Research On Fringe Projection Profilometry Based On Spatiotemporal Hybrid Coding Strategy

With the continuous development of science and technology,2-dimensional image data can no longer meet the needs of people’s production and life.Industrial manufacturing,reverse engineering,product testing,bioengineering,medical imaging,virtual reality and other fields urgently need the support of 3-dimensional(3D)measurement technology.As a typical structured light measurement method,fringe projection profilometry has become one of the mainstream 3D measurement technologies due to its high accuracy and fast speed.Fourier transform profilometry and phase-shifting method are commonly used phase calculation methods in fringe projection profilometry,but in order to achieve high-precision measurement,both methods need to use phase unwrapping algorithms to achieve absolute phase recovery.The mainstream phase unwrapping methods can be divided into spatial methods and temporal methods.The spatial methods are fast and the temporal methods are accurate.In order to ensure the measurement speed and accuracy at the same time,this paper introduces spatial strategies into the temporal methods to achieve phase unwrapping.The main work is as follows:(1)Due to the influence of system noise and random noise,the number of global cycles of the traditional two-frequency method is limited.To use a large number of highfrequency cycles for wide range and high-precision measurements,it is necessary to add additional patterns to improve the stability of the method.But,the addition of patterns means reduction in the measurement speed.In order to solve this problem,this paper proposed a dispersed spatial coding strategy,which can increase the number of global cycles of the two-frequency method without adding additional patterns.In this way,faster measurement of the two-frequency algorithm in wide range and high-precision measurement situations can be realized.(2)The measurement speed of the two-frequency algorithm is closely related to the number of measurement patterns.The traditional two-frequency algorithm requires at least six patterns to be projected and photographed.The improved mixed-intensity twofrequency algorithm still needs five patterns to complete the measurement.In order to further reduce the number of patterns,this paper proposed a mixed-phase two-frequency algorithm,which can reduce the number of patterns to four frames.Furthermore,a continuous spatial coding strategy suitable for the mixed-phase two-frequency algorithm is proposed,which can double the number of global cycles of this algorithm.(3)The phase-coding method encodes the fringe order by using the stair phase,but in actual measurement,noise often leads to a boundary misalignment between the fringe order and the wrapped phase,which makes the absolute phase unwrapped wrongly.In order to solve this problem,this paper proposed a half-period phase-coding method,which can generate two fringe orders with half-period offset.Correct recovery of the absolute phase can be achieved by the complementary nature between the two fringe orders.In addition,the method is combined with the geometric constraint method to avoid the decrease of the measurement accuracy of the algorithm due to the insufficient number of stair phase.