Diffusion-based generative models have achieved promising results recently, but raise an array of open questions in terms of conceptual understanding, theoretical analysis, algorithm improvement and extensions to discrete, structured, non-Euclidean domains. This work tries to re-exam the overall framework, in order to gain better theoretical understandings and develop algorithmic extensions for data from arbitrary domains. By viewing diffusion models as latent variable models with unobserved diffusion trajectories and applying maximum likelihood estimation (MLE) with latent trajectories imputed from an auxiliary distribution, we show that both the model construction and the imputation of latent trajectories amount to constructing diffusion bridge processes that achieve deterministic values and constraints at end point, for which we provide a systematic study and a suit of tools. Leveraging our framework, we present 1) a first theoretical error analysis for learning diffusion generation models, and 2) a simple and unified approach to learning on data from different discrete and constrained domains. Experiments show that our methods perform superbly on generating images, semantic segments and 3D point clouds.

本研究重新考虑扩散模型的总体框架，将其视为具有未观察到扩散轨迹的潜在变量模型，并应用于数据不受限制的领域。利用最大似然估计，我们表明模型构建和潜在路径的插补都可以构造扩散桥过程，实现端点的确定值和约束条件，并提供一套系统的研究和工具，进而提出了学习扩散生成模型的第一个理论误差分析和学习不同离散和受限领域数据的简单而统一的方法。实验证明，我们的方法在生成图像、语义分段和3D点云方面表现出色。

构建桥梁：理解和扩展扩散生成模型