Yanjun Fu

Fringe projection profilometry based on a novel phase shift method

Fringe projection profilometry is generally used to measure the 3D shape of an object. In oblique-angle projection, the grating fringe cycle is broadened on the reference surface. A well-fitted, convenient, and quick cycle correction method is proposed in this study. Based on the proposed method, an accurate four-step phase shift method is developed. Comparative experiments show that the fringe projection profilometry based on the novel phase shift method can eliminate cycle error and significantly improve measurement accuracy. The relative error of the measurement is less than 1.5%. This method can be widely employed for measuring large objects.

Three-dimensional shape measurement of complex surfaces based on defocusing and phase-shifting coding

Binary fringe defocused projection is a promising method for high-speed, high-accuracy three-dimensional (3D) shape measurement because of the advantages of superfast projection and capability to overcome the nonlinear gamma of the projector. However, the application of this approach for complex surface measurement remains a challenge. Thus, a method that combines binary fringe and phase coding fringe based on defocusing is proposed to achieve fast-speed, high-accuracy 3D shape measurement of complex surfaces. However, in the case of defocusing, as the number of phase coding fringe cycles increases, the fringe order k(x, y) becomes difficult to judge, errors occur in the jump area, and the periodic dislocation problem becomes worse, ultimately leading to phase unwrapping error. A phase-shifting coding method is thus presented to resolve these issues. This method not only alleviates the difficulty in judging the fringe order as a result of defocusing but also resolves the correction problem, and in turn, the periodic dislocation problem. Thus, measurement accuracy is improved. Experimental results demonstrate the feasibility and superiority of the proposed method.

A fast 3D shape measurement method based on sinusoidal and triangular fringe projection

Abstract Existing methods to measure 3D shape of complex object involve processing more than six captured images to obtain the absolute phase, which limit the measurement speed. This paper presents two sinusoidal fringes and two triangular wave fringes which is used to measure 3D shape of complex object. The two-step phase-shifting sinusoidal fringes and two-step phase-shifting triangular wave fringes are calculated to obtain the wrapped phase, and then the two-step phase-shifting triangular wave fringes are used to determine the fringe order. Due to decrease the number of projection fringes, the speed of measurement increases. The triangular wave fringe carries more information of the object than linear increasing/decreasing ramp fringe in the actual measurement, more noise in the base phase to be overcome, thus improving the measurement accuracy. The benefits can be widely applied in high-speed, real-time 3D measurement of complex shape. Experimental results have demonstrated that the proposed method is simple, but effective.

Flexible Projector Calibration in the Structured Light 3D Measurement System

In the structured light 3D shape measurement system, the projector plays an essential part for 3D shape reconstruction. As the projector calibration accuracy affects the 3D shape measurement accuracy, a flexible projector calibration approach in the structured light system is proposed. Key to the proposed method is to establish the relationship between the projector coordinate and the world coordinate using the pre-calibrated camera. The process of the system calibration can be divided into three steps. First, a black-and-white (B/W) checkerboard is pasted on the white board plane for camera calibration. Second, the pre-calibrated camera is used to obtain the 3D world coordinates of the checkerboard corners. Here, instead of using complicated process with spatial color illuminations or phase-unwrapping method for the pixel mapping, a projected B/W checkerboard is used for the projector calibration. Unlike some other methods, the non-wide-angle camera can be used to capture the images of both pasted and projected checkerboards. Also the calibration process is not vulnerable to the environment. Finally, the results of the camera and projector calibration are used to reconstruct 3D shape. Experiments show that the reprojection errors for both camera and projector are within ±1 pixel, and the RMS error of 3D reconstruction is 0.25 mm.

A new three-dimensional shape measurement method based on double-frequency fringes

Fringe projection profilometry (FPP) is a rapidly developing technique which is widely used for industrial manufacture, heritage conservation, and medicine etc. because of its high speed, high precision, non-contact operation, full-field acquisition, and easy information processing. Among the various FFP methods, the squared binary defocused projection method (SBM) has been promptly expanding with several advantages: (1) high projection speed because of 1-bit grayscale fringe; (2) eliminating nonlinear gamma of the projector for the defocusing effect. Nevertheless, the method is not trouble-free. When the fringe stripe is wide, it brings down the fringe contrast and is difficult to control the defocused degree, resulting in a low measurement accuracy. In order to further improve high-speed and high-precision three-dimensional shape measurement, this paper presents a new three-dimensional shape measurement method based on double-frequency fringes projection. This new method needs to project two sets of 1-bit grayscale fringe patterns (low-frequency fringe and high-frequency fringe) onto the object surface under slightly defocused projection mode. The method has the following advantages: (1) high projection speed because of 1-bit grayscale fringe; (2) high measurement precision for selectively removing undesired harmonics. Low-frequency fringe is produced by error-diffusion dithering (Dithering) technique and high-frequency fringe is generated by optimal pulse-width modulation (OPWM) technique. The two kinds of fringe patterns have each superiorities and flaws. The low-frequency fringe has a low measurement accuracy, but the continue phase can be easily retrieved. However, the property of high-frequency fringe and low-frequency fringe is the opposite. The general idea of this method proposed is as follows: Because the both fringes test the same object, the height is the same. The low-frequency fringe can be used to assist the high frequency fringe to retrieve continue phase map, then the three-dimensional shape information of the object can be obtained. Theory analyzes the mathematical principle of error-diffusion dithering technique, optimal pulse-width modulation technique and three-dimensional reconstructed algorithm based on double-frequency fringes projection. A second-hand mouse was used to test the proposed method. The experiment results show that the three-dimensional shape measurement method combining OPWM technique and Dithering technique can achieve fast-speed and high-precision three-dimensional shape measurement.

