Dithering
The Voodoo Graphics hardware represents color internally as 32-bit quadruplets in a format specified by the color format argument passed to grSstWinOpen() (see Chapter 3). This color is eventually dithered to 16-bit RGB for storage in the frame buffer, then expanded and (optionally) filtered up to 24-bits for final display. From an application's perspective, the 32-to-16-bit RGB dithering operation is transparent.
Definition : Dithering
Dithering is a technique for increasing the perceived range of colors in an image by applying a pattern to surrounding pixels to modify their color values. 
When viewed from a distance, these colors appear to blend into an intermediate color that can’t be represented directly. Dithering is similar to the half-toning used in black and white publications to produce shades of gray.
void grDitherMode( GrDitherMode_t mode ) 
grDitherMode() selects the form of dithering the Voodoo Graphics subsystem uses when converting 24-bit RGB values to the 16-bit RGB color buffer format.
Valid values are
  • GR_DITHER_DISABLE,
  • GR_DITHER_2x2, and
  • GR_DITHER_4x4.
    • GR_DITHER_DISABLE forces a simple truncation that may result in noticeable banding.
    GR_DITHER_2x2 uses a 2x2 ordered dither matrix, and
    GR_DITHER_4x4 uses a 4x4 ordered dither matrix.

    The default dithering mode is GR_DITHER_4x4.

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