A compact 3-D shape measurement system based on the combined stereovision and phase shifting method has been
developed, which consists of a miniature projector and two small cameras arranged as a stereo pair. The projector
projects sinusoidal phase shifted fringe patterns, which are captured by both cameras simultaneously. The two phase
maps calculated are used for stereo matching. The 3-D shape of the object is then reconstructed by the triangulation
method. This research focuses on improving the resolution and accuracy of the measurement system by using a sub-pixel
stereo matching method, a multi-image averaging method, and an error compensation method. Experimental results are
presented to show the effectiveness of the proposed methods in improving the resolution and accuracy of the system.
In this research, we propose to modify a traditional stereo microscope for quantitative 3-D surface profile measurement
based on the combined stereovision and phase shifting method. A simple optical system and a miniature projector are
used to project phase-shifted fringe patterns to the object surface. Two identical black-and-white cameras are used to
capture the fringe images of the object surface, one for each optical channel of the stereo microscope. The calculated
phase maps are used for stereo matching at the sub-pixel level. The 3-D surface profile is reconstructed using the
triangulation method. Experimental results are presented to demonstrate the feasibility of the proposed method.
Two-dimensional face-recognition techniques suffer from facial texture and illumination variations. Although 3-D techniques can overcome these limitations, the reconstruction and storage expenses of 3-D information are extremely high. We present a novel face-recognition method that directly utilizes 3-D information encoded in face fringe patterns without having to reconstruct 3-D geometry. In the proposed method, a digital video projector is employed to sequentially project three phase-shifted sinusoidal fringe patterns onto the subject's face. Meanwhile, a camera is used to capture the distorted fringe patterns from an offset angle. Afterward, the face fringe images are analyzed by the phase-shifting method and the Fourier transform method to obtain a spectral representation of the 3-D face. Finally, the eigenface algorithm is applied to the face-spectrum images to perform face recognition. Simulation and experimental results demonstrate that the proposed method achieved satisfactory recognition rates with reduced computational complexity and storage expenses.
A portable 3-D shape measurement system based on a combined stereovision and phase shifting method which can
realize big scale objects measurement is proposed. This system uses two pre-calibrated cameras and one projector which
do not need to be calibrated. During the whole measurement procedure, the projector is used to project a visibilitymodulated
fringe pattern on the object and is relatively fixed to the object. The two cameras are set up for stereovision
and grab fringe images simultaneously. The cameras can be moved to as many positions as needed to capture single
views and these single views can then be transformed into the same global coordinate system to reconstruct the whole 3-
D model. Since the phase value at each pixel is used to assist stereo matching only, it does not have to be accurate and
the errors caused by inaccurate phase measurement, for example, periodic errors due to the nonlinearity of the
projector's gamma curve are eliminated. These two high frame rate cameras can grab images as fast as 180 fps. Using
the visibility-modulated fringe pattern the phase information in one direction and fringe visibility information in the
other direction can be obtained simultaneously for stereo matching. Therefore only three images are needed for a single
view, which means the image acquisition time for each view is just 13.9ms. Experimental results are presented to show
the feasibility of this method.
The use of a color visibility-modulated fringe pattern is proposed to further accelerate image acquisition in the newly
proposed combined stereovision and phase shifting method for 3-D shape measurement. The method uses two cameras
and one projector and can eliminate errors caused by inaccurate phase measurement. In order to eliminate the need for
pattern switching and thus make real-time image acquisition possible, the use of a visibility-modulated fringe pattern
was previously proposed. This modified pattern is sinusoidal in one direction as in a conventional fringe pattern, but is
visibility-modulated in the other direction. Using this modified pattern we can achieve pixel-level phase matching in
both directions without changing the fringe pattern. In this paper, a color visibility-modulated fringe pattern is
introduced to further accelerate image acquisition. To obtain the three fringe images simultaneously, we encode the three
phase-shifted fringe patterns into the R, G, and B channels of a color pattern and project it onto the object via a color
projector. The color fringe image is then taken by two single-CCD color cameras simultaneously and each decoded into
three fringe images. The problems specifically associated with color systems, such as color coupling and color
imbalance, will be shown to have much less effect on the measured results. With this technique, the acquisition speed is
limited only by the frame rate of the camera, which significantly reduces the errors caused by object motions.
