Real-time 3D Imaging System

High-speed three-dimensional profilometry for multiple objects with complex shapes

(Real-time 3D Imaging System)

Chao Zuo, Qian Chen, Guohua Gu, Shijie Feng, and Fangxiaoyu Feng

Jiangsu Key Laboratory of Spectral Imaging & Intelligent Sense,

Nanjing University of Science and Technology, Nanjing, Jiangsu Province 210094, China

Abstract

This paper describes an easy-to-implement three-dimensional (3-D) real-time shape measurement technique using our newly developed high-speed 3-D vision system. It employs only four projection fringes to realize full-field phase unwrapping in the presence of discontinuous or isolated objects. With our self-designed pattern generation hardware and a modified low-cost DLP projector, the four designed patterns can be generated and projected at a switching speed of 360 Hz. Using a properly synchronized high-speed camera, the high-speed fringe patterns distorted by measured objects can be acquired and processed in real-time. The resulting system can capture and display high-quality textured 3-D data at a speed of 120 frames per second, with the resolution of 640 × 480 points. The speed can be trebled if a camera with a higher frame rate is employed. We detail our shape measurement technique, including the four-pattern decoding algorithm as well as the hardware design. Some evaluation experiments have been carried out to demonstrate the validity and practicability of the proposed technique.

Files:

              PDF            Media 1        Media 2       Human body   Double hands

Citations

Chao Zuo, Qian Chen, Guohua Gu, Shijie Feng, and Fangxiaoyu Feng, "High-speed three-dimensional profilometry for multiple objects with complex shapes," Opt. Express 20, 19493-19510 (2012).

Results

Fig. 1. Our real-time 3-D measurement system (a) and its key components (b).

Fig. 2. Measurement result for two isolated structures; (a) One captured fringe image. (b)

Wrapped phase map; (c) Base phase map; (d) Absolute phase map; (e) Reconstructed 3-D

result; (f) 3-D result with texture from another angle.

Fig. 3. Real-time measurement results of a moving hand (Media 1). Selected frames of

measured results with depth pseudo color (a) and texture (b). The 3-D shape could be

successfully reconstructed when the illumination intensities were either weak (c) or strong (d).

Fig. 4. Real-time measurement results of spatially isolated objects (Media 2).

Methods

Fig. 1. An example of the proposed patterns and their 8-bit (F = 10, Ap = 127.5, Bp = 127.5)

gray-scale intensity distributions.

Our research focus is to develop a low-cost real-time 3-D shape measurement system that is able to produces high-speed and high accuracy profilometry of general objects. Shortening the pattern sequence required for unwrapping phase images while maintaining its key benefits such as high accuracy and data density is one crucial step toward this goal. To achieve this, we developed a four-pattern strategy (Fig. 1) to decode fewer pattern sets while retaining high resolution and object independence.

Fig. 2. (a) Schematic of the high-speed pattern projection mechanism. (b) Synchronization signal HSYNC and VSYNC of VGA. The values of TS, TPW, TBP, TDP, and TFP for HSYNC and VSYNC can be observed in Table 1 for an 800 × 600 resolution at 120 Hz.

To project the four patterns sequentially and repeatedly at a high speed, we create a sequence of color encoded pattern images, and sent them sequentially and repeatedly at 120 Hz. When the projector receives this pattern sequence, the four patterns can be decoded by the DLP video processor system and then projected them in a correct order. We illustrate this process in Fig. 2(a).

     We use a self-developed Field-Programmable Gate Array (FPGA) board to generate and send the desired patterns to the projector rapidly. Instead of using a computer, the VGA signal is generated by a high-speed RGB digital-to-analog converter (DAC) (Analog Devices ADV7123) controlled by the FPGA (ALTERA Cyclone ®IV EP4CE15). Inside the ADV7123, there are three channels of 10-bit-precision video DAC, which are respectively used to implement the digital-to-analog converting of three channels of RGB digital signals. Besides the three video signals, two main signals are used in VGA to synchronize the information that is sent to the display: The Horizontal Synchronization (HSYNC) is responsible for indicating that a new line is starting, while the Vertical Synchronization (VSYNC) is responsible for indicating that a new screen is starting. The HSYNC and VSYNC representations are shown in Fig. 2(b).

Fig. 3. Timing diagram of the whole system. The darker regions represent the exposure period of the camera.

After removing the color filter of the projector, the feedback trigger signal from the photodiode is no longer available. To keep the DMD working normally, a substitute of feedback trigger signal is generated directly by the FPGA. We use the VSYNC signal as the benchmark to generate the trigger pulses for the camera and the feedback trigger signal for the DMD. The timing signals of the whole system are shown in Fig. 3. The waveform at the top is the VSYNC signal of VGA, whose frequency is identical with the feedback trigger signal to of projector shown in the second row. The third row is the trigger signal to the camera, which is carefully designed in accordance with the projection timing, which is shown at the bottom row. In the projection timing, the G, B, and R, represent the green, blue, and red channels of the projector.

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Contact

Chao Zuo

Associate professor at the school of Electronic and Optical Engineering

Email: surpasszuo@163.com

Nanjing University of Science and Technology, Jiangsu Province (210094), P.R.China

Qian Chen

Dean of the school of Electronic and Optical Engineering

Email: chenqian@njust.edu.cn

Nanjing University of Science and Technology, Jiangsu Province (210094), P.R.China

Shijie Feng

Ph.D. Candidate of NJUST

Email:geniusshijie@163.com ( or 311040574@njust.edu.cn)

Nanjing University of Science and Technology, Jiangsu Province (210094), P.R.China