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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 |
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Actual Size
Dithered Image |
 |
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Using black and white to produce the illusion of a mid gray:
Zoomed In
50% Black, 50% White |
|
Actual Size
Dithered Image |
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Using black and white to produce the illusion of a dark gray:
Zoomed In
75% Black, 25% White |
|
Actual Size
Dithered Image |
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Using red and yellow to produce the illusion of a orange:
Zoomed In
50% Red, 50% Yellow |
|
Actual Size
Dithered Image |
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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:
- 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.
- Various methods of dithering have been devised (some of which
may produce less obvious patterning than others). These include:
- 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.
- 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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