Controlling dither

You may have noticed that certain areas of the image appear mottled or spotty when optimized with different color palettes and numbers of colors. This spotty appearance results from dithering, the technique used to simulate the appearance of colors that are not included in the color palette. For example, a blue color and a yellow color may dither in a mosaic pattern to produce the illusion of a green color that is not included in the color palette.

When optimizing images, keep in mind the two kinds of dithering that can occur:

• Application dither occurs when ImageReady or Photoshop attempts to simulate colors that appear in the original image but not in the optimized color palette you specify. You can control the amount of application dither by dragging the Dither slider in the Optimize palette.

• Browser dither can occur in addition to application dither. A browser dither occurs when a Web browser using an 8-bit (256-color) display simulates colors that appear in the optimized image's color palette but not in the system palette used by the browser. You can control the amount of browser dither by shifting selected colors to Web-safe colors in the Color Table palette.

In the Photoshop Save For Web dialog box and in ImageReady, you can view application dither directly in an optimized image. You can also preview the additional browser dither that will appear in the final image when viewed in a browser using an 8-bit display.

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