SePPO: Semi-Policy Preference Optimization for Diffusion Alignment

Daoan Zhang, Guangchen Lan, Dong-Jun Han, Wenlin Yao, Xiaoman Pan, Hongming Zhang, Mingxiao Li, pengcheng chen, Dong Yu, Christopher Brinton, Jiebo Luo

Keywords: Reinforcement Learning, Diffusion Model, Image Generation, Video Generation

TL;DR: We propose SePPO, a method to align diffusion models without reward models or human-annotated data. SePPO outperforms previous methods, achieving a PickScore of 21.57 on Pick-a-Pic and excels in text-to-video tasks.

Abstract: Reinforcement learning from human feedback (RLHF) methods are emerging as a way to fine-tune diffusion models (DMs) for visual generation. However, commonly used on-policy strategies are limited by the generalization capability of the reward model, while off-policy approaches require large amounts of difficult-to-obtain paired human-annotated data, particularly in visual generation tasks. To address the limitations of both on- and off-policy RLHF, we propose a preference optimization method that aligns DMs with preferences without relying on reward models or paired human-annotated data. Specifically, we introduce a Semi-Policy Preference Optimization (SePPO) method. SePPO leverages previous checkpoints as reference models while using them to generate on-policy reference samples, which replace “losing images” in preference pairs. This approach allows us to optimize using only off-policy “winning images”. Furthermore, we design a strategy for reference model selection that expands the exploration in the policy space. Notably, we do not simply treat reference samples as negative examples for learning. Instead, we design an anchor-based criterion to assess whether the reference samples are likely to be winning or losing images, allowing the model to selectively learn from the generated reference samples. This approach mitigates performance degradation caused by the uncertainty in reference sample quality. We validate SePPO across both text-to-image and text-to-video benchmarks. SePPO surpasses all previous approaches on the text-to-image benchmarks and also demonstrates outstanding performance on the text-to-video benchmarks.

Primary Area: generative models

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Submission Number: 8420