Haihua Cui

Reliability-guided phase-unwrapping algorithm for the measurement of discontinuous three-dimensional objects

Accurate and robust phase unwrapping is an important procedure for three dimensional (3D) profilometry measurement. The mask cut phase-unwrapping algorithm merges the characteristics of Goldsteins branch cut and reliability-guided path-following algorithms, so that it has higher accuracy and stability. However, it cannot handle discontinuous phase regions because it completely relies on the phase quality map to guide the placement of mask cuts, thereby easily causing phase error propagation. To overcome these drawbacks, we propose a new phase-unwrapping method that merges the residue check principle and a Laplace phase derivative variance quality map that we developed. First, the entire phase was divided into several isolated regions in accordance with the quality map. Then, each pixel of the absolute phase marker line was taken as the starting point to unwrap the discontinuous phase regions based on reliability guidance. In addition, a new list-trimming algorithm was employed to guarantee a speedy phase-unwrapping procedure. The entire phase unwrapping and accurate 3D measurement of discontinuous objects was successfully completed. The simulated and experimental data both demonstrate the validity of the proposed phase-unwrapping algorithm.

Calibration Algorithm for Structured Light 3D Vision Measuring System

A novel procedure is proposed to calibrate a structured light 3D vision measuring system. Firstly, robust subpixel and substripe calibration methods are proposed for the camera and the projector. Secondly, a novel approach is introduced to solve the nature restriction of the structure light system: the calibration of the camera and the project is restricted with the locations of them. Thirdly, the systemic calibration procedure based on rigidity constraint to system architecture is developed according to the stereo vision, the L-M optimization algorithm is used to adjust the internal and external parameters of each respective component in this system. Finally, this system calibration algorithm is proved to be robustness and effectiveness by reconstructing a 3-D standard cylinder. The experiment results show that the precision of single view is about 0.06mm over a measurement volume of 250mm 200mm 250mm. It roughly meets the need of the industry.

A three-step system calibration procedure with error compensation for 3D shape measurement

System calibration, which usually involves complicated and time-consuming procedures, is crucial for any three-dimensional (3D) shape measurement system based on vision. A novel improved method is proposed for accurate calibration of such a measurement system. The system accuracy is improved with considering the nonlinear measurement error created by the difference between the system model and real measurement environment. We use Levenberg-Marquardt optimization algorithm to compensate the error and get a good result. The improved method has a 50% improvement of re-projection accuracy compared with our previous method. The measurement accuracy is maintained well within 1.5% of the overall measurement depth range.

A new phase error compensation method of 3-D shape measurement system using DMD projector

Structured light system using a digital micro-mirror device (DMD) projector is increasingly used for a 3-D shape measurement because of its digital nature. When the DMD projector is used in phase measuring profilometer (PMP), the precision of profile measurement will increase with the precision of phase-shift increasing. But the non-sinusoidal nature of the projected fringe patterns causes significant phase measurement error and consequent shape measurement error. In the reality, we find that the non-sinusoidal effect is never caused by only one factor. A real measurement shows that it is a combination of influences by all effects, e.g., the object independent irradiance function, the nonlinear gamma curve of the projector, the spatio-temporal characteristic of DMD, etc. In view of the above factors, a comprehensive compensation method is proposed to compensate these factors for the 3D measurement. If the compensation is well accomplished, the measurement error can be reduced average 6 times. The experiment is carried out to demonstrate the validity of this technique.

An accurate simulation algorithm for focus variation microscopy

The focus variation microscopy is widely used and researched in both industrial and academic field. But the 3D construction quality of surface topography is affected by the noise, the double peak value, and the discontinuous surface, and so on. A simulation method for focus variation is proposed based on the physical model of optical imaging and the Point Spread Model(PSF). At first, the linear relationship between the blur factor σ of Gaussian function and the defocus distance δ is deduced which is called point spread parameter λ, then, considering the positive correlation between blur factorσ and blur degree, the difference between the real defocused image and the calculated image by Gaussian convolution operation to real captured focused image is shown. It is used to the objective to computed the accurate value σ and the spread parameter λ. At last, the difference between focus measure valued of real image sequence and simulation image sequence is used to verify the method. The simulation method is a judgement basis for focus measurement, the single peak of focus curve, and the identification of high-frequency noise.

