"Accurate calibration for 3D shape measurement system using a binary defocusing technique," Opt. Laser Eng., (2013)

L. Merner*, Y. Wang*, and S. Zhang, "Accurate calibration for 3D shape measurement system using a binary defocusing technique," Opt. Laser Eng. 51(5), 514-519, 2013; doi: 10.1016/j.optlaseng.2012.10.015

This paper introduces a novel method to calibrate 3D shape measurement systems that use the binary defocusing technique. Specifically, this method calibrates the pixelwise z as low-order polynomial functions of absolute phase; (x, y) coordinates are calculated from camera calibration with known z value; and the camera is calibrated using the standard flat checkerboard method. Because this method does not require estimation of the projector’s parameters, it can be adopted for any phase measurement system including those employing out-of-focus projectors. Our experiment found that the root-mean squared (rms) error for the depth measurement is less than 70 mm when the measurement depth range is about 100 mm, which is at the same level of the calibration stage +-50 mm.

"Fourier transform profilometry using a binary area modulation technique," Opt. Eng., (2012)

W. Lohry* and S. Zhang, "Fourier transform profilometry using a binary area modulation technique," Opt. Eng. 51(11),113602, 2012; doi: 10.1117/1.OE.51.11.113602

A recent study found that it is very difficult to use the squared binary defocusing technique to eliminate the influence of third-order harmonics without compromising fringe quality, and thus it is challenging to utilize Fourier transform profilometry to achieve high-quality three-dimensional measurement. A novel approach is presented to effectively eliminate the third-order harmonics by modulating the squared binary structured patterns. Both simulation and experiments are presented to verify the performance of the proposed technique.

"Three-dimensional shape measurement with binary dithered patterns," Appl. Opt.,(2012)

Y. Wang* and S. Zhang, "Three-dimensional shape measurement with binary dithered patterns," Appl. Opt. 51(27), 6631-6636, 2012; doi: 10.1364/AO.51.006631

The previously proposed binary defocusing technique and its variations have proven successful for high-quality three-dimensional (3D) shape measurement when fringe stripes are relatively narrow, but they suffer if fringe stripes are wide. This paper proposes to utilize the binary dithering technique to conquer this challenge. Both simulation and experimental results show the phase error is always less than 0.6% even when the fringe stripes are wide and the projector is nearly focused.

"Mapping cardiac surface mechanics with structured light imaging," American Journal of Physiology: Heart and Circular Physiology, (2012)

J. I. Laughner, S. Zhang, H. Li, C. C. Shao, and I. R. Efimov, "Mapping cardiac surface mechanics with structured light imaging," American Journal of Physiology: Heart and Circular Physiology 303(6), H712-H720, 2012 (Image of the week of October 1, 2012, American Journal of Physiology); doi: 10.1152/ajpheart.00269.2012

Cardiovascular disease often manifests as a combination of pathological electrical and structural heart remodeling. The relationship between mechanics and electrophysiology is crucial to our understanding of mechanisms of cardiac arrhythmias and the treatment of cardiac disease. While several technologies exist for describing whole heart electrophysiology, studies of cardiac mechanics are often limited to rhythmic patterns or small sections of tissue. Here, we present a comprehensive system based on ultrafast three-dimensional (3-D) structured light imaging to map surface dynamics of whole heart cardiac motion. Additionally, we introduce a novel nonrigid motion-tracking algorithm based on an isometry-maximizing optimization framework that forms correspondences between consecutive 3-D frames without the use of any fiducial markers. By combining our 3-D imaging system with nonrigid surface registration, we are able to measure cardiac surface mechanics at unprecedented spatial and temporal resolution. In conclusion, we demonstrate accurate cardiac deformation at over 200,000 surface points of a rabbit heart recorded at 200 frames/s and validate our results on highly contrasting heart motions during normal sinus rhythm, ventricular pacing, and ventricular fibrillation.

"Three-dimensional range data compression using computer graphics rendering pipeline," Appl. Opt., (2012)

S. Zhang, "Three-dimensional range data compression using computer graphics rendering pipeline," Appl. Opt. 51(18), 4058-4064, 2012 (Cover feature); doi: 10.1364/AO.51.004058

This paper presents the idea of naturally encoding three-dimensional (3D) range data into regular two dimensional (2D) images utilizing computer graphics rendering pipeline. The computer graphics pipeline provides a means to sample 3D geometry data into regular 2D images, and also to retrieve the depth information for each sampled pixel. The depth information for each pixel is further encoded into red, green, and blue color channels of regular 2D images. The 2D images can further be compressed with existing 2D image compression techniques. By this novel means, 3D geometry data obtained by 3D range scanners can be instantaneously compressed into 2D images, providing a novel way of storing 3D range data into its 2D counterparts. We will present experimental results to verify the performance of this proposed technique.

"Composite phase-shifting algorithm for absolute phase measurement," Opt. Laser Eng., (2012)

S. Zhang, "Composite phase-shifting algorithm for absolute phase measurement," Opt. Laser Eng. 50, 1538-1541, 2012;doi: 10.1016/j.optlaseng.2012.06.005

This paper presents a method to recover absolute phase by using only four images: three phase-shifted patterns and one stair pattern. The stair pattern is designed in such a way that the stair changes are perfectly aligned with the phase jumps, and thus absolute phase can be recovered by referring to the stair pattern. Due to system noises and camera and/or projector blurring, a computational framework is also proposed. Because this technique only requires four fringe images for absolute phase recovery, it has the merit of measurement speed. And since the absolute phase is obtained, this technique is suitable for measuring step-height objects. We have developed a digital fringe projection system to verify the performance of the proposed technique.

