9. One or more non-transitory computer-readable media storing instructions that, when executed by one or more computing devices, cause the computing devices to perform operations comprising: clustering features from one or more images to form feature clusters, wherein the features and the feature clusters are defined in a higher-dimensional space; projecting the feature clusters from the higher-dimensional space to a first lower-dimensional space to form projections of the feature clusters in the first lower-dimensional space; projecting the feature clusters from the higher-dimensional space to a second lower-dimensional space to form projections of the feature clusters in the second lower-dimensional space; and generating buckets in the first lower-dimensional space and in the second lower-dimensional space based on the projections of the feature clusters in the first lower-dimensional space and on the projections of the feature clusters in the second lower-dimensional space.
9. One or more non-transitory computer-readable media storing instructions that, when executed by one or more computing devices, cause the computing devices to perform operations comprising: clustering features from one or more images to form feature clusters, wherein the features and the feature clusters are defined in a higher-dimensional space; projecting the feature clusters from the higher-dimensional space to a first lower-dimensional space to form projections of the feature clusters in the first lower-dimensional space; projecting the feature clusters from the higher-dimensional space to a second lower-dimensional space to form projections of the feature clusters in the second lower-dimensional space; and generating buckets in the first lower-dimensional space and in the second lower-dimensional space based on the projections of the feature clusters in the first lower-dimensional space and on the projections of the feature clusters in the second lower-dimensional space. 14. The one or more non-transitory computer readable media of claim 9 , wherein the buckets are generated further based on a purity measure.