Experimental results are presented to support claims of the proposed method.
In this paper, a quality-guided phase unwrapping method is proposed for a modified Fourier transform method, which
utilizes a fringe image and a flat image. The proposed method takes advantage of the additional information provided by
the flat image to calculate the visibility of the fringe pattern, which is difficult to obtain in the conventional Fourier
transform method. The phase unwrapping process includes two steps. First, the pixels with unreliable phase values in the
wrapped phase map are masked out. Then, visibility is chosen as a quality index to assist the quality-guided unwrapping.
Experimental results show that the proposed method is an effective method to unwrap complex surface's phase map
generated from dense fringe patterns.
A new method, which combines the stereovision and phase shifting techniques, is proposed for more accurate 3-D shape
measurement. This method uses two cameras and one projector and can eliminate errors caused by inaccurate phase
measurement, for example, periodic errors due to the nonlinearity of the projector's gamma curve. The two cameras are
set up for stereovision. The projector is used to project phase-shifted fringe patterns onto the object twice with the fringe
patterns rotated by 90 degrees in the second time. Fringe images are taken by the two cameras from different directions
simultaneously. The resulting phase maps are used to assist stereo matching at the pixel level. The coordinates of the
object surface are calculated based on triangulation. Since the phase value at each pixel is used to assist stereo matching
only, it does not have to be accurate. This means that the projector does not need to be calibrated, which simplifies the
system calibration. Errors due to inaccurate phase measurement are significantly reduced because the two cameras
produce phase maps with the same phase errors. This combined method is better than stereovision method alone because
it provides higher resolution and easier stereo matching. It is better than the phase shifting method alone because it
eliminates the need of accurate phase measurement in order to ensure high measurement accuracy. Experimental results
and comparisons with the typical phase shifting method are presented to show the effectiveness and advantages of this
newly proposed method.
We propose to use a visibility-modulated fringe pattern to enable real-time image acquisition in the newly proposed
combined stereovision and phase shifting method for 3-D shape measurement. The combined stereovision and phase
shifting method uses two cameras and one projector and can eliminate errors caused by inaccurate phase measurement,
such as periodic errors due to the nonlinearity of the projector's gamma curve. In order to achieve pixel-to-pixel
matching between the two cameras, we previously used two phase-shifted fringe patterns, one with fringes in the vertical
direction and the other in the horizontal direction. This means that the projected fringe pattern has to be switched during
the image acquisition process, which slows down the process. As a result, measurement of dynamically changing objects
is difficult. In this paper, we propose to use a visibility-modulated fringe pattern to eliminate the need of the second
fringe pattern. This new fringe pattern is sinusoidal in the horizontal direction as in a conventional fringe pattern, but is
visibility-modulated in the vertical direction. With this new pattern, we can obtain the phase information in one direction
and fringe visibility information in the other direction simultaneously for stereo matching. Since no pattern changing is
necessary during the image acquisition process, the image acquisition time can be reduced to less than half of the time
previously required, thus making the measurement of dynamically changing objects possible. Experimental results are
presented to demonstrate the effectiveness of the proposed method.
The phase-to-coordinate conversion is an important step for accurate 3-D shape measurement by use of structured light methods. We propose to use an absolute phase map in the Fourier transform method to achieve better accuracy in the phase-to-coordinate conversion. A cross-shaped marker is embedded in the fringe pattern that is projected onto the object. The position of the marker in the captured fringe image is detected and utilized to calculate the absolute phase map. For phase analysis of the fringe image, the marker is removed and the sinusoidal intensity distribution of the fringe pattern is restored before the forward and inverse Fourier transforms are applied. Experimental results of absolute phase retrieval and 3-D reconstruction are presented to show the feasibility of the proposed method.
