Dithering
Arranging adjacent pixels of different colors into a pattern which simulates colors that are not available to the computer.
When working with a computer display system that supports 8-bit color (or less), the video card can display only 256 different colors at one time. In order to display colors that are not available in the current color palette, dithering can be used to simulate the display of the unavailable colors.
For instance, if you are viewing an image that contains a specific shade of green on a display that does not support that shade, the computer can use dithering to approximate the missing color. By using available colors, pressed closely together, it can visually “blend” colors, in much the same way an artist blends paints to create a shade he or she does not have. Since the human eye can’t see each individual pixel, dithering can result in a highly-accurate color representation.
Also See: Pixel, File Compression
Frequently Asked Questions
Does dithering still happen with today’s higher-bit displays?
Yes, but not as often. Modern computers are capable of displaying millions of colors, so chances are they have the color you’re looking for. However, there are infinite shades of color in the world, so regardless of how many colors a computer can display, there will still be times when dithering is necessary to get exactly the right shade…though, chances are, our eyes wouldn’t notice the difference anyway. Dithering is also commonly used as part of photo conversion. In order to reduce the size of an image file, many compressed file types, such as GIFs, use dithering to reduce the amount of colors they need to include in the data file, thereby reducing the overall size of the file.
How can I tell if dithering is taking place?
Your eye probably can’t tell. However, if you zoom in far enough on an image, you may be able to see the dithering. This is because you are taking each pixel of color and making them larger by spreading them over more pixels. As the dithered part of the image is spread out over enough pixels, the color mixture used will become visible, allowing you to see how the computer blended colors to create the missing shade.
Is dithering used for anything other than pictures?
Computers use dithering for a variety of audio and visual files, in order to fill in gaps for human perception. For example, dithering is commonly used in audio conversion. Since music files on a computer are stored in binary (1s and 0s), they can only approximate the original sound. As music files are compressed, in order to create files that can be downloaded over the Internet or fit on a CD, additional quality is lost, resulting in a file that sounds even less like the original. That’s because reducing the quality of an audio file results in gaps in the music. Dithering can be used to fill those gaps with white noise and other data, smoothing the sound and making it difficult for the human ear to recognize the difference.