A Multi Camera and Multi Laser Calibration Method for 3D Reconstruction of Revolution Parts

This paper describes a method for calibrating multi camera and multi laser 3D triangulation systems, particularly for those using Scheimpflug adapters. Under this configuration, the focus plane of the camera is located at the laser plane, making it difficult to use traditional calibration methods, such as chessboard pattern-based strategies. Our method uses a conical calibration object whose intersections with the laser planes generate stepped line patterns that can be used to calculate the camera-laser homographies. The calibration object has been designed to calibrate scanners for revolving surfaces, but it can be easily extended to linear setups. The experiments carried out show that the proposed system has a precision of 0.1 mm.

ON-SITE GEOMETRIC CALIBRATION OF THERMAL AND OPTICAL SENSORS FOR UAS PHOTOGRAMMETRY

UAS imagery has become a widely used source of information in geomorphic research. When photogrammetric methods are applied to quantify geomorphic change, camera calibration is essential to ensure accuracy of the image measurements. Insufficient self-calibration based on survey data can induce systematic errors that can cause DEM deformations. The typically low geometric stability of consumer grade sensors necessitates in-situ calibration, as the reliability of a lab based calibration can be affected by transport. In this research a robust on-site workflow is proposed that allows the time-efficient and repeatable calibration of thermal and optical sensors at the same time. A stone building was utilised as calibration object with TLS scans for reference. The approach was applied to calculate eight separate camera calibrations using two sensors (DJI Phantom 4 Pro and Workswell WIRIS pro), two software solutions (Vision Measurement System (VMS) and Agisoft Metashape) and two different subsets of images per sensor. The presented results demonstrate that the approach is suitable to determine camera parameters for pre-calibrating photogrammetric surveys.

Accuracy of measurements performed on digital panoramic radiographs with and without an extra-oral calibration object

Aim: To investigate the accuracy of measurements in vertical and horizontal direction using an extra-oral calibration object placed in different positions on the panoramic radiograph in JPEG and DICOM image format. Methods: Digital panoramic radiographs of a purpose made model with 32 removable teeth replaced with metal balls were taken. The measurements of metal balls were performed with and without the calibration object placed in the middle or on the side of the radiograph in JPEG and DICOM image formats. Results: One sample t-test was used for the analyses of accuracy of measurements in vertical and horizontal direction. The most accurate vertical measurements were achieved in canine group in JPEG (6.02±0.04 mm, P=0.144) and DICOM (6.03±0.07 mm, P=0.104) formats using calibration object placed in the middle of the radiograph. The mean values of measurements in horizontal direction differed significantly from the real values (P<0.05) in all teeth groups regardless of the image format. Conclusion: The most accurate measurements in vertical direction were achieved by placing the metal scale ruler extra-orally in the middle of panoramic radiograph independent of the image format. Reliable clinical accuracy for measurements in horizontal direction was achieved only in canine region (G2) in both image formats.

CAMERA CALIBRATION USING MULTIPLE UNORDERED COPLANAR CHESSBOARDS

The now widely available and highly popular among non-expert users, particularly in the context of UAV photogrammetry, Structure-from-Motion (SfM) pipelines have also further renewed the interest in the issue of automatic camera calibration. The well-documented requirements for robust self-calibration cannot be always met, e.g. due to restrictions in time and cost, absence of ground control and image tilt, terrain morphology, unsuitable flight configuration etc.; hence, camera pre-calibration is frequently recommended. In this context, users often resort to flexible, user-friendly tools for camera calibration based on 2D coded patterns (primarily ordinary chessboards). Yet, the physical size of such patterns poses obvious limitations. This paper discusses the alternative of extending the size of the calibration object by using multiple unordered coplanar chessboards, which might accommodate much larger imaging distances. This is done initially by a detailed simulation to show that – in terms of geometry – this could be a viable alternative to single patterns. A first algorithmic implementation is then laid out, and results from real multi-pattern configurations, both ordered and unordered, are successfully compared. However, aspects of the proposed approach need to be further studied for its reliable practical employment.

Agricultural Robot Localization Based on Camera Ranging

In this paper, a camera ranging method for agricultural robot localization in arable farming is described. Three calibration objects in different directions in the field are set. The distances between the robot are calculated, and the calibration objects are used the camera to obtain the robot position. The benchmarking procedure in Matlab includes image binarization of the calibration object and the curve fitting method was used to establish the relationship between the pixels number of the calibration object and the actual distance. The verification results show that the relative error of ranging is less than 1.5%. Furthermore, the camera ranging method for agricultural robot localization is proved to be easy and feasible.