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Adam7 is the interlacing algorithm specified for use in PNG images. An interlaced PNG image is broken into seven subimages, which are defined by replicating the following 8x8 pattern on the right across the full image. The subimages are stored in the PNG file in numerical order.
Adam7 is a 2D interlacing scheme unlike the 1D scheme used in interlaced GIF. This means the percieved quality of the image and the readablity of text in the image rises much faster in the early passes than with GIF interlacing (demo animation).
Interesting points of note
- Two-dimensional interlacing scheme
- PNG first interlace pass displays 1/64th of the image data
- The GIF format on the other hand displays 1/8th of the image data
- PNG will send 4 passes to GIF's 1st pass
History
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Adam7 is named after Adam M. Costello, who suggested the method on January 30, 1995, based on a very similar five-pass scheme that had earlier been proposed by Lee Daniel Crocker.
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