X-Adapter: Adding Universal Compatibility of Plugins for Upgraded
  Diffusion Model
Authors: name: Lingmin Ran, name: Xiaodong Cun, name: JiaWei Liu, name: Rui Zhao, name: Song Zijie, name: Xintao Wang, name: Jussi Keppo, name: Mike Zheng Shou

We introduce X-Adapter, a universal upgrader to enable the pretrained
plug-and-play modules (e.g., ControlNet, LoRA) to work directly with the
upgraded text-to-image diffusion model (e.g., SDXL) without further retraining.
We achieve this goal by training an additional network to control the frozen
upgraded model with the new text-image data pairs. In detail, X-Adapter keeps a
frozen copy of the old model to preserve the connectors of different plugins.
Additionally, X-Adapter adds trainable mapping layers that bridge the decoders
from models of different versions for feature remapping. The remapped features
will be used as guidance for the upgraded model. To enhance the guidance
ability of X-Adapter, we employ a null-text training strategy for the upgraded
model. After training, we also introduce a two-stage denoising strategy to
align the initial latents of X-Adapter and the upgraded model. Thanks to our
strategies, X-Adapter demonstrates universal compatibility with various plugins
and also enables plugins of different versions to work together, thereby
expanding the functionalities of diffusion community. To verify the
effectiveness of the proposed method, we conduct extensive experiments and the
results show that X-Adapter may facilitate wider application in the upgraded
foundational diffusion model.