Special Features

You may choose to use a PNG (in that perfect world) for some of its advanced features that no other graphic offers, such as variable transparency levels and full color management systems for automatic image correction, including gamma and color balance corrections.

Interlacing (Progressive Display)

Like GIFs, PNGs can be encoded for interlaced display. When this option is selected, the image displays in a series of passes, the first displaying after only a portion of the file has been downloaded, and each subsequent pass increasing in detail and clarity until the whole image is rendered.

Interlaced PNGs display over a series of seven passes (using a method known as “Adam7,” named for its creator, Adam Costello). The first rendering of the image appears after only 1/64 of the file has downloaded (that’s eight times faster than GIF). Unlike GIF, which fills in horizontal rows of information, PNGs fill in both horizontally and vertically (the effect looks more like the display of progressive JPEGs). Interlacing can add to the file size of PNGs, especially on small images (which don’t really need to be interlaced anyway). To keep file sizes as small as possible, turn interlacing off.

Gamma Correction

Briefly stated, gamma refers to the brightness setting of a monitor (for more information on gamma, see Chapter 3). Because gamma settings vary by platform (and even by manufacturer), the graphics you create may not look the way you intend. In general, graphics created ...

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