Dithering
   
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    Graphics Glossary

         
    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 can be an effective technique for making an image look as if it has more colors than actually does, there are a number of issues with dithering:
    1. 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.

      There are various methods of dithering available - and which is best in each case depends on the content of image, and of course, personal taste.

      Shown below is a color image that has been converted to monochrome using some different dithering methods:

      Original Image:
      Original image
      Ordered Dithering:
      Image converted to monochrome using Ordered Dithering

      Jarvis Dithering:
      Image converted to monochrome using Jarvis, Judice & Ninke Dithering
      Floyd-Steinberg Dithering:
      Image converted to monochrome using Floyd-Steinberg Dithering

      Stucki Dithering:
      Image converted to monochrome using Stucki Dithering

      Next, we compare reducing the color depth of an image using different approaches. Shown below is a color image that has been converted from True Color to the greatly reduced color depth of color-safe palette using three different methods: no dithering, Ordered Dithering, and Floyd-Steinberg Dithering.

      Color-Safe
      (No dithering):
      Color-safe image
      Color-Safe
      (Ordered Dithering):
      Color-safe image created using Ordered Dithering

      Color-Safe
      (Jarvis Dithering):
      Color-safe image created using Jarvis, Judice & Ninke Dithering
      Color-Safe
      (Floyd-Steinberg Dithering):
      Color-safe image created using Floyd-Steinberg Dithering

      Color-Safe
      (Stucki Dithering):
      Color-safe image created using Stucki Dithering

    2. 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.

    3. 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 True Color, 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 the PNG file format.
    As mentioned above, there are many methods of dithering. These include:

    Discuss Dithering


       

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