, you may experience performance loss because the two compete for limited resources with other OpenGL applications.