智能计算成像实验室
Smart Computational Imaging (SCI) Lab
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High-speed Real-time 3D Imaging Using Fringe Projection
Fringe projection profilometry (FPP) is a noncontact, optical, active, triangulation-based 3D reconstruction technique that is well known for its simple implementation, low cost, and high accuracy. FPP works on the same principle as stereo vision, where an object's coordinate in 3D space is derived by triangulating between pixels of two cameras. SLI avoids the computational complexities of matching pixels across camera views by replacing one of the two component cameras with a projector that generates a series of fringe patterns. By analyzing the change in the pattern at a particular point on the target object's surface (a process known as phase demodulation and unwrapping of the captured fringes), unique correspondences can be derived between the camera and projector pixels.


However, traditional fringe projection profilometers are usually designed to measure static object shapes under non-time-critical measurement conditions, their measurement of moving objects is limited. Besides, it is difficult to retrieve the absolute phase for spatially isolated surfaces simultaneously and rapidly since in the phase unwrapping, fringe orders will be ambiguous, making the depth difference between spatially isolated surfaces indiscernible. However, many areas of science and industry require 3D measurements made in high-speed or even real-time. One example is a human-to-computer interface where hand gestures are used to control a computer.

Real-time 3D Imaging and System
We are dedicated to innovate high-resolution, high speed , real-time 3-D imaging/sensing methods and systems for numerous applications such as human-to-computer interaction, computer graphics, healthcare, and the manufacturing industry. We have developed a real-time three-dimensional (3D) imaging system based on digital fringe projection, both hardware and software. The system achieves 120 fps real-time data acquisition, processing and visualization. This novel system employs high speed fringe projection [by modified a commercial DLP projector with self-developed Field-Programmable Gate Array (FPGA) circuit], pattern design, direct phase unwrapping (novel 4-pattern strategy to realize phase shifting & temporal phase unwrapping), gamma distortion correction (pre-coding the distorted pattern in the on-chip memory of the FPGA), noise reduction, motion artifacts suppression, filter design and other techniques (implemented in C++ with Visual Studio 2008).  By combining an infrared camera (self-developed 640×512 VOx Microbolometer) with this system, real-time three-dimensional thermal imaging can be realized.

Super-high-speed 3D Imaging with Modulated Patterns
The digital micromirror array (DMD) in a DLP projector is capable of switching mirrors "on" and "off" at very high speeds (106 Hz). An off-the-shelf DLP projector, however, effectively operates at much lower rates (60 - 120Hz) by emitting smaller intensities that are integrated over time by a sensor (eye or camera) to produce the desired brightness value. For example, an 1-bit binary image just need one working cycles of DMD while an 8-bit gray-scales image needs 256 working cycles. So it is preferable that the pattern can be generated using as fewer gray-scales as possible so that it can be reproduced by DMD at a higher switching speed. We are developing techniques to minimize phase error and generate high-quality modulated fringe patterns with only very few gray-levels (2 or 3). Our goal is to realize high-accuracy, super-high-speed (kHz) 3D imaging for fast-moving objects and dynamic scenes.