Since 24-bit color has a palette of millions of colors and 8 and 16-bit color
have much less, the 24-bit image must be dithered when it is viewed on a system
equipped with 8 or 16-bit color. Dithering simulates the colors from the original
24-bit image that are not available in the reduced palette of the 8 or 16-bit
system. Dithering creates smooth, natural looking gradations of color, especially
on photographic images, rather than the sharp edges that can result from a reduction
of palette colors.
The graphic below illustrates the concept of dithering. The green color in
the rectangle on the 24-bit color photograph at the left does not exist in the
256 color palette in the computer on which the photograph will be viewed. The
color must be simulated with the use of colors that are part of the computer
system's color palette. In the picture below, the computer uses dots of a blue
color and a yellow color from its palette and places the colored dots adjacent
to each other. The dots are so small and close together that the eye will not
focus on the individual dots, but will merge them, and see the area as being
the green color shown in the photograph. The dots being so small and close together
also creates a larger file size because instead of storing blocks of solid colors,
the file will contain hundreds of different colored dots in the dithered areas,
which causes the file to be much larger.
The rectangle on the right is a close-up of the corresponding
in the photograph showing the colors that were used to simulate
the green color.