1. A method comprising: at one or more computing devices comprising one or more processors and memory storing one or more computer programs executed to perform the method, performing operations comprising: obtaining a first plurality of digital images clustered into a first cluster, the first cluster associated with a first numeric representation representing one or more similarities identified among at least two digital images of the first plurality of digital images; obtaining a second plurality of digital images clustered into a second cluster, the second cluster associated with a second numeric representation representing one or more similarities identified among at least two digital images of the second plurality of digital images; determining a cluster link density based at least upon the first numeric representation and the second numeric representation, the cluster link density representing one or more similarities identified among the first plurality of digital images and the second plurality of digital images; based on the cluster link density, forming a merged cluster by merging the first cluster and the second cluster; and storing, in the memory, data identifying the first plurality of digital images and the second plurality of digital images in the merged cluster.