20.4. Progressive JPEGs
Progressive JPEGs are just like ordinary JPEGs except they display in a series of passes (like interlacing in the GIF format), each pass containing more detailed information, until the whole image is rendered clearly. Graphics programs allow you to specify the number of passes it takes to fill in the final image (3, 4, or 5 scans). Bear in mind that over a fast Internet connection, the image may load and render so quickly the user may not see any passes at all.
One advantage to using Pro-JPEGs is that like interlaced GIFs, they provide some indication of the full image for the reader to look at without having to wait for the entire image to download. Progressive JPEGs are also generally slightly smaller than standard JPEG files.
One disadvantage to Progressive JPEGs is that they require more processing power to display. The higher the specified number of passes, the more power it takes the user's machine to render them.
The other disadvantage is that they are not supported on older browser versions. Netscape Navigator 2.0 and Internet Explorer 2.0 display Pro-JPEGs inline but may not support the progressive display. Pro-JPEGs are fully supported by Versions 3.0 and higher of both Netscape and MSIE. If a browser cannot identify a Pro-JPEG, it displays a broken graphic image.
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