Cunwei Lu

Intensity-modulated moiré topography

Moiré topography has the advantage of requiring only a single image to obtain a three-dimensional measurement, but it cannot discern the fringe order. Because there is an ambiguity problem when calculating the depth range by use of fringe intensity or phase unwrapping, it is impossible to obtain an absolute phase and an absolute depth range. It is therefore difficult to discern the relation between fringes in the cases in which the fringes are discontinuous or the objects are isolated. An intensity-modulated moiré topography method is presented. By modulation of the transmission factors of the projection and the observation gratings by exponential functions a new moiré pattern whose fringe intensity changes with its order can be produced. The fringe order can be extracted easily from the fringe intensity, and the absolute range of the skeleton line can be obtained solely from its intensity. At the same time, we can segment the moiré pattern by its fringe order. For every segment the absolute phase and the absolute depth range of every point of the moiré pattern can be obtained solely from its intensity with no need for interaction with the user.

Optimal intensity-modulation projection technique for three-dimensional shape measurement

A new pattern projection technique for measuring three-dimensional topography is presented, called the optimal intensity-modulation projection technique. The proposed technique dramatically shortens the measurement time and improves stripe detection accuracy compared with previous methods. Furthermore, the method deals reliably with discontinuous patterns and multiple objects.

Projection pattern intensity control technique for 3-D optical measurement

A new projection pattern control technique is presented in an attempt to solve the problem whereby an image having an ideal intensity distribution cannot be photographed when measurement conditions, such as object color or object surface reflection, change. The proposed technique can adjust the intensity distribution of a projection pattern automatically, according to changes in the measurement conditions. An image with an ideal intensity distribution can then be obtained in a short time, approximately three projections on average. Thus, the speed, robustness, and practicality of 3-D image measurement can be improved.

An absolute depth range measurement of 3-D objects based on modulation Moire topography

It is the foundation that the absolute fringe order is discerned in 3-D image measurement methods, such as spatial pattern projection technique and Moire topography. The reason is that the depth range of the object is computed from the correspondence relation between the fringe order of a reflective pattern and the depth of an object. Multiple images are need by the conventional method for discerning the fringes order. Moreover, it has a problem of phase unwrapping in the phase analysis method of a Moire pattern or other interfering pattern. So it is difficult to calculate the absolute phase or to discern the relation between the fringes, especially in cases of which the fringes are discontinuous or the objects are isolated, and so forth. The technique of using modulation projection with 3-D information is presented. A new fringes pattern whose fringe intensity or color changes with its order is produced. Thus the fringe order can be extracted easily from the fringe intensity or color, and then the absolute phase and the absolute depth range can be obtained also. This technique needs only a single image or double images.

Intensity modulated moire and its intensity-phase analysis

Moire topography has the advantage that it just requires a single image, but it cannot discern the fringe order. So, it is difficult to discern the connection among fringes in the case where the fringe orders are not continuous. In our work, an intensity modulated moire is presented. By modulating the transmission of projection and observation grating, the moire pattern whose fringe intensity changes with its order can be produced. The fringe order can be obtained easily from its intensity, so the skeleton of fringes in every order can be extracted to discern the global 3-D shape by intensity analysis. In the same time, we can segment the moire pattern by its fringe order. In every segment, the accurate 3-D shape among the skeletons can be calculated by phase analysis.

3-D face recognition method based on optimum 3-D image measurement technology

Most face recognition systems employ 2-D color or gray-scale images. However, face recognition based on 2-D images is adversely affected by 3-D movement, variable lighting, and the use of cosmetics. 3-D image measurement technology has the potential to overcome these limitations of face recognition based on 2-D images since it can perform geometric analysis. We propose a method that is capable of recognizing a person from a 3-D facial image obtained using a 3-D shape measurement system by employing a technique that optimizes the intensity-modulation pattern projection. This face recognition method is based on the iterative closest point algorithm. It is robust to changes in reflectivity and color. Since the 3-D facial information can be registered, this method can estimate rotations and translations to compensate for different positions or directions. In order to prove the validity of the proposed technique, a verification experiment was conducted which used 105 sample 3-D images obtained from 15 subjects. It achieved a detection rate of 96% when heads were turned at an angle of 20° or less relative to the camera.

Practical 3-D Shape Measurement Using Optimal Intensity-Modulated Projection and Intensity-Phase Analysis Techniques

In the field of 3D image measurement, a technique based on stripe pattern projection and observation image intensity analysis is expected to be able to detect several stripes by a single projection. It is necessary to increase the stripe number of the projection pattern in order to improve the measurement accuracy of depth distance. However, when the stripe number is increased, the difference of the intensity between stripes will be reduced and stripe detection will become difficult. In order to improve the detection accuracy of the stripe order and shorten the 3D measurement time, we use the optimal intensity-modulation projection (OIMP) technique, and in order to improve the depth distance measurement accuracy, we propose an intensity-phase analysis (IPA) technique. In the proposed IPA technique, the observation pattern be segmented by the intensity of the intensity-modulated stripe, and in every segmentation the depth distance of all pixels are obtained by phase analysis. By using a combination of the OIMP and IPA techniques, high-speed (single projection and double image captures) and high-accuracy (error less than 0.1%) practical 3D shape measurement can be realized

O(1) time algorithm on BSR for constructing a binary search tree with best frequencies

Constructing a binary search tree of n nodes with best frequencies needs Ω(n log n) time on RAM, and Ω(log n) time on n-processor EREW, CREW, or CRCW PRAM. In this paper, we propose an O(1) time algorithm on n-processor BSR PRAM for the problem, which is the first constant time solution to the problem on any model of computation.

A high-speed 3-D image measurement method

With the recent popularization of digital cameras and projectors, practical applications of three-dimensional (3-D) image measurements based on intensity-modulation pattern projection are eagerly anticipated. Such an approach would permit a projection pattern to contain more measurement data, allowing 3-D data to be calculated with greater accuracy. However, to achieve a high level of accuracy using this approach, it is essential that an ideal observation pattern image should be obtained with a certain minimum number of stripes and a certain intensity distribution. To satisfy these requirements, an intensity correction method with two observation pattern images has previously been used. It is difficult to measure a fast-moving object in such calculations because the correspondence relation of the measured object is not established between the two photographic images, and consequently the pattern intensity is not corrected accurately. This study proposes an image analysis method for correcting intensity using an image synthesis technique that extracts precise stripes from a single observation pattern image. This analysis method allows 3-D shape measurements to be performed from a single projection pattern and a single image capture.

Stripes extraction technique of projection pattern in frequency domain for 3D shape measurement based pattern projection technique

Because the 3-D shape measurement technique using optimal intensity-modulation pattern projection (OIMP) method can detect a lot of stripes numbers by one projection, the application is expected by the spread of the digital camera and the pattern projector. However, there is a problem that the accuracy of stripes recognition decreases in the case of the intensity range of the observation image of the measurement object is insufficient. Or since two images are needed to convert the observation image by the image correction method, saying, OIMP method, the measurement time is long. In order to solve above problem, we propose an image analysis method based on Fourier transform technique to correct the intensity of the observation image. By this method, the stripes can be extracted only using a single observation image, and then, a fast 3-D shape measurement is realizable.