In this paper, a novel modified Fourier transform method is proposed, which employs a fringe image and a flat image to eliminate the background and in the mean time facilitate the retrieval of the absolute phase map. Both the fringe and flat patterns are projected onto the object by a digital video projector. With the subtraction of the flat image from the fringe image, the background is completely removed and the spectrum overlapping in the frequency domain is prevented. The flat image is also employed for hole and shadow detection. Two cross-shaped markers are embedded in the flat and fringe image respectively for absolute phase retrieval. Experimental results showed that the proposed method produced better shape measurement results when measuring fast moving or changing objects, compared to the phase shifting method. The proposed method has the potential to boost the speed of our real-time 3-D shape measurement system to 120 fps with better measurement accuracy.
The difficulty of implementing the phase shifting method in shadow moiré lies in the fact that the phase shift due to the displacement of the light source, the imaging sensor, or the grating is non-uniform across the field of view. Typical phase shifting algorithms fail to produce accurate results. In the past few decades, various approximation methods have been developed to overcome this difficulty. In this paper, we describe an elegant solution that provides exact close-form result. In our proposed system, the grating is translated in equal steps to introduce phase shifts. The phase value at each point is determined by the Carré algorithm, which only requires uniform phase shifts for each point, instead of in the whole field of view. The 3-D shape of the object is then reconstructed from the phase map retrieved from the Carré algorithm. The simulation results demonstrate the effectiveness of the Carré algorithm for shadow moiré.
Accurate 3D shape measurement via fringe analysis methods requires the determination of the absolute phase map of the
object. In this paper, we present a novel method for absolute phase retrieval developed for use with the Fourier transform
method for fringe analysis. A cross-shaped marker is embedded in the fringe pattern that is projected to the object. The
position of the marker in the captured fringe image is detected and later used in calculating the absolute phase map. For
phase analysis of the fringe image, the marker is removed and the sinusoidal intensity distribution of the fringe pattern is
restored before the Fourier transform method is applied. This paper focuses on the concept of absolute phase retrieval
from a single fringe pattern as well as techniques on marker detection and removal. Experimental results on absolute
phase retrieval and 3D reconstruction are also presented to show the feasibility of the proposed method.
This paper describes a novel phase error compensation method for reducing the measurement error caused by nonsinusoidal waveforms in phase-shifting methods. For 3-D shape measurement systems using commercial video projectors, the nonsinusoidal waveform of the projected fringe patterns as a result of the nonlinear gamma of projectors causes significant phase measurement error and therefore shape measurement error. The proposed phase error compensation method is based on our finding that the phase error due to the nonsinusoidal waveform depends only on the nonlinearity of the projector's gamma. Therefore, if the projector's gamma is calibrated and the phase error due to the nonlinearity of the gamma is calculated, a lookup table that stores the phase error can be constructed for error compensation. Our experimental results demonstrate that by using the proposed method, the measurement error can be reduced by 10 times. In addition to phase error compensation, a similar method is also proposed to correct the nonsinusoidality of the fringe patterns for the purpose of generating a more accurate flat image of the object for texture mapping. While not relevant to applications in metrology, texture mapping is important for applications in computer vision and computer graphics.
We describe a high-resolution, real-time 3-D shape measurement system based on a digital fringe projection and phase-shifting technique. It utilizes a single-chip digital light processing projector to project computer-generated fringe patterns onto the object, and a high-speed CCD camera synchronized with the projector to acquire the fringe images at a frame rate of 120 frames/s. A color CCD camera is also used to capture images for texture mapping. Based on a three-step phase-shifting technique, each frame of the 3-D shape is reconstructed using three consecutive fringe images. Therefore the 3-D data acquisition speed of the system is 40 frames/s. With this system, together with the fast three-step phase-shifting algorithm and parallel processing software we developed, high-resolution, real-time 3-D shape measurement is realized at a frame rate of up to 40 frames/s and a resolution of 532×500 points per frame.
