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DiffusionNFT: Online Diffusion Reinforcement with Forward Process

About

Online reinforcement learning (RL) has been central to post-training language models, but its extension to diffusion models remains challenging due to intractable likelihoods. Recent works discretize the reverse sampling process to enable GRPO-style training, yet they inherit fundamental drawbacks, including solver restrictions, forward-reverse inconsistency, and complicated integration with classifier-free guidance (CFG). We introduce Diffusion Negative-aware FineTuning (DiffusionNFT), a new online RL paradigm that optimizes diffusion models directly on the forward process via flow matching. DiffusionNFT contrasts positive and negative generations to define an implicit policy improvement direction, naturally incorporating reinforcement signals into the supervised learning objective. This formulation enables training with arbitrary black-box solvers, eliminates the need for likelihood estimation, and requires only clean images rather than sampling trajectories for policy optimization. DiffusionNFT is up to $25\times$ more efficient than FlowGRPO in head-to-head comparisons, while being CFG-free. For instance, DiffusionNFT improves the GenEval score from 0.24 to 0.98 within 1k steps, while FlowGRPO achieves 0.95 with over 5k steps and additional CFG employment. By leveraging multiple reward models, DiffusionNFT significantly boosts the performance of SD3.5-Medium in every benchmark tested.

Kaiwen Zheng, Huayu Chen, Haotian Ye, Haoxiang Wang, Qinsheng Zhang, Kai Jiang, Hang Su, Stefano Ermon, Jun Zhu, Ming-Yu Liu• 2025

Related benchmarks

TaskDatasetResultRank
Text-to-Image GenerationGenEval (test)--
169
Compositional Image GenerationGenEval
Overall Score0.94
22
Text-to-Image GenerationGenEval official (val)
Object Presence (1 Obj)100
15
Text-to-Image GenerationTMDB
OCR Accuracy91
12
Text-to-Image GenerationDrawBench
PickScore23.63
12
Text-to-Image GenerationEOpenLib
OCR Accuracy86
12
Text-to-Image GenerationFlowGRPO OCR-1k (val)
OCR Accuracy89
11
Text-to-Image AlignmentPick-a-Pic (test)
Pick Score2.38e+3
10
Text-to-Image AlignmentHPD v2.1 (test)
HPS-v2.10.331
10
Text-to-Image AlignmentImageReward (test)
Image Reward1.49
10
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