In fringe projection technique, system calibration is a tedious task to establish the mapping relationship between the object depths and the fringe phases. Especially, it is not easy to accurately determine the parameters of the projector in this system, which may induce errors in the measurement results. To solve this problem, this paper proposes a new calibration by using the cross-ratio invariance in the system geometry for determining the phase-to-depth relations. In it, we analyze the epipolar eometry of the fringe projection system. On each epipolar plane, the depth variation along an incident ray induces the pixel movement along the epipolar line on the image plane of the camera. These depth variations and pixel movements can be connected by use of the projective transformations, under which condition the cross-ratio for each of them keeps invariant. Based on this fact, we suggest measuring the depth map by use of this cross-ratio invariance. Firstly, we shift the reference board in its perpendicular direction to three positions with known depths, and measure their phase maps as the reference phase maps; and secondly, when measuring an object, we calculate the object depth at each pixel by equating the cross-ratio of the depths to that of the corresponding pixels having the same phase on the image plane of the camera. This method is immune to the errors sourced from the projector, including the distortions both in the geometric shapes and in the intensity profiles of the projected fringe patterns.The experimental results demonstrate the proposed method to be feasible and valid.
In fringe projection profilometry, the phase sensitivity of a fringe pattern to depth variation of the measured surface is vital to measurement accuracy and resolution. This paper represents the implementation of the optimal fringe pattern with the best phase sensitivities over the whole fringe pattern, and deduces an efficient calibration method to determine the relationship between the phase-difference distribution and the depth variation. In it, first we find the epipole location by projecting sets of horizontal and vertical fringe patterns on several depth-known reference planes, and meanwhile determine the parameters of the measurement system calibration by analyzing the geometry of measurement system. And then project the optimal fringe pattern onto the object to measure. Experimental results demonstrate that this method is very efficient and easy to implement.
As the most important core part of stereo vision, there are still many problems to solve in stereo matching technology.
For smooth surfaces on which feature points are not easy to extract, this paper adds a projector into stereo vision
measurement system based on fringe projection techniques, according to the corresponding point phases which extracted
from the left and right camera images are the same, to realize rapid matching of stereo vision. And the mathematical
model of measurement system is established and the three-dimensional (3D) surface of the measured object is
reconstructed. This measurement method can not only broaden application fields of optical 3D measurement technology,
and enrich knowledge achievements in the field of optical 3D measurement, but also provide potential possibility for the
commercialized measurement system in practical projects, which has very important scientific research significance and
Spatial carrier fringe pattern analysis is an effective tool in optical measurement, e.g. in interferometry and fringe projection technique. With it, the very large phase deformations in a spatial carrier fringe pattern may increases the bandwidth of fringe component thus leading to difficulties in retrieving its phase map. In order to overcome this problem, many local-adaptive methods have been developed for processing the spatial carrier fringe pattern with large phase variations, and in fact, the local spatial frequency estimation is central to these methods. This paper introduces a simple algorithm for estimating the local frequencies of a fringe pattern with spatial carrier. First, the intensity gradients of the fringe pattern are calculated, and then the standard deviations (SDs) of the intensity gradients at each pixel are estimated from its neighborhood. Finally the local frequencies are estimated from the SDs just calculated simply using an arccosine function. This algorithm is potential in developing effective techniques for retrieving phases from a spatial carrier fringe pattern with large phase variations. For example, we can recover the phase map by directly integrating the local frequencies or by use of an adaptive spatial carrier phase shifting algorithm (SCPS) with the local frequencies being the local phase shifts. It can also be used in Fourier transform method for exactly determining the carrier frequencies, or for extrapolating aperture in order to reduce the boundary effect. Combined with time-frequency techniques such as windowed Fourier transform and wavelet transform methods, it is helpful for alleviating the computational burdens.
Wireless sensor network consisting of a large number of small sensors with low-power transceiver can be an effective
tool for gathering data in a variety of environment. The collected data must be transmitted to the base station for further
processing. Since a network consists of sensors with limited battery energy, the method for data gathering and routing
must be energy efficient in order to prolong the lifetime of the network. In this paper, we presented an energy-efficient
data gathering protocol in wireless sensor network. The new protocol used data fusion technology clusters nodes into
groups and builds a chain among the cluster heads according to a hybrid of the residual energy and distance to the base
station. Results in stochastic geometry are used to derive the optimum parameter of our algorithm that minimizes the
total energy spent in the network. Simulation results show performance superiority of the new protocol.