Optimizing Images for the Web > Previewing and controlling dithering
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Previewing and controlling dithering


Most images viewed on the Web are created using 24-bit color displays (millions of colors mode), but many Web browsers are used on computers using only 8-bit color displays (256-color mode), so that Web images often contain colors not available to many Web browsers. Computers use a technique called dithering to simulate colors not available in the color display system. Dithering creates adjacent pixels of different colors to give the appearance of a third color. For example, a red color and a yellow color may dither in a mosaic pattern to produce the illusion of an orange color that does not appear in the color palette.

When optimizing images, keep in mind that two kinds of dithering can occur:
  • Application dither occurs in GIF and PNG-8 images when Photoshop Elements attempts to simulate colors that appear in the original image but not in the color lookup table. You can choose a dithering pattern to be applied to the image. In addition, you can create customized dither patterns for GIF or PNG-8 images using the DitherBoxTM filter. (See Creating and applying custom dither patterns.)
  • Browser dither occurs when a Web browser using an 8-bit color display (256-color mode) attempts to simulate colors that appear in an optimized image but not in the color palette used by the browser. Browser dither can occur with GIF, PNG, or JPEG images and can occur in addition to application dither in GIF or PNG-8 images. You can control the amount of browser dither by shifting selected colors in the image to Web-safe colors. Options in the color picker let you specify Web-safe colors when choosing a color.

    • Optimizing Images for the Web > Previewing and controlling dithering

    Related subtopics:

    Previewing and controlling application dither
    Previewing browser dither
    Minimizing browser dither
    Creating and applying custom dither patterns