# Fast Back-Projection for Non-Line of Sight Reconstruction

## TL;DR

This paper introduces a significantly faster back-projection method for non-line of sight reconstruction using space-time manifold voxelization, enabling real-time performance with minimal quality loss.

## Contribution

A novel back-projection technique that accelerates NLOS reconstruction by up to a thousand times through space-time manifold voxelization, suitable for GPU implementation.

## Key findings

- Achieves up to 1000x speedup over previous methods
- Maintains high reconstruction quality with minimal loss
- Effective on both real captured and synthetic data

## Abstract

Recent works have demonstrated non-line of sight (NLOS) reconstruction by using the time-resolved signal frommultiply scattered light. These works combine ultrafast imaging systems with computation, which back-projects the recorded space-time signal to build a probabilistic map of the hidden geometry. Unfortunately, this computation is slow, becoming a bottleneck as the imaging technology improves. In this work, we propose a new back-projection technique for NLOS reconstruction, which is up to a thousand times faster than previous work, with almost no quality loss. We base on the observation that the hidden geometry probability map can be built as the intersection of the three-bounce space-time manifolds defined by the light illuminating the hidden geometry and the visible point receiving the scattered light from such hidden geometry. This allows us to pose the reconstruction of the hidden geometry as the voxelization of these space-time manifolds, which has lower theoretic complexity and is easily implementable in the GPU. We demonstrate the efficiency and quality of our technique compared against previous methods in both captured and synthetic data