Dithering
   
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GraphicsAcademy.com  >  Glossary  >  Dithering

Dithering

Dithering is an imaging technique which allows a bitmap (or a bitmapped device, such as a screen or printer) to appear to display more colors than are actually possible.
  • If the bitmap (or bitmapped device) only allows black and white, dithering can be used to produce an illusion of gray.

  • If the bitmap (or bitmapped device) allows a palette of colors, then dithering can produce an illusion of more colors being displayed than are in the palette.
Let's look at some examples (on the left I show a zoomed in section of bitmap, on the right I show a larger section of bitmap at actual size.

Using black and white to produce the illusion of light gray:

Zoomed In
25% Black, 75% White
  Actual Size
Dithered Image
   

Using black and white to produce the illusion of a mid gray:

Zoomed In
50% Black, 50% White
  Actual Size
Dithered Image
   

Using black and white to produce the illusion of a dark gray:

Zoomed In
75% Black, 25% White
  Actual Size
Dithered Image
   

Using red and yellow to produce the illusion of a orange:

Zoomed In
50% Red, 50% Yellow
  Actual Size
Dithered Image
   

While dithering is an effective technique for making an image look as if it has more colors, there are a number of issues with dithering:
  1. Some methods of dithering can produce the illusion of patterns within the colored areas. This can be a significant problem when dithering images containing naturalistic images (for example, photographs), and tends to be particularly noticeable when dithering is used on relatively low resolution devices.

  2. Various methods of dithering have been devised (some of which may produce less obvious patterning than others). These include:


  3. Once an image has been dithered, it can be extremely hard to edit.

    For example, if we had an image which contained areas that had been dithered (like the examples above) to produce orange and various shades of gray, there is no simple way to adjust the color of those areas, because each area contains a mix of pixels of different colors.

    As a result, in an ideal world, images would be prepared and stored in full color, and then only dithered when displayed/printed on a device which did not support all the necessary colors. However, in the real world, the limitations of networks, storage, memory, and graphics file formats, frequently means that despite this significant disadvantage, images are often prepared and stored already dithered.

  4. Dithered images contain lots of fine detail that may not compress well, particularly in file formats that use LZW or Run-length Compression.

    As a result, in some cases, it may actually take less storage and bandwidth to store and transmit images in full TrueColor, and allow the output device to perform any necessary dithering upon receiving this image. An example where this may occur is if web publishing images in PNG format.

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