progressive coding

progressive coding

(graphics, file format, algorithm)(Or "interlacing") Anaspect of a graphics storage format or transmissionalgorithm that treats bitmap image data non-sequentiallyin such a way that later data adds progressively greaterresolution to an already full-size image. This contrastswith sequential coding.

Progressive coding is useful when an image is being sentacross a slow communications channel, such as the Internet,as the low-resolution image may be sufficient to allow theuser to decide not to wait for the rest of the file to bereceived.

In an interlaced GIF89 image, the pixels in a row arestored sequentially but the rows are stored in interlacedorder, 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 thegaps left by the previous one.

PNG interlaces both horizontally and vertically using the"Adam7" method, a seven pass process named after AdamM. Costello.

Interlacing is also supported by other formats. JPEGsupports a functionally similar concept known as Progressive JPEG. 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].