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Diffusion Schrödinger Bridge with Applications to Score-Based Generative Modeling

This repository contains the implementation for the paper Diffusion Schrödinger Bridge with Applications to Score-Based Generative Modeling.

If using this code, please cite the paper:

    @article{de2021diffusion,
              title={Diffusion Schr$\backslash$" odinger Bridge with Applications to Score-Based Generative Modeling},
              author={De Bortoli, Valentin and Thornton, James and Heng, Jeremy and Doucet, Arnaud},
              journal={arXiv preprint arXiv:2106.01357},
              year={2021}
            }

Contributors

• Valentin De Bortoli
• James Thornton
• Jeremy Heng
• Arnaud Doucet

What is a Schrödinger bridge?

The Schrödinger Bridge (SB) problem is a classical problem appearing in applied mathematics, optimal control and probability; see [1, 2, 3]. In the discrete-time setting, it takes the following (dynamic) form. Consider as reference density p(x_0:N) describing the process adding noise to the data. We aim to find p*(x_0:N) such that p*(x₀) = p_data(x₀) and p*(x_N) = p_prior(x_N) and minimize the Kullback-Leibler divergence between p* and p. In this work we introduce Diffusion Schrodinger Bridge (DSB), a new algorithm which uses score-matching approaches [4] to approximate the Iterative Proportional Fitting algorithm, an iterative method to find the solutions of the SB problem. DSB can be seen as a refinement of existing score-based generative modeling methods [5, 6].

Installation

This project can be installed from its git repository.

1. Obtain the sources by:

   git clone https://github.com/anon284/schrodinger_bridge.git

or, if git is unavailable, download as a ZIP from GitHub https://github.com/<repository>.

2. Install:

   conda env create -f conda.yaml

   conda activate bridge

3. Download data examples:

   • CelebA: python data.py --data celeba --data_dir './data/'
   • MNIST: python data.py --data mnist --data_dir './data/'

How to use this code?

3. Train Networks:

• 2d: python main.py dataset=2d model=Basic num_steps=20 num_iter=5000
• mnist python main.py dataset=stackedmnist num_steps=30 model=UNET num_iter=5000 data_dir=<insert filepath of data dir <local paths/data/>
• celeba python main.py dataset=celeba num_steps=50 model=UNET num_iter=5000 data_dir=<insert filepath of data dir <local paths/data/>

Checkpoints and sampled images will be saved to a newly created directory. If GPU has insufficient memory, then reduce cache size. 2D dataset should train on CPU. MNIST and CelebA was ran on 2 high-memory V100 GPUs.

References

.. [1] Hans Föllmer Random fields and diffusion processes In: École d'été de Probabilités de Saint-Flour 1985-1987

.. [2] Christian Léonard A survey of the Schrödinger problem and some of its connections with optimal transport In: Discrete & Continuous Dynamical Systems-A 2014

.. [3] Yongxin Chen, Tryphon Georgiou and Michele Pavon Optimal Transport in Systems and Control In: Annual Review of Control, Robotics, and Autonomous Systems 2020

.. [4] Aapo Hyvärinen and Peter Dayan Estimation of non-normalized statistical models by score matching In: Journal of Machine Learning Research 2005

.. [5] Yang Song and Stefano Ermon Generative modeling by estimating gradients of the data distribution In: Advances in Neural Information Processing Systems 2019

.. [6] Jonathan Ho, Ajay Jain and Pieter Abbeel Denoising diffusion probabilistic models In: Advances in Neural Information Processing Systems 2020