Interlace (bitmaps)
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Interlacing is a method of encoding a bitmap image such that a person who has partially received it sees a degraded copy of the entire image. When communicating over a slow communications link, this is often preferable to seeing a perfectly clear copy of one part of the image, as it helps the viewer decide more quickly whether to abort or continue the transmission.
Interlacing is supported by the following formats:
- GIF stores the lines in the order 0, 8, 16, ..., 4, 12, ..., 2, 6, 10, 14, ..., 1, 3, 5, 7, 9, ....
- PNG uses the Adam7 algorithm
- JPEG and JPEG 2000
- PGF
Interlacing is also known as "progressive" encoding, because the image becomes progressively clearer as it is received. Note that this terminology has nothing to do with interlaced and progressive scan CRTs[dubious ].
For example: Interlaced GIF is a GIF image that seems to arrive on your display like an image coming through a slowly-opening Venetian blind. A fuzzy outline of an image is gradually replaced by seven successive waves of bit streams that fill in the missing lines until the image arrives at its full resolution.
