An effective method for encoding image-input blocks in vector quantisation (VQ) is proposed. For each codeword in the codebook, a group of Peano scannings of selected feature vectors is computed. Each Peano scanning acts as a transform from a higher dimension to one dimension, while preserving neighbourhood adjacency. An ordered list of the Peano scannings and their link to the codebook is stored. Coding is conducted by restricting the search to two windows of codewords with the closest Peano scannings to that of the input block. Each window centre is found in logarithmic time proportional to the codebook size. The number of codewords to be searched is fixed, and is determined by some additional distortion that is acceptable over exhaustive search methods. Coded images show no significant degradation, while maintaining considerable constant search-time savings over exhaustive search methods. The algorithm can be used with other fast full-search equivalent methods, and can use savings from other methods in searching within the windows as well.
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