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Chapter 22. Saving Images for the Web > Image Formats for the Web

Image Formats for the Web

Of the hundreds of file formats available for recording digital information, only a few are appropriate for graphics on the Web. The two most common are JPEG and GIF. Improved compatibility with the latest Web browsers from Microsoft and Netscape has allowed the PNG format to be more widely used, but it is still not universally supported. Other major formats require that a plug-in be available to the browser in order for a graphic to be properly displayed. Flash (SWF) (discussed in Chapter 23, “Flash and SVG Support”) and PDF are two examples.

GIF

GIF (Graphical Interchange Format), also known as CompuServe GIF and in a variation as GIF89a, is an 8-bit color format designed for illustration-type graphics. This format excels at storing and reproducing images with large areas of solid color but, because it is 8-bit, is limited to 256 colors. GIF does not do a good job with continuous tone images, such as photographs, having to reduce them to the smaller number of colors. Areas in an image that should blend smoothly from one color to another become banded or dithered. Banding occurs when a large number of colors across an area must be reduced to a smaller number. The colors are then grouped into stripes of a color that is available in the reduced palette. Dithering is the technique of applying a pattern of two colors to represent a third, intermediate color. See Figure 22.2 and Figure 22.3 for examples of banding and dithering. Banding can be controlled, to a large degree, with dithering. See “Types of Dithering” later in this chapter. GIF is available in Illustrator through the command Save for Web and will be discussed further in that section of this chapter.


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