We propose a nonlinear calibration method for improving the accuracy of structured light systems that use cameras and
projectors. Previously we have developed a systematic method for the calibration of projectors as well as structured light
systems for 3-D shape measurement. However, we used only a linear model and did not consider lens distortions of the
camera and the projector. As a result, measurement accuracy was limited. In this paper, we develop nonlinear models for
both the camera and the projector and apply them to develop a nonlinear algorithm for 3-D shape measurement. The aim is to improve system accuracy by reducing the nonlinear error caused by lens distortion. Experimental results on
nonlinear camera and projector calibration, comparison of 3-D measurement errors with linear and nonlinear camera and
projector models, as well as 3-D shape measurement of some sample objects are presented.
System calibration, which usually involves complicated and time-consuming procedures, is crucial for any 3-D shape measurement system. In this work, a novel systematic method is proposed for accurate and quick calibration of a 3-D shape measurement system we developed based on a structured light technique. The key concept is to enable the projector to "capture" images like a camera, thus making the calibration of a projector the same as that of a camera. With this new concept, the calibration of structured light systems becomes essentially the same as the calibration of traditional stereovision systems, which is well established. The calibration method is fast, robust, and accurate. It significantly simplifies the calibration and recalibration procedures of structured light systems. This work describes the principle of the proposed method and presents some experimental results that demonstrate its performance.
An automated pavement inspection system consists of image acquisition and distress image processing. The former is accomplished with imaging sensors, such as video cameras and photomultiplier tubes. The latter includes distress detection, isolation, classification, evaluation, segmentation, and compression. We focus on wavelet-based distress detection, isolation, and evaluation. After a pavement image is decomposed into different-frequency subbands by the wavelet transform, distresses are transformed into high-amplitude wavelet coefficients and noise is transformed into low-amplitude wavelet coefficients, both in the high-frequency subbands, referred to as details. Background is transformed into wavelet coefficients in a low-frequency subband, referred to as approximation. First, several statistical criteria are developed for distress detection and isolation, which include the high-amplitude wavelet coefficient percentage (HAWCP), the high-frequency energy percentage (HFEP), and the standard deviation (STD). These criteria are tested on hundreds of pavement images differing by type, severity, and extent of distress. Experimental results demonstrate that the proposed criteria are reliable for distress detection and isolation and that real-time distress detection and screening is currently feasible. A norm for pavement distress quantification, which is defined as the product of HAWCP and HFEP, is also proposed. Experimental results show that the norm is a useful index for pavement distress evaluation.
A color phase-shifting technique has been recently developed for high-speed 3-D shape measurement. In this technique, three sinusoidal phase-shifted images used for a measurement cycle in a traditional grayscale phase-shifting technique are encoded into one color image. Therefore, only a single color image is needed for reconstructing the 3-D surface shape of an object. The measurement speed can then be increased up to the frame rate of the camera. However, previous experimental results showed that the measurement accuracy of this technique was initially low, due largely to the coupling and imbalance of color channels. In this paper, two solutions, one software-based and one hardware-based, are proposed to compensate for these errors. Experimental results show that the second solution—modification of the camera together with an imbalance compensation algorithm—would effectively reduce the errors and produce better measurement results than the software-based compensation method. This technique has many potential applications in high-speed measurement, such as highway inspection and dynamic measurement of human body.
We propose a novel structured light method, namely a trapezoidal phase-shifting method, for 3-D shape measurement. This method uses three patterns coded with phase-shifted, trapezoidal-shaped gray levels. The 3-D information of the object is extracted by direct calculation of an intensity ratio. Compared to traditional intensity-ratio-based methods, the vertical or depth resolution is six times better. Also, this new method is significantly less sensitive to the defocusing effect of the captured images, which makes large-depth 3-D shape measurement possible. If compared to sinusoidal phase-shifting methods, the resolution is similar, but the data processing speed is at least 4.5 times faster. The feasibility of this method is demonstrated in a previously developed real-time 3-D shape measurement system. The reconstructed 3-D results show similar quality to those obtained by the sinusoidal phase-shifting method. However, since the data processing speed is much faster (4.6 ms per frame), both image acquisition and 3-D reconstruction can be done in real time at a frame rate of 40 fps and a resolution of 532×500 points. This real-time capability allows us to measure dynamically changing objects, such as human faces. The potential applications of this new method include industrial inspection, reverse engineering, robotic vision, computer graphics, medical diagnosis, etc.
