Time-of-Flight non-line-of-sight (ToF NLOS) imaging techniques provide state-of-the-art reconstructions of scenes hidden around corners by inverting the optical path of indirect photons scattered by visible surfaces and measured by picosecond resolution sensors. The emergence of a wide range of ToF NLOS imaging methods with heterogeneous formulae and hardware implementations obscures the assessment of both their theoretical and experimental aspects. We present a comprehensive study of a representative set of ToF NLOS imaging methods by discussing their similarities and differences under common formulation and hardware. We first outline the problem statement under a common general forward model for ToF NLOS measurements, and the typical assumptions that yield tractable inverse models. We discuss the relationship of the resulting simplified forward and inverse models to a family of Radon transforms, and how migrating these to the frequency domain relates to recent phasor-based virtual line-of-sight imaging models for NLOS imaging that obey the constraints of conventional lens-based imaging systems. We then evaluate performance of the selected methods on hidden scenes captured under the same hardware setup and similar photon counts. Our experiments show that existing methods share similar limitations on spatial resolution, visibility, and sensitivity to noise when operating under equal hardware constraints, with particular differences that stem from method-specific parameters. We expect our methodology to become a reference in future research on ToF NLOS imaging to obtain objective comparisons of existing and new methods.

翻译：飞行时间非视距（ToF NLOS）成像技术通过反演由可见表面散射并被皮秒级分辨率传感器探测的间接光子光路，实现了对拐角后隐藏场景的最先进重建。当前涌现出大量具有不同数学表达和硬件实现的ToF NLOS成像方法，这给评估其理论与实验特性带来了困难。本文通过对代表性ToF NLOS成像方法在统一数学框架和硬件条件下进行相似性与差异性分析，开展了系统性研究。我们首先基于通用的ToF NLOS测量前向模型概述问题定义，并讨论为获得可处理逆模型所采用的典型假设。随后分析了所得简化前向与逆模型与Radon变换族的关系，并阐明将其转换至频域如何关联到近期基于相量的虚拟视距NLOS成像模型——这类模型遵循传统透镜成像系统的约束条件。最后，我们在相同硬件配置与近似光子计数条件下，评估了所选方法对隐藏场景的成像性能。实验表明，在同等硬件约束下，现有方法在空间分辨率、可见度及噪声敏感性方面存在相似的局限性，其特定差异主要源于方法自身的参数设置。我们预期本研究方法将成为未来ToF NLOS成像研究的参照基准，为现有及新方法的客观比较提供依据。