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Title: Optimal Transport Maps For Distribution Preserving Operations on Latent Spaces of Generative Models

Paper content:
Generative models such as Variational Auto Encoders (VAEs) and Generative Adversarial Networks (GANs) are typically trained for a fixed prior distribution in the latent space, such as uniform or Gaussian.
After a trained model is obtained, one can sample the Generator in various forms for exploration and understanding, such as interpolating between two samples, sampling in the vicinity of a sample or exploring differences between a pair of samples applied to a third sample.
However, the latent space operations commonly used in the literature so far induce a distribution mismatch between the resulting outputs and the prior distribution the model was trained on.
Previous works have attempted to reduce this mismatch with heuristic modification to the operations or by changing the latent distribution and re-training models.
In this paper, we propose a framework for modifying the latent space operations such that the distribution mismatch is fully eliminated.
Our approach is based on optimal transport maps, which adapt the latent space operations such that they fully match the prior distribution, while minimally modifying the original operation.
Our matched operations are readily obtained for the commonly used operations and distributions and require no adjustment to the training procedure.
Generative models such as Variational Autoencoders (VAEs) BID7 and Generative Adversarial Networks (GANs) BID3 have emerged as popular techniques for unsupervised learning of intractable distributions.
In the framework of Generative Adversarial Networks (GANs) BID3 , the generative model is obtained by jointly training a generator G and a discriminator D in an adversarial manner.
The discriminator is trained to classify synthetic samples from real ones, whereas the generator is trained to map samples drawn from a fixed prior distribution to synthetic examples which fool the discriminator.
Variational Autoencoders (VAEs) BID7 are also trained for a fixed prior distribution, but this is done through the loss of an Autoencoder that minimizes the variational lower bound of the data likelihood.
For both VAEs and GANs, using some data X we end up with a trained generator G, that is supposed to map latent samples z from the fixed prior distribution to output samples G(z) which (hopefully) have the same distribution as the data.In order to understand and visualize the learned model G(z), it is a common practice in the literature of generative models to explore how the output G(z) behaves under various arithmetic operations on the latent samples z.
However, the operations typically used so far, such as linear interpolation BID3 , spherical interpolation BID20 , vicinity sampling and vector arithmetic BID12 , cause a distribution mismatch between the latent prior distribution and the results of the operations.
This is problematic, since the generator G was trained on a fixed prior and expects to see inputs with statistics consistent with that distribution.To address this, we propose to use distribution matching transport maps, to obtain analogous latent space operations (e.g. interpolation, vicinity sampling) which preserve the prior distribution of samples from prior linear matched (ours) spherical (a) Uniform prior: Trajectories of linear interpolation, our matched interpolation and the spherical interp.
BID20 .
(e) Spherical midpoint distribution BID20 Figure 1: We show examples of distribution mismatches induced by the previous interpolation schemes when using a uniform prior in two dimensions.
Our matched interpolation avoids this with a minimal modification to the linear trajectory, traversing through the space such that all points along the path are distributed identically to the prior.
the latent space, while minimally changing the original operation.
In Figure 1 we showcase how our proposed technique gives an interpolation operator which avoids distribution mismatch when interpolating between samples of a uniform distribution.
The points of the (red) matched trajectories are obtained as minimal deviations (in expectation of l 1 distance) from the the points of the (blue) linear trajectory.
We proposed a framework that fully eliminates the distribution mismatch in the common latent space operations used for generative models.
Our approach uses optimal transport to minimally modify (in l 1 distance) the operations such that they fully preserve the prior distribution.
We give analytical formulas of the resulting (matched) operations for various examples, which are easily implemented.
The matched operators give a significantly higher quality samples compared to the originals, having the potential to become standard tools for evaluating and exploring generative models.

assistant: text: We propose a framework for modifying the latent space operations such that the distribution mismatch between the resulting outputs and the prior distribution the generative model was trained on is fully eliminated.