What is spanning dictionary (Global Dictionary) support?
Significant compression is possible if one or more Global dictionaries are used to hold symbols for all pages in a document. Typically multi-page documents use the same font type and size. There is no need in this case to recreate a new symbol dictionary for each page. The JBIG2 standard allows one or more Global dictionaries for the entire document and one or more local dictionaries for each page. Dictionaries can also refer to other dictionaries. It is up to the implementer to decide how many and what type of symbol dictionaries to create based on the type of document. Documents using a similar font type and size on each page compress efficiently using spanning dictionaries while documents with highly variable font types are best compressed using local dictionaries.
