6. The method of claim 4 wherein performing heterogeneous affinity propagation comprises computing candidate exemplars for each image and each word, including by aggregating received homogeneous messages and received heterogeneous messages.
4. The method of claim 1 wherein processing the image-to-image data structure, the word-to-word data structure, and the image-to-word data structure simultaneously comprises performing heterogeneous affinity propagation to determine a set of exemplars.