We review some of our most recent works on 3-D shape measurement using the digital fringe projection and phase-shifting method. First, we introduce the measurement principle and phase-shifting algorithms. Then we discuss an effective method for phase error compensation and a novel idea for system calibration. Finally, we describe a 3-D shape measurement system for high-resolution, real-time 3-D shape acquisition, reconstruction and display.
This paper describes a novel phase error compensation method for reducing the measurement error caused by non-sinusoidal waveforms in the phase-shifting method. For 3D shape measurement systems using commercial video projectors, the non-sinusoidal nature of the projected fringe patterns as a result of the nonlinear gamma
curve of the projectors causes significant phase measurement error and therefore shape measurement error. The proposed phase error compensation method is based on our finding that the phase error due to the non-sinusoidal waveform of the fringe patterns depends only on the nonlinearity of the projector's gamma curve. Therefore, if the projector's gamma curve is calibrated and the phase error due to the nonlinearity of the gamma curve is calculated, a look-up-table (LUT) that stores the phase error can be constructed for error compensation. Our experimental results demonstrate that by using the proposed method, the measurement error can be reduced by 10 times. In addition to phase error compensation, a similar method is also proposed to correct the nonsinusoidality of the fringe patterns for the purpose of generating a more accurate flat image of the object for texture mapping. While not relevant to applications in metrology, texture mapping is important for applications in computer vision and computer graphics.
We propose a new three-step phase-shifting algorithm, which is much faster than the traditional three-step algorithm. We achieve the speed advantage by using a simple intensity ratio function to replace the arctangent function in the traditional algorithm. The phase error caused by this new algorithm is compensated for by use of a look-up-table (LUT). Our experimental results show that both the new algorithm and the traditional algorithm generate similar results, but the new algorithm is 3.4 times faster. By implementing this new algorithm in a high-resolution, real-time 3D shape measurement system, we were able to achieve a measurement speed of 40 frames per second (fps) at a resolution of 532 × 500 pixels, all with an ordinary personal computer.
For any automated distress inspection system, typically a huge number of pavement images are collected. Use of an appropriate image compression algorithm can save disk space, reduce the saving time, increase the inspection distance, and increase the processing speed. In this research, a modified EZW (Embedded Zero-tree Wavelet) coding method, which is an improved version of the widely used EZW coding method, is proposed. This method, unlike the two-pass approach used in the original EZW method, uses only one pass to encode both the coordinates and magnitudes of wavelet coefficients. An adaptive arithmetic encoding method is also implemented to encode four symbols assigned by the modified EZW into binary bits. By applying a thresholding technique to terminate the coding process, the modified EZW coding method can compress the image and reduce noise simultaneously. The new method is much simpler and faster. Experimental results also show that the compression ratio was increased one and one-half times compared to the EZW coding method. The compressed and de-noised data can be used to reconstruct wavelet coefficients for off-line pavement image processing such as distress classification and quantification.
