Rezaei Sina, Arefi Hossein
i3mainz-Institute for Spatial Information and Surveying Technology, School of Technology, Mainz University of Applied Sciences, D-55118 Mainz, Germany.
Sensors (Basel). 2025 Mar 13;25(6):1789. doi: 10.3390/s25061789.
The evolution of photogrammetry has been significantly influenced by advancements in camera technology, particularly the emergence of spherical cameras. These devices offer extensive photographic coverage and are increasingly utilised in many photogrammetry applications due to their significant user-friendly configuration, especially in their low-cost versions. Despite their advantages, these cameras are subject to high image distortion. This necessitates specialised calibration solutions related to fisheye images, which represent the primary geometry of the raw files. This paper evaluates fisheye calibration processes for the effective utilisation of low-cost spherical cameras, for the purpose of 3D reconstruction and the verification of geometric stability. Calibration optical parameters include focal length, pixel positions, and distortion coefficients. Emphasis was placed on the evaluation of solutions for camera calibration, calibration network design, and the assessment of software or toolboxes that support the correspondent geometry and calibration for processing. The efficiency in accuracy, correctness, computational time, and stability parameters was assessed with the influence of calibration parameters based on the accuracy of the 3D reconstruction. The assessment was conducted using a previous case study of graffiti on an underpass in Wiesbaden, Germany. The robust calibration solution is a two-step calibration process, including a pre-calibration stage and the consideration of the best possible network design. Fisheye undistortion was performed using OpenCV, and finally, calibration parameters were optimized with self-calibration through bundle adjustment to achieve both calibration parameters and 3D reconstruction using Agisoft Metashape software. In comparison to 3D calibration, self-calibration, and a pre-calibration strategy, the two-step calibration process has demonstrated an average improvement of 2826 points in the 3D sparse point cloud and a 0.22 m decrease in the re-projection error value derived from the front lens images of two individual spherical cameras. The accuracy and correctness of the 3D point cloud and the statistical analysis of parameters in the two-step calibration solution are presented as a result of the quality assessment of this paper and in comparison with the 3D point cloud produced by a laser scanner.
摄影测量学的发展受到相机技术进步的显著影响，尤其是球形相机的出现。这些设备提供了广泛的摄影覆盖范围，并且由于其显著的用户友好配置，特别是低成本版本，越来越多地用于许多摄影测量应用中。尽管它们具有优势，但这些相机存在高图像失真问题。这就需要与鱼眼图像相关的专门校准解决方案，鱼眼图像代表原始文件的主要几何形状。本文评估鱼眼校准过程，以有效利用低成本球形相机，用于三维重建和几何稳定性验证。校准光学参数包括焦距、像素位置和畸变系数。重点在于评估相机校准解决方案、校准网络设计以及支持相应几何形状和校准处理的软件或工具箱。基于三维重建的精度评估校准参数的影响，对精度、正确性、计算时间和稳定性参数方面的效率进行了评估。评估是通过德国威斯巴登一个地下通道涂鸦的先前案例研究进行的。稳健的校准解决方案是一个两步校准过程，包括预校准阶段和考虑最佳可能的网络设计。使用OpenCV进行鱼眼去畸变，最后通过光束法平差进行自校准来优化校准参数，以使用Agisoft Metashape软件实现校准参数和三维重建。与三维校准、自校准和预校准策略相比，两步校准过程在三维稀疏点云中平均提高了2826个点，并且从两个单独球形相机的前镜头图像得出的重投影误差值降低了0.22米。本文通过质量评估以及与激光扫描仪生成的三维点云进行比较，给出了两步校准解决方案中三维点云的精度和正确性以及参数的统计分析结果。