Flexible point cloud matching method based on three-dimensional image feature points

Flexible and robust point cloud matching is important for three-dimensional surface measurement. This article proposes a new matching method based on three-dimensional image feature points. First, an intrinsic shape signature algorithm is used to detect the key shape feature points, using a weighted three-dimensional occupational histogram of the data points within the angular space, which is a view-independent representation of the three-dimensional shape. Then, the point feature histogram is used to represent the underlying surface model properties at a point whose computation is based on the combination of certain geometrical relations between the point’s nearest k-neighbors. The two-view point clouds are robustly matched using the proposed double neighborhood constraint of minimizing the sum of the Euclidean distances between the local neighbors of the point and feature point. The proposed optimization method is immune to noise, reduces the search range for matching points, and improves the correct feature point matching rate for a weak surface texture. The matching accuracy and stability of the proposed method are verified using experiments. This method can be used for a flat surface with weak features and in other applications. The method has a larger application range than the traditional methods.

High precision surface measurement with a trans-scale optical measurement method

Increasing requirements on the complexity and accuracy of dimensional metrology demand the application of aerospace turbine blade, cutting tools, and so on. The multi-scale data of holistic geometrical is needed. In order to obtain all meaningful details of the surface at various required scales, a novel trans-scale optical measurement method mixing macro and micro measurement technology is proposed. The optical scanning system is composed of a variable-focus structured light sensor fitted with zoom lens camera and a focus variation microscopy sensor. The structured light sensor is used to acquire the form of the object. The focus variation microscopy sensor is used to acquire the waviness or roughness of the object. The macro structured light sensor can flexibly zoom in or out to measure a 3D object profile in sections according to the approximate surface profile and the view of the micro measurement system. It originally connects the macro and micro scale at view, resolution, precision. The different scale measurement data are registered and fusion with multi-dimensional images which includes 2D image, 2.5D range image, and 3D point cloud image. Experimental measurement results show that macro holistic geometry profile and micro surface texture can be acquired with the developed method in a single frame system.

A colorful codification method for structured light measurement based on the hue of single image

To measure the shape of object with high-speed and avoid the disturbance of the vibration is remaining challenges faced by structured-light projection method. This paper proposes a high-speed optical metrology by coding three cosine patterns into three channels of RGB model to form the pattern. When the color image is obtained by camera, it will be transformed to HSI color model (hue, saturation and intensity). The hue component is regarded as the phase information to retrieve the 3D shape of object with single image, while the saturation and intensity are applied to avoiding phase errors caused by height steps or spatially isolated surfaces. This method can be used to measure object with non-monochromatic surfaces after the color compensated. Experimental results verify the feasibility of the developed method.

A flexible new method for 3D measurement based on multi-view image sequences

Three-dimensional measurement is the base part for reverse engineering. The paper developed a new flexible and fast optical measurement method based on multi-view geometry theory. At first, feature points are detected and matched with improved SIFT algorithm. The Hellinger Kernel is used to estimate the histogram distance instead of traditional Euclidean distance, which is immunity to the weak texture image; then a new filter three-principle for filtering the calculation of essential matrix is designed, the essential matrix is calculated using the improved a Contrario Ransac filter method. One view point cloud is constructed accurately with two view images; after this, the overlapped features are used to eliminate the accumulated errors caused by added view images, which improved the camera’s position precision. At last, the method is verified with the application of dental restoration CAD/CAM, experiment results show that the proposed method is fast, accurate and flexible for tooth 3D measurement.

A robust real-time laser measurement method based on noncoding parallel multi-line

Single line scanning is the main method in traditional 3D hand-held laser scanning, however its reconstruction speed is very slow and cumulative error is very large. Therefore, we propose a method to reconstruct the 3D profile by parallel multi-line 3D hand-held laser scanning. Firstly, we process the two images that contain multi-line laser stripes shot by the binocular cameras, and then the laser stripe centers will be extracted accurately. Then we use the approach of stereo vision principle, polar constraint and laser plane constraint to match the laser stripes of the left image and the right image correctly and reconstruct them quickly. Our experimental results prove the feasibility of this method, which improves the scanning speed and increases the scanning area greatly.