  Recent advancements in human preference optimization, initially developed for
Language Models (LMs), have shown promise for text-to-image Diffusion Models,
enhancing prompt alignment, visual appeal, and user preference. Unlike LMs,
Diffusion Models typically optimize in pixel or VAE space, which does not align
well with human perception, leading to slower and less efficient training
during the preference alignment stage. We propose using a perceptual objective
in the U-Net embedding space of the diffusion model to address these issues.
Our approach involves fine-tuning Stable Diffusion 1.5 and XL using Direct
Preference Optimization (DPO), Contrastive Preference Optimization (CPO), and
supervised fine-tuning (SFT) within this embedding space. This method
significantly outperforms standard latent-space implementations across various
metrics, including quality and computational cost. For SDXL, our approach
provides 60.8\% general preference, 62.2\% visual appeal, and 52.1\% prompt
following against original open-sourced SDXL-DPO on the PartiPrompts dataset,
while significantly reducing compute. Our approach not only improves the
efficiency and quality of human preference alignment for diffusion models but
is also easily integrable with other optimization techniques. The training code
and LoRA weights will be available here:
https://huggingface.co/alexgambashidze/SDXL\_NCP-DPO\_v0.1
