  We present Direct Reward Fine-Tuning (DRaFT), a simple and effective method
for fine-tuning diffusion models to maximize differentiable reward functions,
such as scores from human preference models. We first show that it is possible
to backpropagate the reward function gradient through the full sampling
procedure, and that doing so achieves strong performance on a variety of
rewards, outperforming reinforcement learning-based approaches. We then propose
more efficient variants of DRaFT: DRaFT-K, which truncates backpropagation to
only the last K steps of sampling, and DRaFT-LV, which obtains lower-variance
gradient estimates for the case when K=1. We show that our methods work well
for a variety of reward functions and can be used to substantially improve the
aesthetic quality of images generated by Stable Diffusion 1.4. Finally, we draw
connections between our approach and prior work, providing a unifying
perspective on the design space of gradient-based fine-tuning algorithms.