Color coding has been used for 3-D shape measurement in many recently developed fringe projection techniques. Use of color allows for more information to be coded in the same number of patterns as compared to the black-and-white techniques. However, one major problem of using color is that the appearance of the color fringe patterns projected onto the object can be affected by the color of the object surface itself. Thus, correctly decoding the fringe patterns can be difficult and sometimes even impossible. We describe a color-coded binary fringe projection technique that solves this problem. The use of an adaptive threshold scheme enables the extraction of the 3-D information and texture of an object without being affected by the color of the object surface. The development of a color gray-code concept, which is an extension of the gray-code technique, further reduces decoding errors. In addition, this technique can be used to measure objects with discontinuous features. The system has small digitizing errors and its measurement accuracy is hardly affected by system noise and nonlinearity errors. The system setup, color pattern design, shape reconstruction, and experimental results are presented
We propose a novel structured light method, namely trapezoidal
phase-shifting method, for 3-D shape measurement. This method uses
three patterns coded with phase-shifted, trapezoidal-shaped gray
levels. The 3-D information of the object is extracted by direct
calculation of an intensity ratio. Theoretical analysis showed
that this new method was significantly less sensitive to the
defocusing effect of the captured images when compared to the
traditional intensity-ratio based methods. This important
advantage makes large-depth 3-D shape measurement possible. If
compared to the sinusoidal phase-shifting method, the resolution
is similar, but the processing speed is at least 4.5 times faster.
The feasibility of this method was demonstrated in a previously
developed real-time 3-D shape measurement system. The
reconstructed 3-D results showed similar quality as those obtained
by the sinusoidal phase-shifting method. However, since the
processing speed was much faster, we were able to not only acquire
the images in real time, but also reconstruct the 3-D shapes in
real time (40 fps at a resolution of 532 x 500 pixels).
This real-time capability allows us to measure dynamically
changing objects, such as human faces. The potential applications
of this new method include industrial inspection, reverse
engineering, robotic vision, computer graphics, medical diagnosis,
Color-encoded digital fringe projection is a recently developed technique for high-speed 3-D shape measurement. In this technique, only a single image is needed for reconstructing the 3-D surface shape of an object. Thus the measurement speed can be increased up to the frame rate of the camera. However, previous experimental results showed that the measurement accuracy of this technique was lower than that of the traditional grayscale technique due largely to the color coupling and imbalance problems. In this research, we propose an improved system with a high brightness LCD projector and a 3-CCD video camera to address these problems. A Look-Up-Table (LUT) method is developed to compensate for errors caused by color coupling and imbalance. In addition, a nonlinearity compensation method is used to further reduce measurement errors. Experimental results showed that the measurement error was significantly reduced.
A wavelet-based pavement distress detection and evaluation method is proposed. This method consists of two main parts, real-time processing for distress detection and offline processing for distress evaluation. The real-time processing part includes wavelet transform, distress detection and isolation, and image compression and noise reduction. When a pavement image is decomposed into different frequency subbands by wavelet transform, the distresses, which are usually irregular in shape, appear as high-amplitude wavelet coefficients in the high-frequency details subbands, while the background appears in the low-frequency approximation subband. Two statistical parameters, high-amplitude wavelet coefficient percentage (HAWCP) and high-frequency energy percentage (HFEP), are established and used as criteria for real-time distress detection and distress image isolation. For compression of isolated distress images, a modified EZW (Embedded Zerotrees of Wavelet coding) is developed, which can simultaneously compress the images and reduce the noise. The compressed data are saved to the hard drive for further analysis and evaluation. The offline processing includes distress classification, distress quantification, and reconstruction of the original image for distress segmentation, distress mapping, and maintenance decision-making. The compressed data are first loaded and decoded to obtain wavelet coefficients. Then Radon transform is then applied and the parameters related to the peaks in the Radon domain are used for distress classification. For distress quantification, a norm is defined that can be used as an index for evaluating the severity and extent of the distress. Compared to visual or manual inspection, the proposed method has the advantages of being objective, high-speed, safe, automated, and applicable to different types of pavements and distresses.