"Novel phase coding method for absolute phase retrieval," Opt. Lett., (2012)

Y. Wang* and S. Zhang, "Novel phase coding method for absolute phase retrieval," Opt. Lett. 37(11), 2067-2069, 2012;doi: 10.1364/OL.37.002067

This Letter presents a novel absolute phase recovery technique with phase coding. Unlike the conventional graycoding method, the codeword is embedded into the phase and then used to determine the fringe order for absolute phase retrieval. This technique is robust because it uses phase instead of intensity to determine codewords, and it could achieve a faster measurement speed, since three additional images can represent more than 8
 unique codewords for phase unwrapping. Experimental results will be presented to verify the performance of the proposed technique. 

"3D shape measurement with 2D area modulated binary patterns," Opt. Laser Eng.,(2012)

W. Lohry* and S. Zhang, "3D shape measurement with 2D area modulated binary patterns," Opt. Laser Eng. 50(7), 917-921, 2012; doi: 10.1016/j.optlaseng.2012.03.002

This paper presents a novel area-modulation technique for three-dimensional (3D) shape measurement with binary defocusing. Specifically, this technique modulates local 2*2 pixels to create five grayscale values to enhance fringe quality when the projector is not perfectly in focus. With this novel technique, we will show that the phase error is approximately 1/3 of the square binary method when fringe pattern is dense and the projector is nearly focused. 

"Uniaxial three-dimensional shape measurement with projector defocusing," Opt. Eng., (2012)

Y. Xu* and S. Zhang, "Uniaxial three-dimensional shape measurement with projector defocusing," Opt. Eng. 51(2) 023604, 2012; doi: 10.1117/1.OE.51.2.023604

Our study shows that the phase error caused by improperly defocused binary structured patterns correlates to the depth z. This finding leads to a novel uniaxial three-dimensional shape measurement technique without triangulation. Since the measurement can be performed from the same viewing angle, this proposed method overcomes some limitations of the triangulation-based techniques, such as the problem of measuring deep holes. Our study explains the principle of the technique and presents some experimental results to verify its feasibility.

"Comparion among square binary, sinusoidal pulse width modulation, and optimal pulse width modulation methods for three-dimensional shape measurement," Appl. Opt. (2012)

Y. Wang* and S. Zhang, "Comparion among square binary, sinusoidal pulse width modulation, and optimal pulse width modulation methods for three-dimensional shape measurement," Appl. Opt. 51(7), 861-872, 2012; doi:10.1364/AO.51.000861

This paper presents a comparative study on three sinusoidal fringe pattern generation techniques with projector defocusing: the squared binary defocusing method (SBM), the sinusoidal pulse width modulation (SPWM) technique, and the optimal pulse width modulation (OPWM) technique. Because the phase error will directly affect the measurement accuracy, the comparisons are all performed in the phase domain. We found that the OPWM almost always performs the best, and SPWM outperforms SBM to a great extent, while these three methods generate similar results under certain conditions. We will briefly explain the principle of each technique, describe the optimization procedures for each technique, and finally compare their performances through simulations and experiments. © 2012 Optical Society of America

"Holovideo: Real-time 3D range video encoding and decoding on GPU," Opt. Laser Eng. (2012)

N. Karpinsky* and S. Zhang, "Holovideo: Real-time 3D range video encoding and decoding on GPU," Opt. Laser Eng.50(2), 280-286, 2012; doi: 10.1016/j.optlaseng.2011.08.002

We present a 3D video-encoding technique called Holovideo that is capable of encoding high-resolution 3D videos into standard 2D videos, and then decoding the 2D videos back into 3D rapidly without significant loss of quality. Due to the nature of the algorithm, 2D video compression such as JPEG encoding with QuickTime Run Length Encoding (QTRLE) can be applied with little quality loss, resulting in an effective way to store 3D video at very small file sizes. We found that under a compression ratio of 134:1, Holovideo to OBJ file format, the 3D geometry quality drops at a negligible level. Several sets of 3D videos were captured using a structured light scanner, compressed using the Holovideo codec, and then uncompressed and displayed to demonstrate the effectiveness of the codec. With the use of OpenGL Shaders (GLSL), the 3D video codec can encode and decode in realtime. We demonstrated that for a video size of 512 512, the decoding speed is 28 frames per second (FPS) with a laptop computer using an embedded NVIDIA GeForce 9400 m graphics processing unit (GPU). Encoding can be done with this same setup at 18 FPS, making this technology suitable for applications such as interactive 3D video games and 3D video conferencing. 

"High-resolution, real-time 3-D imaging with fringe analysis," Real Time Image Processing, (2012)

[25] N. Karpinsky* and S. Zhang, "High-resolution, real-time 3-D imaging with fringe analysis," Real Time Image Processing,7(1) 55-66, 2012; doi: 10.1007/s11554-010-0167-4

Real-time 3D imaging is becoming increasingly important in areas such as medical science, entertainment, homeland security, and manufacturing. Numerous 3D imaging techniques have been developed, but only a few of them have the potential to achieve realtime. Of these few, fringe analysis based techniques stand out, having many advantages over the rest. This paper will explain the principles behind fringe analysis based techniques, and will provide experimental results from systems using these techniques.