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 ProgressiveJPEG. [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].

(2000-09-12)