Three-dimensional shape measurement based on a combination of gray-code and phase-shift light projection

Blade is the key component of the aero-engine. In generally, it requires precise size and accurate shape, so the three-dimensional shape measurement of the blade is very important. Fringe projection profilometry is generally used to measure the 3D shape of an object, because of the advantages of non-contact operation, full-field acquisition, high resolution, and fast data processing. In this paper, 3D shape measurement method based on surface structured light that combination of gray-code and phase-shift projection is proposed. The wrapped phase of the blade is got by the four-step phase-shift method. In the projection process of the gray-code, due to the ambient light, different surface reflectance and surface physical discontinuous, the edge of gray-code becomes smooth, leading to decode error, so that the gray-code pattern must be binaried before decode. Through the accurate binarization of the blade gray-code pattern, the decoding cycle of the blade is achieved, and the unwrapped phase is achieved by phase unwrapping. Then the unwrapped phase difference between the blade and reference plane is got, the height of the blade can be obtained by the relationship between the phase difference and the height. The experimental and simulation results show that the proposed method can achieve a high precision, high speed and low cost 3D shape measurement of the blade. The measurement accuracy reaches 0.03 mm. The proposed method extends the reliability and practicality of the fringe projection profilometry.

The modeling research of blade based on UG

Blade is one of the most important work pieces in Aero-engine. Owing to its structural complexity and irregularity, the three-dimensional digital point cloud of blade is obtained by projecting grating method, and UG software is used to carry on impeller and blade of solid modeling via data processing and 3D model of improvement. The design accuracy and efficiency of blade are improved through the surface modeling and virtual assembly capabilities of UG software fully applied.

Research of Image Retrieval Technology based on Color Feature

Recently, with the development of the communication and the computer technology and the improvement of the storage technology and the capability of the digital image equipment, more and more image resources are given to us than ever. And thus the solution of how to locate the proper image quickly and accurately is wanted.The early method is to set up a key word for searching in the database, but now the method has become very difficult when we search much more picture that we need. In order to overcome the limitation of the traditional searching method, content based image retrieval technology was aroused. Now, it is a hot research subject.Color image retrieval is the important part of it. Color is the most important feature for color image retrieval. Three key questions on how to make use of the color characteristic are discussed in the paper: the expression of color, the abstraction of color characteristic and the measurement of likeness based on color. On the basis, the extraction technology of the color histogram characteristic is especially discussed. Considering the advantages and disadvantages of the overall histogram and the partition histogram, a new method based the partition-overall histogram is proposed. The basic thought of it is to divide the image space according to a certain strategy, and then calculate color histogram of each block as the color feature of this block. Users choose the blocks that contain important space information, confirming the right value. The system calculates the distance between the corresponding blocks that users choosed. Other blocks merge into part overall histograms again, and the distance should be calculated. Then accumulate all the distance as the real distance between two pictures. The partition-overall histogram comprehensive utilizes advantages of two methods above, by choosing blocks makes the feature contain more spatial information which can improve performance; the distances between partition-overall histogram make rotating and translation does not change. The HSV color space is used to show color characteristic of image, which is suitable to the visual characteristic of human. Taking advance of humans feeling to color, it quantifies color sector with unequal interval, and get characteristic vector. Finally, it matches the similarity of image with the algorithm of the histogram intersection and the partition-overall histogram. Users can choose a demonstration image to show inquired vision require, and also can adjust several right value through the relevance-feedback method to obtain the best result of search.An image retrieval system based on these approaches is presented. The result of the experiments shows that the image retrieval based on partition-overall histogram can keep the space distribution information while abstracting color feature efficiently, and it is superior to the normal color histograms in precision rate while researching. The query precision rate is more than 95%. In addition, the efficient block expression will lower the complicate degree of the images to be searched, and thus the searching efficiency will be increased. The image retrieval algorithms based on the partition-overall histogram proposed in the paper is efficient and effective.