Dithering selects between two color values or indices.
In RGBA mode, consider the value of any of the color
components as a fixed-point value with m bits to the
left of the binary point, where m is the number of bits
allocated to that component in the framebuffer;
call each such value c.
For each c,
dithering selects a value 
such that
(after this selection, treat 
as a fixed point value in [0,1] with
m bits).
This selection may depend on the 
and 
coordinates of the pixel.
In color index mode,
the same rule applies with c being a single color index.
c must not be larger than the maximum value representable
in the framebuffer for either the component or the index, as appropriate.
Many dithering algorithms are possible, but a dithered value produced by any algorithm must depend only the incoming value and the fragment's x and y window coordinates. If dithering is disabled, then each color component is truncated to a fixed-point value with as many bits as there are in the corresponding component in the framebuffer; a color index is rounded to the nearest integer representable in the color index portion of the framebuffer.
Dithering is enabled with Enable and disabled with Disable using the symbolic constant DITHER. The state required is thus a single bit. Initially, dithering is enabled.