Rephrase this text: Figure 2 shows a set of top-ranked discriminative patch clusters discovered with our approach.
To perform classification, top 210 discovered patches of each scene are aggregated into a spatial pyramid using maxpooling over the discriminative patch scores as in [3] .
For each cluster, we asked human labelers to mark which of the cluster's top ten firings on the validation set are visually consistent with the cluster.
Therefore, we performed an informal perceptual experiment with human subjects, measuring the visual purity of our clusters.
Initialization: The input to our discovery algorithm is a \discovery dataset\ D of unlabeled images as well as a much larger \natural world dataset\ N (in this paper we used 6,000 images randomly sampled from Flickr.com).