Non-redundant 2D strings

Introduces a variation of the 2D string representation for symbolic pictures, the non-redundant 2D string, and analyze it with respect to compactness and non-ambiguity. It results that the non-redundant 2D string is a more compact representation than the 2D string, and that the class of unambiguous pictures under the non-redundant 2D string is almost equal to the class of unambiguous pictures under the reduced 2D string, up to a special case. Moreover, we show that the compactness of the new index does not affect the time complexity of picture retrieval.<
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