  Text-to-image diffusion models are typically trained to optimize the
log-likelihood objective, which presents challenges in meeting specific
requirements for downstream tasks, such as image aesthetics and image-text
alignment. Recent research addresses this issue by refining the diffusion U-Net
using human rewards through reinforcement learning or direct backpropagation.
However, many of them overlook the importance of the text encoder, which is
typically pretrained and fixed during training. In this paper, we demonstrate
that by finetuning the text encoder through reinforcement learning, we can
enhance the text-image alignment of the results, thereby improving the visual
quality. Our primary motivation comes from the observation that the current
text encoder is suboptimal, often requiring careful prompt adjustment. While
fine-tuning the U-Net can partially improve performance, it remains suffering
from the suboptimal text encoder. Therefore, we propose to use reinforcement
learning with low-rank adaptation to finetune the text encoder based on
task-specific rewards, referred as \textbf{TexForce}. We first show that
finetuning the text encoder can improve the performance of diffusion models.
Then, we illustrate that TexForce can be simply combined with existing U-Net
finetuned models to get much better results without additional training.
Finally, we showcase the adaptability of our method in diverse applications,
including the generation of high-quality face and hand images.
