A method for processing video data defining a series of images, comprising the steps of: defining a first series of nodes in a first multidimensional space, each of the first series of nodes corresponding to an image of the series of images and its location in the first space being defined in dependence on features of the respective image; defining a transformation function that maps each of the nodes in the first multidimensional space on to a respective node in a second multidimensional space having a lower dimensionality than the first multidimensional space, in such a way that neighbourhood relationships between nodes in the first multidimensional space are preserved between the respective nodes in the second multidimensional space; defining a second series of nodes in the second multidimensional space, each of the second series of nodes corresponding to one of the first set of nodes and its location in being the second multidimensional space being defined in accordance with the transformation function; and performing a clustering analysis in dependence on the nodes of the second multidimensional space to identify clusters of nodes therein.