DjVu File Structure


o DjVu Image Files
o IW44 Image Files
o Experimental Features.
DjVu files are organized according to the ``EA IFF 85'' layout. Pointers to the appropriate reference document are provided in section IFFByteStream.h. IFF files are logically composed of a sequence of data chunks. Each chunk comes with a four character chunk identifier describing the type of the data stored in the chunk. A few special chunk identifiers, for instance "FORM", are reserved for so called composite chunks containing a sequence of data chunks. This convention effectively provides IFF files with a hierarchical structure. Composite chunks are further identified by a secondary chunk identifier. For convenience, both identifiers are gathered as an extended chunk identifier such as "FORM:DJVU".

The four octets "AT&T" may be inserted in front of the IFF compliant byte stream. The decoder simply ignores these four octets when they are present. These four octets are not part of the IFF format and are not required components of a valid DjVu file. Certain versions of MSIE incorrectly recognize any IFF file as a Microsoft AIFF sound file. The presence of these four octets prevents this incorrect identification.

The DjVu specification mandates that the decoder should silently skip chunks whose identifier is not recognized. This mechanism provides a backward compatible way to extend the initial format by allocating new chunk identifiers.

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