Font compression and retrieval

Abstract

Method and apparatus for compressing data representing a set of symbols such that each symbol of the set can be separately accessed and decompressed. Each symbol of the set of symbols is encoded in the form of a two-pair code wherein a first part of the code is common for all encoded symbols and a second part of the code encodes the data representing a symbol. An identifier is given for each symbol for permitting each encoded symbol to be separately accessed and decompressed. The invention is particularly useful for storing large fonts such as a Chinese or Japanese character set.

Description

[0001] 1. Field of the Invention[0002] The present invention relates generally to the compression and retrieval of data representing a font or other set of symbols; and, more particularly, to a method and apparatus for storing a large font, such as a Chinese or Japanese character set, in a compressed form while retaining access to individual symbols of the font.[0003] 2. Description of the Prior Art[0004] In order to display messages in languages such as Chinese or Japanese on a CRT or an LCD display, a large set of symbols, or glyphs, is required. For example, the Chinese Unicode standard character set contains about 21,000 different Chinese symbols. Furthermore, each symbol is the size of at least some hundreds of pixels; and, as a result, to store a complete Chinese font requires a large amount of memory. Being able to store the glyphs in a more compact format than pure bitmaps will substantially reduce memory requirements.[0005] For laser printers or high resolution displays, a ...

AI Technical Summary

Benefits of technology

[0013] To decode a particular glyph, given the identifier for the glyph, the indexing tables are first searched for a matching entry. From the table lengths and the position in the table, the position or location of the particular glyph in the code set can be computed, and this permits the desired codeword for that glyph to then be extracted and decoded. Because, in the present invention, a two-part code is used where the first part of the code is common for all the encoded glyphs; indexing is greatly simplified inasmuch as for each glyph it is only necessary to locate the codeword for that particular glyph.

[0014] In accordance with one presently preferred embodiment of the invention, font compression is achieved utilizing an arithmetic encoder with a fixed probability table. This procedure provides near optimal compression of the glyphs, given the probability table, without the need of additional tables. According to an alternative embodiment of the invention, font compression is by means of a predictive encoding scheme with a fixed prediction table followed by a fixed Huffman coding. This procedure makes it possible to have a very fast decompression while retaining reasonable compression speeds. This embodiment is also particularly suitable for hardware implementation.

Problems solved by technology

Furthermore, each symbol is the size of at least some hundreds of pixels; and, as a result, to store a complete Chinese font requires a large amount of memory.

Such methods are inappropriate for font compression, however; where, ideally, only a single glyph should be decompressed at a time.

If sequential methods of this type are employed, some blocking of the glyphs is required, and a trade-off must be made between the two extremes of compressing the entire font as one block, thus losing random access capability, and compressing each symbol separately, in which case overall performance becomes quite poor.

In general, however, none of these prior art methods describes a font compression and retrieval technique that provides complete random access of individual symbols of the font, which is important to permit high-speed access of the symbols by modern high-speed equipment.

Images