Among the chief applications of the MPEG-7 visual descriptors is image indexing and retrieval. And among the colour image descriptors in the standard having the best retrieval performance is the colour structure (CS) descriptor, a descriptor that uses no more storage than an ordinary colour histogram but that substantially out-performs it. The basis of the CS descriptor is the CS histogram, a generalisation of the colour histogram. The CS histogram encodes information about the spatial structure of the colours in an image as well as their frequency of occurrence. Performance is further enhanced by extracting the CS histogram in the novel nonlinear HMMD colour space and by non-uniformly quantising histogram amplitudes to reflect the bin amplitude statistics derived from consumer oriented image databases. The CS descriptor is scalable in the sense that smaller, more compact, CS descriptors can be easily derived from the largest, highest performance descriptor without re-extraction from the image.
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