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.
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
KEYWORDS: Projection systems, Cameras, Fringe analysis, 3D metrology, LCDs, Phase shifts, Video, 3D image reconstruction, 3D image processing, RGB color model
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.
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