Ψ-Sampler: Initial Particle Sampling for SMC-Based Inference-Time
  Reward Alignment in Score Models
Authors: name: Taehoon Yoon, name: Yunhong Min, name: Kyeongmin Yeo, name: Minhyuk Sung

We introduce Psi-Sampler, an SMC-based framework incorporating pCNL-based
initial particle sampling for effective inference-time reward alignment with a
score-based generative model. Inference-time reward alignment with score-based
generative models has recently gained significant traction, following a broader
paradigm shift from pre-training to post-training optimization. At the core of
this trend is the application of Sequential Monte Carlo (SMC) to the denoising
process. However, existing methods typically initialize particles from the
Gaussian prior, which inadequately captures reward-relevant regions and results
in reduced sampling efficiency. We demonstrate that initializing from the
reward-aware posterior significantly improves alignment performance. To enable
posterior sampling in high-dimensional latent spaces, we introduce the
preconditioned Crank-Nicolson Langevin (pCNL) algorithm, which combines
dimension-robust proposals with gradient-informed dynamics. This approach
enables efficient and scalable posterior sampling and consistently improves
performance across various reward alignment tasks, including layout-to-image
generation, quantity-aware generation, and aesthetic-preference generation, as
demonstrated in our experiments.