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In this paper, the first application of utilizing a unique 3D sensor for sequential 3D map building in unknown cluttered urban search and rescue (USAR) environments is proposed. The sensor utilizes a digital fringe projection and phase shifting technique to provide real-time 2D and 3D sensory information of the environment. The proposed sensor is unique over current technologies in that high-resolution 3D information of rubble filled environments can be acquired from the single sensor at a speed of 30 frames per second (fps). Furthermore, we propose the development of a novel robust and reliable landmark identification technique that utilizes both 2D and 3D depth images taken by the sensor for 3D mapping. Preliminary experiments show the potential of the real-time 3D sensory system and landmark identification scheme for robotic 3D mapping in unknown cluttered USAR-like environments.

We present a novel automatic method for high resolution, non-rigid dense 3D point tracking. High quality dense point clouds of non-rigid geometry moving at video speeds are acquired using a phase-shifting structured light ranging technique. To use such data for the temporal study of subtle motions such as those seen in facial expressions, an efficient non-rigid 3D motion tracking algorithm is needed to establish inter-frame correspondences. The novelty of this paper is the development of an algorithmic framework for 3D tracking that unifies tracking of intensity and geometric features, using harmonic maps with added feature correspondence constraints. While the previous uses of harmonic maps provided only global alignment, the proposed introduction of interior feature constraints allows to track non-rigid deformations accurately as well. The harmonic map between two topological disks is a diffeomorphism with minimal stretching energy and bounded angle distortion. The map is stable, insensitive to resolution changes and is robust to noise. Due to the strong implicit and explicit smoothness constraints imposed by the algorithm and the high-resolution data, the resulting registration/deformation field is smooth, continuous and gives dense one-to-one inter-frame correspondences. Our method is validated through a series of experiments demonstrating its accuracy and efficiency.

Synthesis and re-targeting of facial expressions is central to facial animation and often involves significant manual work in order to achieve realistic expressions, due to the difficulty of capturing high quality dynamic expression data. In this paper we address fundamental issues regarding the use of high quality dense 3-D data samples undergoing motions at video speeds, e.g. human facial expressions. In order to utilize such data for motion analysis and re-targeting, correspondences must be established between data in different frames of the same faces as well as between different faces. We present a data driven approach that consists of four parts: 1) High speed, high accuracy capture of moving faces without the use of markers, 2) Very precise tracking of facial motion using a multi-resolution deformable mesh, 3) A unified low dimensional mapping of dynamic facial motion that can separate expression style, and 4) Synthesis of novel expressions as a combination of expression styles. The accuracy and resolution of our method allows us to capture and track subtle expression details. The low dimensional representation of motion data in a unified embedding for all the subjects in the database allows for learning the most discriminating characteristics of each individual's expressions as that person's “expression style”. Thus new expressions can be synthesized, either as dynamic morphing between individuals, or as expression transfer from a source face to a target face, as demonstrated in a series of experiments. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Animation; I.3.5 [Computer Graphics]: Curve, surface, solid, and object representations; I.3.3 [Computer Graphics]: Digitizing and scanning; I.2.10 [Artificial intelligence]: Motion ; I.2.10 [Artificial intelligence]: Representations, data structures, and transforms; I.2.10 [Artificial intelligence]: Shape; I.2.6 [Artificial intelligence]: Concept learning

In this paper, a vision-based method is proposed for three-degree-of-freedom (3-DOF) planar position () measurement. This method uses a single camera to capture the image of a 2D periodic pattern and then uses the 2D discrete Fourier transform (2D DFT) method to estimate the phase of its fundamental frequency component for position measurement. To improve position measurement accuracy, the phase estimation error of 2D DFT is analyzed and a phase estimation method is proposed. Different simulations are done to verify the feasibility of this method and study the factors that influence the accuracy and precision of phase estimation. To demonstrate the performance of the proposed method for position measurement, a prototype encoder consisting of a black-and-white industrial camera with VGA resolution (480 × 640 pixels) and an iPhone 4s has been developed. Experimental results show the peak-to-peak resolutions to be 3.5 nm in X axis, 8 nm in Y axis and 4 in axis. The corresponding RMS resolutions are 0.52 nm, 1.06 nm, and 0.60 respectively.

