openGL and JOGL

Colormode: RGBA versus Color-Index Mode

[IMAGE]

RGBA Mode

The R, G, and B values are typically stored as scaled integers.
If a system has 8 bits available for the R component, integers between 0 and 255 can be stored; thus, 0, 1, 2, ..., 255 in the bitplanes would correspond to R values of 0/255 = 0.0, 1/255, 2/255, ..., 255/255 = 1.0. Regardless of the number of bitplanes, 0.0 specifies the minimum intensity, and 1.0 specifies the maximum intensity.

The alpha value (the A in RGBA) has no direct effect on the color displayed on the screen. It can be used for many things, including blending and transparency, and it can have an effect on the values of R, G, and B that are written.

Color-Index Mode

With color-index mode, OpenGL uses a color map (or lookup table), which is similar to using a palette to mix paints to prepare for a paint-by-number scene.

A computer stores the color index in the bitplanes for each pixel. Then those bitplane values reference the color map, and the screen is painted with the corresponding red, green, and blue values from the color map

In color-index mode, the number of simultaneously available colors is limited by the size of the color map and the number of bitplanes available. The size of the color map is determined by the amount of hardware dedicated to it. Typical sizes range from 256 (2^8) to 4096 (2^12). The size of the color map is a power of 2, indexed by the number of bitplanes available in color-index mode. If there are 2n indices in the color map and m available bitplanes, the number of usable entries is the smaller of 2n and 2m.

RGBA versus Color-Index

With RGBA mode, each pixel's color is independent of other pixels. However, in color-index mode, each pixel with the same index stored in its bitplanes shares the same color-map location. If the contents of an entry in the color map change, then all pixels of that color index change their color.

You might prefer to use color-index mode in the following cases:

If you have only a small number n of bitplanes available and if you need fewer than 2n different colors, you should consider color-index mode.
If you have a limited number of bitplanes available, RGBA mode might produce noticeably coarse shading.
Color-index mode can be useful for various tricks, such as color-map animation and drawing in layers.
When porting an existing application that makes significant use of color-index mode.

In general, use RGBA mode: It works with texture mapping and works better with lighting, shading, fog, antialiasing, and blending.