Progressive coding
<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 with
sequential coding.
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 received.
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.
Interlacing is also supported by other formats.
JPEG supports a functionally similar concept known as
Progressive JPEG.
[How does the algorithm differ?]
JBIG uses
progressive coding.
See also
progressive/sequential coding.
["Progressive Bi-level Image Compression, Revision 4.1", ISO/IEC JTC1/SC2/WG9, CD 11544, 1991-09-16].
