When do we need dithering?


  • We can discern approximately 100 brightness levels
    (depends on hue and ambient lighting)
  • True-color displays are usually adequate under normal indoor lighting
    (when the nonlinearities of the display are properly compensated for).
  • High-color displays provide only 32 shades of a each primary.
    Without dithering you will see contours.
  • Made worse by Mach-banding
  • Worse on indexed displays
  • Largest use of dithering is in printed media (newsprint, laser printers)
Lecture 4   Slide 22   6.837 Fall '00




When do we need dithering?

Under fixed lighting conditions the typical person can discern approximately 100 different brightness levels. This number varies slightly with hue (for instance we can see more distinct shades of green than blue). Which particular set of 100 levels also changes as a function of the ambient lighting conditions.

The 256 colors available for each primary in a true color display are usually adequate for representing these 100 levels under normal indoor lighting (when the nonlinearities of the display are properly compensated for). Thus, there is usually no need to dither a true color display.

A high-color display, however, only allows 32 shades of a given primary, and without dithering you will usually be able to detect visible contours between two colors that vary by only one level. Our visual system happens to be particularly sensitive to this, and it even amplifies the variation so that it is more pronounced than the small intensity difference would suggest. This apparent amplification of contours is called Mach-banding, and it is named for the psycho physical researcher who first described it.

On index displays dithering is frequently used to represent color images. Given a 256 entry color map you can only represent approximately 6 colors per red, green, and blue primary (6x6x6=216). However, if just one or two hues are used it is possible to allocate enough color table entries (~50 per hue) so that dithering can be avoided.

By far the largest customer of dithering is in printed media. You probably seen dithering yourselves on newsprint or in the printing on continuous-tone images on a laser printer. In most printing technologies there is very little control of the shade of ink that can be deposited at a particular point. Instead only the density of ink is controlled.