Sample and Map from a Single Convex Potential: Generation using Conjugate Moment Measures

The canonical approach in generative modeling is to split model fitting into two blocks: define first how to sample noise (e.g. Gaussian) and choose next what to do with it (e.g. using a single map or flows). We explore in this work an alternative route that ties sampling and mapping. We find inspiration in moment measures, a result that states that for any measure ρ, there exists a unique convex potential u such that ρ = ∇u♯e^-u. While this does seem to tie effectively sampling (from log–concave distribution e^-u) and action (pushing particles through ∇u), we observe on simple examples (e.g., Gaussians or 1D distributions) that this choice is ill–suited for practical tasks. We study an alternative factorization, where ρ is factorized as ∇w*♯e^-w, where w* is the convex conjugate of a convex potential w. We call this approach conjugate moment measures, and show far more intuitive results on these examples. Because ∇w* is the Monge map between the log–concave distribution e^-w and ρ, we rely on optimal transport solvers to propose an algorithm to recover w from samples of ρ, and parameterize w as an input–convex neural network. We also address the common sampling scenario in which the density of ρ is known only up to a normalizing constant, and propose an algorithm to learn w in this setting.

• † CREST–ENSAE, Institut Polytechnique de Paris