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. This combined method uses two cameras and one projector and can significantly reduce errors caused by inaccurate phase measurement. 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. With this modified fringe pattern, pixel-level stereo matching can be achieved in both directions without having to change 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 to form a color visibility-modulated fringe pattern and project it onto the object via a color projector. The color fringe image is then captured by two single-CCD color cameras simultaneously, each subsequently split into three grayscale fringe images. With this technique, only a single color fringe image is needed for 3-D reconstruction and the image acquisition speed is limited only by the frame rate of the camera, which makes it possible to measure rapidly changing objects. 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, which contributes to the improvement of the system accuracy. Experimental results are presented to demonstrate the feasibility and performance of the proposed method.

The difficulty of implementing the phase shifting method in shadow moire 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 Carre 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 Carre algorithm. The simulation results demonstrate the effectiveness of the Carre algorithm for shadow moire.

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.

A method for precision small-angle measurement is proposed. This method is based on the total-internal-reflection effect of a light beam at a pair of glass prisms. Angular displacement of the light beam is measured when the intensity change of the reflected beam is detected as a result of the relative phase shift between the s- and the p-polarized beams. An initial phase shift between the s- and the p-polarized components is introduced to increase measurement sensitivity. For increased measurement linearity and reduced effect of laser power fluctuation on the output, a differential method is used in which the light beam is split equally into two beams, each reflected at a prism and detected by a photodiode. The output is obtained as the difference of the two detected intensities divided by their sum. A prototype device was built, which demonstrated a nonlinearity error of 1.3% in a measurement range of ±0.6° or 0.4% in ±0.3°. The peak-to-peak noise level was found to be at approximately 0.5 arc sec. This noise level can be reduced further and resolution increased by a reduction of the measurement range.

A new method of angle measurement based on the internal reflection effect is proposed that uses a single right-angle prism. We measure the angular displacement between a laser beam and the prism by detecting the changes in reflectance as a function of the angle of incidence. We achieve high linearity of measurement by taking the inverse of reflectance as the output. The inverse of reflectance is obtained from the intensities of the reflected and the transmitted beams measured by two photodiodes. Experiments with a prototype device have demonstrated that angle measurement with a range of ?500 arc sec, a nonlinearity error of ?0.1%, and a resolution of 0.1 arc sec can be readily achieved. The measurement range can be further increased with some sacrifice of linearity.

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.

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.

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 lookup table. 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 three-dimensional shape measurement system, we were able to achieve a measurement speed of 40 frames per second at a resolution of 532 x 500 pixels, all with an ordinary personal computer.

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.

In this paper we describe a high-resolution, real-time 3D shape acquisition system based on structured light techniques. This system uses a color pattern whose RGB channels are coded with either sinusoidal or trapezoidal fringe patterns. When projected by a modified DLP projector (color filters removed), this color pattern results in three grayscale patterns projected sequentially at a frequency of 240 Hz. A high-speed B/W CCD camera synchronized with the projector captures the three images, from which the 3D shape of the object is reconstructed. A color CCD camera is also used to capture images for texture mapping. The maximum 3D shape acquisition speed is 120 Hz (532 × 500 pixels), which is high enough for capturing the 3D shapes of moving objects. Two coding methods, sinusoidal phase-shifting method and trapezoidal phase-shifting method, were tested and results with good accuracy were obtained. The trapezoidal phase-shifting algorithm also makes real-time 3D reconstruction possible.

We present a novel fully automatic method for high resolution, nonrigid dense 3D point tracking. High quality dense point clouds of nonrigid geometry moving at video speeds are acquired using a phase-shifting structured light ranging technique. To use such data for the temporal study of subtle motions such as those seen in facial expressions, an efficient nonrigid 3D motion tracking algorithm is needed to establish inter-frame correspondences. The novelty of this paper is the development of an algorithmic framework for 3D tracking that unifies tracking of intensity and geometric features, using harmonic maps with added feature correspondence constraints. While the previous uses of harmonic maps provided only global alignment, the proposed introduction of interior feature constraints guarantees that nonrigid deformations are accurately tracked as well. The harmonic map between two topological disks is a diffeomorphism with minimal stretching energy and bounded angle distortion. The map is stable, insensitive to resolution changes and is robust to noise. Due to the strong implicit and explicit smoothness constraints imposed by the algorithm and the high-resolution data, the resulting registration/deformation field is smooth, continuous and gives dense one-to-one inter-frame correspondences. Our method is validated through a series of experiments demonstrating its accuracy and efficiency.

