Adam7 algorithm

Adam7 is an interlacing algorithm for raster images, best known as the optional interlacing scheme of the PNG image format. An interlaced image is split into seven subimages, defined by repeating an 8 by 8 pattern across the whole image, and the subimages are stored in the file in numerical order. Because it works in both dimensions over seven passes, where GIF interlacing uses only four passes in the vertical direction, a viewer can show a rough approximation of the entire image much sooner, especially when the early passes are interpolated, for example bicubically. It is named after Adam M. Costello, who suggested the method on 2 February 1995, and after its seven steps. It is a rearrangement of an earlier five-pass scheme proposed by Lee Daniel Crocker. It can be seen as iterating a folding pattern in both dimensions, and it behaves like a coarse multiscale model of the image, similar to a discrete wavelet transform with Haar wavelets but with worse frequency behaviour, showing pixelation in the early passes in return for a simpler implementation.