1. A method for processing video data defining a series of images, comprising: configuring and using at least one digital data processor and associated memory to effect 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 appearance components 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 neighborhood 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 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; wherein the step of defining the transformation function comprises: defining a graph representing the relationships between neighboring nodes of the first space; assigning a weight to each such relationship; and calculating the transformation function in dependence on the determined mapping.