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, etc.

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 3-D shape measurement has been developed and experimented. A DigitalMicromirror 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. Theprojector has a color filter with four segments, RGB and clear, which controls the color of the projected image. When aspecific segment is in the light path, the DMD forms the corresponding image of that color. A 24-bit color image is formedwhen the RGB color channels are sequentially projected in a cycle time of 10 ms. When the color filter in the projector isremoved, 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 forprojection control. When a CCD camera used to capture the fringe patterns is synchronized with the DMD, the individualchannels of RGB or the three phase-shifted fringe patterns can be captured in less than 10 ms, thus making high-speed phaseshifting possible. Experimental results demonstrated the feasibility of this technique for high-speed 3-D shape measurement.Keywords: 3-D shape measurement, fringe projection, phase shifting, video projector, CCD camera, digital lightprocessing (DLP), digital micromirror device (DMD).

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.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A novel optical sensor for precision small-angle measurement is proposed. It is based on the total internal reflection effect of a light beam at a glass prism. Angular displacement of the light beam is measured by detecting the intensity change of the reflected beam as a result of the relative phase shift between the s- and p-polarized beams. A differential method is developed in which the light beam is equally split into two, each reflected at a prism and detected by a photodiode. The output is the difference of the two detected intensities divided by their sum. It was found that this differential method significantly improves the linearity of sensor output. Both theoretical and experimental results are presented.

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.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.

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.

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.

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 estab- lished. The calibration method is fast, robust, and accurate. It signifi- cantly simplifies the calibration and recalibration procedures of struc- tured light systems. This work describes the principle of the proposed method and presents some experimental results that demonstrate its performance. © 2006 Society of Photo-Optical Instrumentation Engineers.

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 trap- ezoidal 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 mea- surement 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 previ- ously developed real-time 3-D shape measurement system. The recon- structed 3-D results show similar quality to those obtained by the sinu- soidal phase-shifting method. However, since the data processing speed is much faster 4.6 ms per frame, both image acquisition and 3-D re- construction can be done in real time at a frame rate of 40 fps and a resolution of 532500 points. This real-time capability allows us to mea- sure 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. © 2005 Society of Photo-Optical Instrumentation Engineers. DOI: 10.1117/1.2147311 Subject terms: 3-D shape measurement; structured light; trapezoidal phase shifting; intensity ratio.

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

A 3-D surface shape measurement 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 x , y , and z coordinates of the object surface by a conversion algorithm. To determine the accurate values of the system parameters as required by the conversion algorithm, a two-step calibration procedure was developed. The parameters were first measured to determine their approximate values, then a calibration plate was measured by the system at various positions, and an iteration algorithm used to estimate the system parameters. Measurement results of several objects are presented. The standard deviation of the measurement error was found to be 0.23 mm.

We present an error compensation method for a full-field 3-D shape measurement system based on a digital fringe projection and phase shifting technique. The error map of the system is first established by comparing the measured coordinates with the coordinates defined by a coordinates measuring machine (CMM) at preselected sample points within the measurement volume. An eight-point interpolation algorithm based on the Shepard's method is then used to compensate for the errors in the measured coordinates. Experimental results showed that the accuracy of the system was improved by more than 60% after error compensation.

