Results and Discussions In computational imaging of out-of-sight targets by virtue of reflective relay walls, the scattering characteristics of the relay wall and the measured target surface directly affect the imaging results. However, it is difficult to find a relay wall material with standardized scattering characteristics in reality. Therefore, this paper firstly proposes a scattering characteristic description method, namely describing with the composition of specular reflection, Lambert scattering and Gaussian scattering, abstracting the scattering characteristics of materials as scattering components, and simulating the scattering characteristics of different materials through computer simulation. Secondly, by tracking a large number of rays and observing the scattered light spots on the relay walls, it is found that the subjective visual effect of NLOS results is poor when the scattering characteristics of the target and the relay wall are similar. Finally, the images of scattered spots are measured by standard deviation, and a multi-factor analysis of variance is used to analyze the relationship between the scattering composition and the standard deviation of the NLOS image. The experiment suggests that the size of the Gaussian scattering angle has a significant effect on the NLOS imaging quality under low specular reflection composition conditions (≤5%).

Applying computational imaging to behind the obstruction or other out-of-sight targets by virtue of relay walls is called non-line-of-sight imaging (NLOS). NLOS technology has great application potential in the fields of medical care, national defense, road safety, and scientific research. It can extend the range of human observation in scenarios where devices or human eyes cannot see. The existing NLOS technologies mainly include transient imaging, range-gated imaging, and passive NLOS imaging. These methods are mostly dedicated for Lambert reflector relay walls, featuring complex system structure, low imaging speed and high cost. However, the common materials in application scenarios are all non-Lambert reflectors. To this end, based on the bidirectional reflection distribution theory on relay wall materials, this paper proposes a material scattering characteristic description method, which realizes light intensity signal tracking and simulation of targets out of sight by configuring different scattering components of the relay wall and conducting massive ray tracing. The simulation work can provide a theoretical basis and experimental basis for the practical application of passive NLOS technologies, and provide a reference for relay wall selection, so it is of practical application significance.

Since there are many kinds of relay walls actually used for NLOS, with quite different scattering characteristics, it is difficult to find a standardized material. Therefore, this paper proposes a material scattering description model based on the traditional bidirectional reflectance distribution function (BRDF) to define the scattering characteristics by composition. Firstly, it is assumed that the scattering characteristics of relay wall materials are described by a combination of specular reflection, Lambert scattering, and Gaussian scattering, with transmission light and superficial stochastic scattering ignored. Then, different combinations of the scattering components are set separately to image the scattered light spots on relay walls, and the imaging results are evaluated by the standard deviations of the images. Finally, the scattering composition is taken as the independent variable and the standard deviation of the image is taken as the dependent variable, multi-factor analysis of variance is used to quantitatively analyze the impact of the scattering compositions of relay walls on the light signals of targets out of sight.

NLOS imaging technology for targets out of sight via relay walls has received wide attention in recent years. This paper proposes a material scattering characteristics description method for non-Lambert scattering relay walls in passive NLOS imaging scenarios and an NLOS simulation method, and analyzes the simulation results by variance analysis. Firstly, based on the material scattering principle, the optical scattering characteristics of some materials in nature are expressed as a combination of diffuse reflection, specular reflection and Gaussian scattering. Secondly, computer simulation is used to simulate the effect of different scattering characteristics of the relay wall and the target surface on the quality of NLOS imaging. Finally, multi-factor analysis of variance suggests a significant effect of Gaussian scattering in the scattering characteristics of the material on the standard deviation of the NLOS images. The proposed analysis method can provide prior knowledge for passive NLOS imaging algorithm under the condition that the scattering characteristics of the relay wall are certain. Besides, it can give an ideal transmission result of the optical signals of an out-of-sight target to compare with the actual result, reconstruct ideal NLOS signals to verify the effectiveness of actual reconstruction algorithm, provide a relay wall material selection scheme for NLOS imaging, and provide an analysis method for passive NLOS imaging conditions.