Measuring the similarity between data points often requires domain knowledge. This can in parts be compensated by relying on unsupervised methods such as latent-variable models, where similarity/distance is estimated in a more compact latent space. Prevalent is the use of the Euclidean metric, which has the drawback of ignoring information about similarity of data stored in the decoder, as captured by the framework of Riemannian geometry. Alternatives---such as approximating the geodesic---are often computationally inefficient, rendering the methods impractical. We propose an extension to the framework of variational auto-encoders allows learning flat latent manifolds, where the Euclidean metric is a proxy for the similarity between data points. This is achieved by defining the latent space as a Riemannian manifold and by regularising the metric tensor to be a scaled identity matrix. Additionally, we replace the compact prior typically used in variational auto-encoders with a recently presented, more expressive hierarchical one---and formulate the learning problem as a constrained optimisation problem. We evaluate our method on a range of data-sets, including a video-tracking benchmark, where the performance of our unsupervised approach nears that of state-of-the-art supervised approaches, while retaining the computational efficiency of straight-line-based approaches.

本研究提出了一种基于Riemannian geometry的扩展的变分自编码器框架，可以学习平面的潜在流形，通过约束优化问题和使用更具表达力的层次先验代替紧凑先验，使得在保留直线状方法的计算效率的同时，能够在视频跟踪基准测试中接近监督学习方法的性能。

用VAEs学习平坦的潜在流形