k-means clustering
Partitions data points into k groups by minimizing distance to cluster centers. Used in computer vision to build visual vocabularies by clustering image feature descriptors.
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Bag of Words Applied to Computer Vision (Visual Vocabulary / Bag of Visual Words)
Josef Sivic and Andrew Zisserman at the University of Oxford applied the Bag of Words text-retrieval model to visual features in their 2003 ICCV paper 'Video Google', representing image regions as a vocabulary of visual words to enable efficient object retrieval from video.