The present work proposes a methodology for 3D shape acquisition of objects through rotation-based Laser Line Scanning. This
enables the acquisition of an object's 3D shape from multiple views,
which leads to a more complete and accurate model

João Manuel R. S. Tavares

Paulo J. S. Tavares

Pedro J. Sousa

Pedro M. G. P. Moreira

English

Repositório Aberto da Universidade do Porto

Shape acquisition of rotating objectsbased on Laser Line ScanningPedro J. Sousa1,2, João Manuel R. S. Tavares1,2, Paulo J. S.Tavares2, and Pedro M. G. P. Moreira21 Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465Porto, Portugal2 Instituto de Ciência e Inovação em Engenharia Mecânica e Engenharia Industrial,Rua Dr. Roberto Frias 400, 4200-465 Porto, PortugalAbstract. The present work proposes a methodology for 3D shape ac-quisition of objects through rotation-based Laser Line Scanning. Thisenables the acquisition of an object's 3D shape from multiple views,which leads to a more complete and accurate model.Keywords: Computer vision · Laser line scanning · 3D shape acquisi-tion.1 IntroductionA methodology was developed to acquire the 3D shape of a ro-tating object. It is based on Laser Line Scanning, a widespread [6,1]shape acquisition technique that uses a laser line to obtain the 3Dshape of a moving object. The traditional method, using linear mo-tion, usually has the drawback that only a single view of the objectcan be acquired with a system. By rotating the object, the currentmethodology aims to allow the acquisition of the whole shape.The application of the proposed methodology starts with a cal-ibration procedure, involving three components: a Camera, a Laserplane and Motion.The camera calibration denes the intrinsic and extrinsic param-eters and the distortion coecients and is based on the acquisitionof multiple images of a calibration pattern [2].The second step involves dening the laser plane, i.e. the spatiallocation of the laser. This can be done using three images wherethe laser is aligned with notable locations of the calibration pattern,where the world coordinates of those notable points are extracted.Using these, it is possible to dene a plane by plane tting [3].The motion calibration is performed at a constant frame rate,acquiring an image sequence of the same calibration pattern per-forming the target motion. From a subset of evenly spaced frames,the axis, angle and centre of rotation between each pair of sequen-tial frames are calculated. This includes the use of a Least-Squaresmethod developed by Spoor and Veldpaus [5] to calculate a best-trotation matrix.Having completed the calibration procedure successfully, the 3Dshape of the object under study is obtained using an image sequenceof the laser line on the surface of the object under rotation, acquiredat a constant frame rate.The image coordinates of the laser line are extracted from eachframe [4] and the laser plane calibration is used to extract the worldcoordinates of the line, as dened in [3]. This originates a partialmodel of the object, i.e. a small subset of points, acquired from thelaser line at a particular angular position, which is combined withthe others to form the nal point cloud.An incremental rotation matrix, i.e. from one frame to the next,is dened from the calibrated data and then the reconstruction ofthe point cloud from the partial models is performed incrementally.The reconstruction steps can be summarized as:1. Add a new partial model to the point cloud (matrix [p]);2. Rotate the current state of the point cloud using the incrementalrotation matrix (R) and the rotation centre (X0): [p] = R · ([p]−X0) +X0;3. Repeat steps (1) and (2) for every partial model.At the end of the procedure, a point cloud is obtained for theobject under reconstruction.2 Experimental Setup and ResultsThe experimental implementation of a 3D laser line scanningbased approach requires at least three dierent components: a cam-era, a laser line and a positioning system, Figure 1.The camera was positioned in front of the positioner, formingan angle with the laser line, which was approximately aimed at thecentre of rotation.Laser lineCameraCentre ofRotationPositionerObjectPositionerControllerFig. 1: Used Experimental SetupThe laser line was created using a 520 nm, 50 mW laser fromCivilLaser (China) and the positioning system includes a gonio-metric cradle, BG50-0.01 from Micro-Controle (France). This cra-dle provides the needed motion, but does not reach a 360◦ rotation.Nonetheless, for proof of concept, it is enough.The methodology was applied to the acquisition of a small statueof Leonardo Da Vinci and the results were promising, having ob-tained a shape that is close to the reality, Figure 2.Fig. 2: Experimental object used (left) and 3D acquired shape (right)For validation, the width of the statue's nametag, which measuresapproximately 40mm, was compared to the acquired shape. Theobtained value was 40.4596mm, indicating an error below 1.5%.3 ConclusionsThis work involved the development and application of a method-ology for 3D shape acquisition of rotating objects that is based onlaser line scanning.The proposed methodology was applied to the acquisition of asmall statue of Leonardo Da Vinci and the results were promising,having obtained a shape that is close to the reality with small errors,around 1.5%. Meanwhile, the angular resolution of the experimentalapplication was around 0.13◦ and the radial resolution was approxi-mately 0.35mm.The stability of the image acquisition frame rate and of the mo-tion are important factors to consider in order to minimize the errors.Fluctuations in either of those parameters can lead to inaccurate cal-ibrations.Finally, considering the obtained results, it is possible to concludethat the proposed methodology is a very capable tool for shape acqui-sition, specially when a 360◦ 3D shape reconstruction is important.AcknowledgmentsPedro J. Sousa gratefully acknowledges the Fundação para aCiência e a Tecnologia (FCT) for the funding of the PhD scholarshipSFRH/BD/129398/2017.The authors gratefully acknowledge the funding of ProjectNORTE-01-0145-FEDER-000022  SciTech  Science and Tech-nology for Competitive and Sustainable Industries, conanced byPrograma Operacional Regional do Norte (NORTE2020), throughFundo Europeu de Desenvolvimento Regional (FEDER).References1. Feng, Q.: Practical Application of 3D Laser Scanning Techniques to UndergroundProjects. Tech. rep., MultiInfo 3D Laser Scan Solution AB (2012)2. MathWorks: Calibrate a single or stereo camera - MATLAB esti-mateCameraParameters, https://www.mathworks.com/help/vision/ref/estimatecameraparameters.html3. Niola, V., Rossi, C., Savino, S.: A new real-time shape acquisition with a laserscanner: First test results. Robotics and Computer-Integrated Manufacturing 26(6),543550 (2010). https://doi.org/10.1016/j.rcim.2010.06.0264. SICK IVP: Ranger E/D Reference Manual (2008)5. Spoor, C.W., Veldpaus, F.E.: Rigid body motion calculated from spatialco-ordinates of markers. Journal of Biomechanics 13(4), 391393 (1980).https://doi.org/10.1016/0021-9290(80)90020-26. Wehr, A., Lohr, U.: Airborne laser scanning - an introduction and overview.ISPRS Journal of Photogrammetry & Remote Sensing 54, 6882 (1999).https://doi.org/Doi 10.1016/S0924-2716(99)00011-8

Shape acquisition of rotating objects based on Laser Line Scanning

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Shape acquisition of rotating objects based on Laser Line Scanning

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