A microscopic 3-D shape measurement system based on a new high-speed phase shifting technique is developed and experimented. By taking advantage of the color channel switching characteristic of a commercial Digital Micromirror Device (DMD) based video projector, a potential 3-D shape measurement speed of up to 100 frames/sec can be achieved. A computer generated color fringe pattern is projected onto an object surface. The red, green and blue channels of this pattern are programmed to be sinusoidal fringe patterns with 120 degrees in phase difference. When the color filter of the projector is removed, three grayscale fringe patterns are actually projected in sequence with a cycle time of approximately 10 ms. Through a stereomicroscope, the fringe patterns generated by the DMD is projected onto a small surface area of the object and then captured by a CCD camera via the same microscope objective. The 3-D microscopic shape of the object surface is reconstructed by using a phase wrapping and unwrapping algorithm. Experimental results demonstrated the feasibility of this technique for high-speed surface profile measurement.
A high-speed phase-shifting technique for 3D shape measurement has been developed and experimented. A Digital Micromirror Device (DMD) based video projector is used to project sinusoidal fringe patterns onto the object surface. Its red, green and blue (RGB) channels are programmed to be sinusoidal fringe patterns with 120-degree phase difference. The projector has a color filter with four segments, RGB and clear, which controls the color of the projected image. When a specific segment is in the light path, the DMD forms the corresponding image of that color. A 24-bit color image is formed when the RGB color channels are sequentially projected in a cycle time of 10 ms. When the color filter in the projector is removed, the DMD still projects the three color channels sequentially, but the actual projected images are now in grayscale. The timing control mechanism of the DMD is studied and a circuit is designed to provide an external timing signal for projection control. When a CCD camera used to capture the fringe patterns is synchronized with the DMD, the individual channels of RGB or the three phase-shifted fringe patterns can be captured in less than 10mn, thus making high- speed phase shifting possible. Experimental results demonstrated the feasibility of this technique for high-speed 3D shape measurement.
A new 3D surface contouring and ranging system based on a digital fringe projection and phase shifting technique is described. In this system, three phase-shifted fringe patterns and a centerline pattern are used to determine the absolute phase map of the object. This phase map is then converted to the absolute x, y, and z coordinates of the object surface by a transformation algorithm. To determine the accurate values of the system parameters as required by the transformation algorithm, a two-step calibration procedure was developed. First the parameters were indirectly measured through experiments to determine their approximate values. Second, a calibration plate whose features were calibrated by a coordinate measuring machine was measured by the system at various positions. An iteration algorithm was then used to estimate the system parameters. Measurements of the calibration plate, a sheet- metal panel, and a Ford master gauge showed results consistent with the actual surface contours of the objects.
This paper describes a new 3D surface contouring system based on a digital fringe projection and phase shifting technique. The new system takes full advantage of a novel projection display technology to provide the capability of digital fringe projection with high brightness and contrasts ratio. This capability helps improve system resolution and accuracy and make the full-field contouring of large objects possible. Fringe patterns with any cross-sectional intensity profile and spacing can be created digitally by software on a computer, limited only by the resolution and bit depth of the projection system. Also, purely software-based digital phase shifting technique can be applied to improve the resolution of the contouring technique. This eliminates the need for physically shifting the grating or other optical components and makes phase shifting digitally accurate. The concept of this new 3D surface contouring system is first introduced. Then issues related to projector nonlinearity, background noise, and phase unwrapping are addressed. FInally, some experimental results are presented.
A color-encoded fringe projection and phase shifting technique is proposed for rapid 3-D surface contouring applications. A color fringe pattern whose RGB components comprise three phase-shifted fringe patterns is created by software on a computer screen and then projected to an object by a novel computer-controlled digital projection system. The image of the object is captured by a digital camera positioned at an angle different from that of the projection system. The image is then separated into its RGB components, creating three phase-shifted images of the object. These three images are used to retrieve the 3-D surface contour of the object through the use of a phase wrapping and unwrapping algorithm. Only one image of the object is required to obtain its 3-D surface contour. Thus contouring speed, limited only by the frame rate of the camera, can be dramatically increased as compared to that of the traditional phase shifting techniques. The technique is especially useful in applications where the object being contoured is going through quasi-static or dynamic changes. This paper describes the principle of the technique and presents some preliminary experimental results.