Byte-aligned bitmap compression

Abstract

Summary form only given. Bitmap compression reduces storage space and transmission time for unstructured bit sequences like in inverted files, spatial objects, etc. On the down side, the compressed bitmaps loose their functional properties. For example, checking a given bit position, set intersection, union, and difference can be performed only after full decoding, thus causing a many-folded operational speed degradation. The proposed byte-aligned bitmap compression method (BBC) aims to support fast set operations on the compressed bitmap formats and, at the same time, to retain a competitive compression rate. To achieve this objective, BBC abandons the traditional approach of encoding run-lengths (distances between two ones separated by zeros). Instead, BBC deals only with byte aligned byte-size bitmap portions that are easy to fetch, store, AND, OR, and convert. The bitmap bytes are classified as gaps containing only zeros or only ones and maps containing a mixture of both. We also introduced a simple extension mechanism for existing methods to accommodate a dual-gap (zeros and ones) run-length encoding. With this extension, encoding of long "one" sequences becomes as efficient and better than arithmetic encoding.