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arXiv · 2609.34071

SNaP: One-Step Posterior Sampling for Noisy Inverse Problems

Shirin Shoushtari·Edward P. Chandler·Xiao Shi·Ulugbek S. Kamilov
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Abstract

Diffusion and flow-matching models can produce high-quality posterior samples for inverse problems, but typically require tens to thousands of network evaluations per draw. MeanFlow enables one-step generation, yet applying it to inverse problems leaves no intermediate steps at which to enforce measurement consistency. We introduce SNaP, a one-step MeanFlow posterior sampler for linear inverse problems with Gaussian noise. Its central innovation is a measurement-adapted source: a Gaussian distribution whose mean and anisotropic covariance are determined by the measurement operator, observation, and noise level. The source anchors well-measured directions while preserving variation where the measurements are weak or uninformative. We show that the exact conditional flow transports this source to the true posterior. Across natural-image restoration and multi-coil MRI, SNaP produces diverse, high-quality samples with one network evaluation per draw, 30 to 2250 $\times$ faster than iterative samplers.

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Shirin Shoushtari, Edward P. Chandler, Xiao Shi, Ulugbek S. Kamilov. 2026-09-28. SNaP: One-Step Posterior Sampling for Noisy Inverse Problems. https://arxiv.org/abs/2609.34071

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