The OpenGL Graphics System: A Specification

3.9. COLOR SUM 191
results of texture blending are undefined.
The required state, per texture unit, is four bits indicating whether each of one-,
two-, three-dimensional, or cube map texturing is enabled or disabled. In the intial
state, all texturing is disabled for all texture units.
3.9 Color Sum
At the beginning of color sum, a fragment has two RGBA colors: a primary color
cpri (which texturing, if enabled, may have modified) and a secondary color csec.
If color sum is enabled, the R, G, and B components of these two colors are
summed to produce a single post-texturing RGBA color c. The A component of c
is taken from the A component of cpri; the A component of csec is unused. The
components of c are then clamped to the range [0, 1]. If color sum is disabled, then
cpri is assigned to c.
Color sum is enabled or disabled using the generic Enable and Disable commands,
respectively, with the symbolic constant COLOR SUM. If lighting is enabled
and if a vertex shader is not active, the color sum stage is always applied, ignoring
the value of COLOR SUM.
The state required is a single bit indicating whether color sum is enabled or
disabled. In the initial state, color sum is disabled.
Color sum has no effect in color index mode, or if a fragment shader is active.
3.10 Fog
If enabled, fog blends a fog color with a rasterized fragment’s post-texturing color
using a blending factor f. Fog is enabled and disabled with the Enable and Disable
commands using the symbolic constant FOG.
This factor f is computed according to one of three equations:
f = exp(−d · c), (3.30)
f = exp(−(d · c) 2 ), or (3.31)
f =
e − c
e − s
(3.32)
If a vertex shader is active, or if the fog source, as defined below, is FOG COORD,
then c is the interpolated value of the fog coordinate for this fragment. Otherwise,
if the fog source is FRAGMENT DEPTH, then c is the eye-coordinate distance from
Version 2.0 - October 22, 2004