复用加权二进制编码条纹三维测量方法

Three-Dimensional Measurement Method of Reusing Weighted Binary Coded Stripes

二值条纹不受三维测量系统非线性伽马效应的影响,同时能显著提高投影效率。基于此,提出一种复用加权二进制编码条纹方法以实现三维测量。首先,对传统正弦条纹的灰度进行特定均匀采样,使用二进制编码条纹方法生成8幅二值条纹。然后,对二值条纹组合进行唯一化处理,将4幅相异的二值条纹投影到被测物体表面,对采集到带有物体表面调制信息的二值条纹进行复用二进制加权运算,生成调制后的正弦条纹。最后,对该方法开展了多组对照实验。实验结果表明,所提方法将用于加权生成正弦条纹的二值条纹优化为4幅后,仍然可以保持良好的测量精度和效果。所得标准球的局部点云与拟合标准球的平均距离,相比于传统三步相移技术降低了72.3%。

Objective There are two primary threedimensional(3D)measurement error sources of structural light based on the phase shift method:phase shift error and nonlinear error.With the development of a digital projector,the computer can produce standard sinusoidal stripes to eliminate the phase shift error.However,the nonlinear error of the projector and camera will cause the stripe to lose particular sinusoidal properties,affecting the measurement accuracy and effect.To reduce the nonlinear error of the system,global scholars have put forward various solutions,among which the binary stripe method is the most widely studied.The binary stripe is not affected by nonlinearity because it has only two gray values.The projection speed can be significantly improved by a digital projector projecting 1-bit binary stripes.In the study of many binary stripes,the binary coded stripe method uses multiple binary stripes to generate a sinusoidal stripe,which avoids defocusing projection,effectively reduces the nonlinear effect of the system,and improves measurement accuracy and projection efficiency.Based on the binary coded stripe method,this paper proposes a method of reusing weighted binary coded stripes,which significantly reduces the number of binary stripes weighted with a sinusoidal stripe and further improves the projection efficiency.Methods This paper proposes a method of reusing weighted binary coded stripes.The binary coded stripe method samples the gray value of sinusoidal stripes uniformly to obtain the discrete decimal gray values of a sinusoidal stripe.The gray values are then processed by binary coding,and all of the code words with the same level of binary code words are combined to generate binary stripes.After sequential projection by a digital projector,the collected stripe images are weighted in binary to generate a sinusoidal stripe modulated by object depth information.In the implementation process of binary coded stripes,a certain number of repeated binary stripes will appear under a specific grayvalue sampling number.After comparison,12 gray values are uniformly sampled within a sinusoidal period,and the binary coding generates binary stripes.After unique processing,the same binary stripes only need to be projected once,and repeated weighting is performed in the calculation process.As a result,only four binary stripes are required to generate a sinusoidal stripe.Finally,the method is combined with the threestep phase shift technique.The complementary gray code method is also used to carry out the phase unwrapping,which can realize an efficient 3D measurement with 20 binary stripes in the state of constant focus.Results and Discussions This paper uses the proposed method,threestep phase shift method,and the fourstep phase shift method to measure different objects and carry out comparison experiments of different gray values,and the comparison experiment with the traditional methods aims to verify the superiority of the method of reusing weighted binary coded stripes.In the first experiment,through the measurement of a standard sphere,the average distance difference between local point cloud data obtained by different gray values and fitted standard sphere is small.The average distance obtained by the threestep phase shift optimization method is 0.0153 mm,and that by the fourstep phase shift optimization method is 0.0107 mm(Fig.6 and Table 3).As shown in Fig.7,the actual item is measured,and the reconstructed results are comparable,which verifies that the proposed method of reusing weighted binary coded stripes can still maintain high accuracy and measurement impact after reducing the number of projections.In the comparison experiment with the traditional methods,sinusoidal fitting is carried out on the sinusoidal stripe obtained by the traditional methods and the proposed method.The rootmeansquare error(RMSE)of the sinusoidal fitting is 3.6082 and 3.3125,and the sum of squared errors(SSE)is 3529.3 and 3028.4(Fig.8 and Table 4).Linear fitting is performed on the measurement results of the highprecision plane.The RMSE is 0.0415 and 0.0388 mm,respectively,and the SSE is 0.4804 and 0.4493 mm(Fig.9 and Table 5).As demonstrated in Fig.10 and Table 6,after measuring the standard sphere,the average distance between the local point cloud and the fitted standard sphere is reduced by 72.3%when compared with that by the traditional threestep phase shift method.For the measurement of the plaster head,whose surface depth varies greatly,the traditional threestep and fourstep phase shift methods have striplike systematic errors due to the nonlinear effects of the system.However,the surface reconstruction effect of the proposed method is better(Fig.11).Conclusions This paper proposes a method of reusing weighted binary coded stripes.A better sampling scheme is designed by further studying the principle of the binary coded stripe method.After the unique processing of binary coding and binary stripes,the method of reusing weighted binary coded stripes generates sinusoidal stripes to reduce the actual projection number of binary stripes.In order to generate a sinusoidal stripe,eight weighted binary stripes are reduced to four binary stripes.As a result,only 20 binary stripes are required to complete the 3D measurement by the threestep phase shift method and the complementary gray code phase unwrapping method.The results of comparison experiments show that the proposed method will not reduce the measurement accuracy and effect while significantly reducing the projection number by the binary coded stripe method.Compared with the traditional phase shift method,the proposed method can significantly reduce the nonlinear effect of the system and further improve the projection rate of the DLP projector.In conclusion,the proposed method effectively reduces the projection number required by the binary coded stripe method and provides technical support for highspeed 3D measurement based on phase shift stripe analysis.