Web Design in a Nutshell, 3rd Edition

Progressive Display (Interlacing)

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