Maximum Likelihood on the Joint (Data, Condition) Distribution for Solving Ill-Posed Problems with Conditional Flow Models
This provides a simpler and more robust approach for solving uncertain, ill-posed problems like image generation and super-resolution, though it appears incremental as an extension of existing flow model techniques.
The paper tackles the problem of training conditional flow models for ill-posed problems by introducing a method based on maximum likelihood on the joint distribution of data and conditions, which simplifies training without needing explicit condition distributions or auxiliary components. The result is robust models that successfully generate class-conditional images and reconstruct degraded images via super-resolution, as demonstrated on toy and practical tasks.
I describe a trick for training flow models using a prescribed rule as a surrogate for maximum likelihood. The utility of this trick is limited for non-conditional models, but an extension of the approach, applied to maximum likelihood of the joint probability distribution of data and conditioning information, can be used to train sophisticated \textit{conditional} flow models. Unlike previous approaches, this method is quite simple: it does not require explicit knowledge of the distribution of conditions, auxiliary networks or other specific architecture, or additional loss terms beyond maximum likelihood, and it preserves the correspondence between latent and data spaces. The resulting models have all the properties of non-conditional flow models, are robust to unexpected inputs, and can predict the distribution of solutions conditioned on a given input. They come with guarantees of prediction representativeness and are a natural and powerful way to solve highly uncertain problems. I demonstrate these properties on easily visualized toy problems, then use the method to successfully generate class-conditional images and to reconstruct highly degraded images via super-resolution.