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Computing (FOLDOC) dictionary
progressive coding
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graphics, file format, algorithm (Or "interlacing") An
aspect of a graphics storage format or transmission
algorithm that treats bitmap image data non-sequentially
in such a way that later data adds progressively greater
resolution to an already full-size image. This contrasts
Progressive coding is useful when an image is being sent
across a slow communications channel, such as the Internet,
as the low-resolution image may be sufficient to allow the
user to decide not to wait for the rest of the file to be
In an interlaced GIF89 image, the pixels in a row are
stored sequentially but the rows are stored in interlaced
order, e.g. 0, 8, 4, 12, 2, 6, 8, 10, 14, 1, 3, 5, 7, 9, 11,
13, 15. Each vertical scan adds rows in the middle of the
gaps left by the previous one.
PNG interlaces both horizontally and vertically using the
"Adam7" method, a seven pass process named after Adam
M. Costello.