Research On Speckle Displacement Measurement Technology Based On Optical Flow

Deformation measurement and surface shape measurement are the most basic and important measurement technology in the field of industry and scientific research.They involve the reliability of high-tech fields and industrial production,and are closely related to the safety of our daily work and life.As the information basis for other research and mechanical property analysis,they are one of the important demands of today’s scientific and technological development.Speckle is a speckle caused by coherent light reflected from a rough surface.The speckle based deformation measurement technique has good performance in both one-and two-dimensional deformation measurement,but its measurement sensitivity and accuracy in three-dimensional surface need to be improved.Optical flow is an apparent motion pattern of objects,surfaces and edges in a visual scene caused by the relative change of position between the viewing point and the object.It can also be defined as the distribution of apparent moving speed of brightness patterns in an image.The essence of optical flow field is the two-dimensional projection of motion displayed on the observed surface when the object is moving.Optical flow detection technology has the advantages of high sensitivity,high precision,and includes the time factor,which is suitable for dynamic measurements.It is widely used in target detection and tracking,image dominance plane extraction,motion detection,robot navigation and visual odometer.The displacement measurement techniques based on optical flow mainly include differential method,the phase method,the energy method and region matching method,among which the most widely used methods are HS method,LK method and Brox method.The research group takes the lead in the research of fringe optical flow displacement measurement technology and fringe based surface shape measurement technology,and has obtained a series of research results.Recently,it has been reported that speckle optical flow is used to measure in-plane displacement,but the characteristics of speckle optical flow displacement measurement are not discussed in depth,and the three-dimensional surface shape measurement of objects is not reported by using speckle optical flow combination.This paper intends to study the measuring range of speckle optical flow and use speckle optical flow to measure surface shape.The research content of this paper is as follows:1.Simulation of precise speckle displacement.The speckle field and the speckle field with small displacement is generated by computer simulation to prepare the speckle field displacement detection by optical flow.2.Discussion on the displacement measurement and measuring range of speckle field by optical flow technology.Two optical flow methods,LK local optical flow and Brox optical flow,were used to measure the speckle field displacement and discuss the measuring range.A series of speckle images with different displacement were generated by simulation.The displacement between them was calculated by using the LK optical flow method and Brox optical flow method,and the measuring range of optical flow in the speckle displacement was discussed.3.The influence of noise on speckle optical flow displacement measurement is discussed.Noise is added to the speckle pattern before and after displacement and displacement is calculated by optical flow.The robustness of speckle optical flow method against noise is discussed by comparing with the calculation results without noise.4.Research on calibration technology of projector.Relative to camera calibration,projector calibration is much more difficult.Brox optical flow method is intended to be used to calibrate the position of projector.Firstly,the position calibration of projector using two calibration images is analyzed theoretically.Secondly,simulation experiments are used to prove the feasibility of the calibration theory.Finally,experimental verification.It is proved that this calibration method can be used for actual measurement.This new calibration method has fast computation speed,good accuracy and good robustness to noise,so it can be used in the experiment of speckle optical flow measurement of three-dimensional surface shape in this paper.5.Discussion on 3D surface shape measurement technology based on speckle and optical flow.The theory of three-dimensional surface shape measurement technology of speckle optical flow is discussed theoretically,and the experimental system parameters are set up based on the theory.The methods used to verify the theory are simulation and experiment.The steps of simulation and experiment are as follows: First,according to the principle of optical flow displacement measurement and experimental parameters,the measurement system is built;secondly,the spherical crown with shape changing slowly and the exact solution is selected as the measurement object to facilitate the evaluation of the measurement results.Thirdly,the speckle patterns before and after height modulation were simulated or collected experimentally.Through two speckle patterns before and after deformation,the optical flow algorithm was used to restore the three-dimensional surface shape of the specimen.Fourthly,the influence of different speckle particle size,speckle number and Gaussian white noise on experimental results was compared by root-mean-square error and absolute error,and the optimal speckle size,speckle number and signal-to-noise ratio was obtained.The optimal algorithm was obtained by comparing the simulation results of LK and Brox optical flow methods.Finally,experimental verification is carried out.Aiming at the surface shape measurement experiment system,Brox method projector calibration technology was used to calibrate the position of projector.Using the optimal optical flow algorithm and parameter size,the 3D surface shape measurement experiment of the actual object(cald mask,whiteboard with holes and fox mask)is carried out to verify that the optical flow method based on speckle projection can accurately restore the height of the measured object.