2. The method of claim 1 , further comprising: selecting, from the set of images, a plurality of first image triplets that each include a triplet reference image and two unlabeled images; determining, for each first image triplet, a distance between feature vectors of the unlabeled images and a feature vector for the triplet reference image; and selecting subsets of unlabeled images based, at least in part, on the distance for each first image triplet.
6. The method of claim 2 , further comprising selecting a plurality of second image triplets from the set of images, each second image triplet including the second reference image and two labeled images.