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Compression

If you download many programs and files from the Internet, you've probably encountered ZIP files before. This compression system is a very handy invention, especially for Web users, because it lets you reduce the overall number of bits and bytes in a file so it can be transmitted faster over slower Internet connections, or take up less space on a disk. Once you download the file, your computer uses a program such as WinZip or Stuffit to expand the file back to its original size. If everything works correctly, the expanded file is identical to the original file before it was compressed.

At first glance, this seems very mysterious. How can you reduce the number of bits and bytes and then add those exact bits and bytes back later? The basic idea behind the process is fairly straightforward. We are not interested here in general file compression we are concerned about the compression of picture files. But most types of computer files are fairly redundant -- they have the same information listed over and over again. File-compression programs of all sorts simply get rid of the redundancy. Instead of listing a piece of information over and over again, a file-compression program lists that information once and then refers back to it whenever it appears in the original program.

In Image compression two types of compression are used. "Lossy" compression is where some to the data is premanently lost when the file is compressed. Lossless compresion is where the data can all be kept.

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

Interlacing is also known as "progressive" encoding, because the image becomes progressively clearer as it is received. The word interlacing is also used to describe a separate process that improves picture quality on video, If this is your interest see Lukes pages
   

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