3D Reconstruction Based On High-speed DMD Grating Projection System

As the mainstream technology of 3D measurement,optical 3D measurement technology has the characteristics of simple and easy to carry,high efficiency,non-contact,non-destructive,large working distance and etc.More than that,it can measure static or dynamic objects,and thus is widely used in medical,Reverse Engineering,3D Printing,Video Games,Industrial Design,Heritage Conservation,Robotics,Driverless and etc.Among them,the grating phase shift method based on time-phase unwrapping is favored due to its high accuracy and insensitivity to environmental disturbances.However,the grating phase shift method needs to project a plurality of stripe patterns,and the commercial projector projection refresh frequency is generally 50-60 Hz,which limits the projection speed and cannot meet the real-time highprecision three-dimensional measurement needs.In addition,The Jedi phase value of the grating phase shift method is an arctangent function,which will lead to the jump of the phase value in the 2? interval.The measuring effect on the surface with abrupt change and the multiple isolated objects at the same time as well as the resistance to environmental interference are not satisfactory.To this end,the study points in this paper based on the domestic and foreign researches are as following:(1)Aiming at the problem of slow projector projection,this paper designs a set of projection scheme based on DMD control system.Results show the projection speed has been highly improved.The projection of binary raster can reach as fast as 10K sheets per second and the fastest raster can reach 140 sheets per second.(2)Aiming at the problem of non-linear gamma error existing in sinusoidal stripe grating,a dithering technique based on Stucki error diffusion is adopted,that is,the sinusoidal grating is dithered to binary image by Stucki error diffusion function,and then the binary image is defocused.Results show this method can not only further improve the projection speed,but also eliminate the gamma non-linearity error in the projection equipment.(3)Aiming at the measurement accuracy and anti-environmental interference,this paper adds the phase coding method based on the original three-step phase shift method.Firstly,the gray-scale steps are set according to the pitch of the sine stripes,and each gray-scale step information is regarded as a code word.Afterwards,the order of the stripes can be determined according to the code words.The 2? periodic jump in the package phase is eliminated point by point according to the code word set in the stripe,and finally the absolute phase would be obtained.On this basis,we design fewer code words,and accordingly increase the difference between the width and the grayscale of the coding stripes,making it easier to distinguish the edges of adjacent code words.Compared with the traditional phase coding method,this method can not only improve the measurement accuracy and reducing noise interference of the camera,but also improve the robustness of measuring complex shapes and isolated objects.Through the analysis of multiple sets of target depth measurements,the proposed method can achieve the depth measurement with high accuracy,high speed and robustness.