Learning similarity preserving representations with neural similarity encoders.

Many dimensionality reduction or manifold learning algorithms optimize for retaining the pairwise similarities, distances, or local neighborhoods of data points. Spectral methods like Kernel PCA (kPCA) or isomap achieve this by computing the singular value decomposition (SVD) of some similarity matrix to obtain a low dimensional representation of the original data. However, this is computationally expensive if a lot of training examples are available and, additionally, representations for new (out-of-sample) data points can only be created when the similarities to the original training examples can be computed. We introduce similarity encoders (SimEc), which learn similarity preserving representations by using a feed-forward neural network to map data into an embedding space where the original similarities can be approximated linearly. The model optimizes the same objective as kPCA but in the process it learns a linear or non-linear embedding function (in the form of the tuned neural network), with which the representations of novel data points can be computed - even if the original pairwise similarities of the training set were generated by an unknown process such as human ratings. By creating embeddings for both image and text datasets, we demonstrate that SimEc can, on the one hand, reach the same solution as spectral methods, and, on the other hand, obtain meaningful embeddings from similarities based on human labels.