A high-speed 3-D shape measurement technique based on digital fringe projection has been developed and experimented. This technique uses a computer-generated color fringe pattern whose red, green, and blue channels are sinusoidal fringe patterns with a 120-deg phase shift between neighboring channels. When this color fringe pat- tern is sent to a digital-micromirror-device (DMD) based video projector with no color filter, three gray-scale fringe patterns are repeatedly pro- jected to an object surface in sequence with a cycle time of approxi- mately 10 ms. The three phase-shifted fringe patterns deformed by the object surface are captured by a CCD camera with proper synchroniza- tion between the camera and the projector. The 3-D shape of the object surface is reconstructed by using a phase wrapping and unwrapping algorithm and a phase-coordinate conversion algorithm. Experimental results demonstrated the feasibility of this technique for high-speed 3-D shape measurement with a potential measurement speed up to 100 Hz. © 2003 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1525272) Subject terms: fringe projection; phase shifting; digital micromirror device; 3-D shape measurement.

A microscopic three-dimensional (3-D) shape measurement system based on digital fringe projection has been developed and experimentally investigated. A Digital Micromirror Device along with its illumination optics is integrated into a stereomicroscope, which projects computer-generated fringe patterns with a sinusoidal intensity profile through the microscope objective onto the object surface being measured. The fringe patterns deformed by the object surface are recorded by a CCD camera. The microscopic 3-D shape of the object surface is measured and reconstructed by use of a phase-shifting technique. We discuss design considerations and error analysis of the system. Experimental results successfully demonstrate the capability of this technique for surface profile measurement of rough surfaces at the micrometer level.

We describe what we believe is a new phase-shifting algorithm called a double three-step algorithm developed to reduce the measurement error of a three-dimensional shape-measurement system, which is based on digital fringe-projection and phase-shifting techniques. After comparing the performance of different existing phase-shifting algorithms, we present the new double three-step algorithm based on the error analysis of the standard three-step algorithm. In this algorithm, three-step phase shifting is done twice with an initial phase offset of 60 degrees between them, and the two obtained phase maps are averaged to generate the final phase map. Both theoretical and experimental results showed that this new algorithm worked well in significantly reducing the measurement error.

An autofocusing system used for a laser burst-cutting-area (BCA) marking system for a DIVX production line has been developed. The principle of focus error detection is based on a method that takes advantage of the focusing properties of simple optical elements. A differ- ential detection method is used to achieve high resolution, and a position servo is designed to accomplish the autofocusing. Some design consid- erations, system modeling, simulation, and experiments are described. Experimental results are presented to show the feasibility of this system. © 2001 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1383993) Subject terms: autofocusing; detection; feedback control; optical disk; laser mark- ing.

We propose to use thin films to provide a drastic improvement of measurement sensitivity in the recently developed small-angle measurement method, namely, angle measurement based on the internal-reflection effect. By designing the thin films (single layer or multiple layers) so that they provide an antireflection effect in the vicinity of the critical angle, we show that the sensitivity of angle measurement can be increased exponentially with the increase of the number of thin-film layers. This method provides a new means of designing angle sensors with increased sensitivities without having to increase the number of reflections and therefore the physical size and the required fabrication accuracy of the reflection prisms. We describe the design of the thin films for this particular application and the analysis of measurement sensitivity and range as determined by the material and the number of layers of the thin films. Selection of the optimal initial angle for high linearity performance is also discussed.

We propose a novel optical probe for surface profile mea- surement of optical surfaces. This probe uses a laser beam to measure local slopes of a surface and obtains the surface profile through numeri- cal integration of the measured slopes. Slope measurement is based on a new angle measurement method, namely, angle measurement based on the internal-reflection effect (AMIRE). The probe is a high-resolution (0.1-arcsec), compact (80360340 mm) device with a simple measure- ment principle. For surface profile measurement, the probe is installed on a coordinate measuring machine (CMM), which provides the neces- sary scanning motion. A commercial laser interferometer is used to mea- sure the rotational error of the CMM in real time. The result is subtracted from the slope data collected by the probe for error compensation. Mea- surement results of a spherical mirror are presented that show a nanometer-order resolution in surface profile measurement. © 1999 Soci- ety of Photo-Optical Instrumentation Engineers. (S0091-3286(99)01607-4)

A color-encoded digital fringe projection technique is proposed for high-speed 3-D surface contouring applications. in this technique, 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 the 3-D surface contour of the object. 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. The principle of the technique is described and some preliminary experimental results are presented.