Absolute phase measurement in fringe projection using multiple perspectives

Geometric-feature-based approach to human face reconstruction with high measurement speed

This paper presents a method based on geometry for three-dimensional (3D) face reconstruction without the need for additional images, hardware components, or objects. In our proposed method, we consider part of the nose as the feature region because its shape remains almost constant during the measurement. The geometry of this region was used to provide cues for phase unwrapping. We first spatially unwrap the phase and determine the integer multiple of 2π to be added by comparing the recovered result of the feature region and its actual shape. Then, the face can be reconstructed with the acquired absolute phase. Experimental results demonstrated that our method is capable of reconstructing a dynamic face with high measurement speed, and only three phase-shifted fringes are required per frame.

Phase Demodulation Method for Fringe Projection Measurement Based on Improved Variable-Frequency Coded Patterns

The phase-to-height imaging model, as a three-dimensional (3D) measurement technology, has been commonly applied in fringe projection to assist surface profile measurement, where the efficient and accurate calculation of phase plays a critical role in precise imaging. To deal with multiple extra coded patterns and 2π jump error caused to the existing absolute phase demodulation methods, a novel method of phase demodulation is proposed based on dual variable-frequency (VF) coded patterns. In this paper, the frequency of coded fringe is defined as the number of coded fringes within a single sinusoidal fringe period. First, the effective wrapped phase (EWP) as calculated using the four-step phase shifting method was split into the wrapped phase region with complete period and the wrapped phase region without complete period. Second, the fringe orders in wrapped phase region with complete period were decoded according to the frequency of the VF coded fringes and the continuous characteristic of the fringe order. Notably, the sampling frequency of fast Fourier transform (FFT) was determined by the length of the decoding interval and can be adjusted automatically with the variation in height of the object. Third, the fringe orders in wrapped phase region without complete period were decoded depending on the consistency of fringe orders in the connected region of wrapped phase. Last, phase demodulation was performed. The experimental results were obtained to confirm the effectiveness of the proposed method in the phase demodulation of both discontinuous objects and highly abrupt objects.

State-of-the-art active optical techniques for three-dimensional surface metrology: a review [Invited]

This paper reviews recent developments of non-contact three-dimensional (3D) surface metrology using an active structured optical probe. We focus primarily on those active non-contact 3D surface measurement techniques that could be applicable to the manufacturing industry. We discuss principles of each technology, and its advantageous characteristics as well as limitations. Towards the end, we discuss our perspectives on the current technological challenges in designing and implementing these methods in practical applications.

Influence of projector pixel shape on ultrahigh-resolution 3D shape measurement

The state-of-art three-dimensional (3D) shape measurement with digital fringe projection (DFP) techniques assume that the influence of projector pixel shape is negligible. However, our research reveals that when the camera pixel size is much smaller than the projector pixel size in object space (e.g., 1/5), the shape of projector pixel can play a critical role on ultimate measurement quality. This paper evaluates the performance of two shapes of projector pixels: rectangular and diamond shaped. Both simulation and experimental results demonstrated that when the camera pixel size is significantly smaller than the projector pixel size, it is advantageous for ultrahigh resolution 3D shape measurement system to use a projector with rectangular-shaped pixels than a projector with diamond-shaped pixels.

Improved unwrapped phase retrieval method of a fringe projection profilometry system based on fewer phase-coding patterns

In this paper, based on two additional phase-coding patterns, an improved phase demodulation method is proposed. First, six equally spaced coding phases in the interval [$ - \pi $-π, $\pi $π] are embedded in different periods of the coded fringes following a certain sequence. Subsequently, since a group of phase orders can be uniquely determined by the four adjacent coding phases, the phase-order map of the object can be generated. To ensure the accuracy of decoding results, the interference coding numbers should be corrected in advance. In the meantime, the connected regions exhibiting the same orders are classified and then labeled for simplifying the decoding process. The simulation results verify the feasibility of the proposed method. By two groups of 3D imaging experiments, the applicability of this method to multiple objects and discontinuous objects is confirmed.

FULLY AUTOMATIC DIGITAL FRINGE PROJECTION MEASUREMENT FOR 3D FACIAL SURFACE

Digital fringe projection technique is widely used in industrial applications with high accuracy and measurement speed. In this study, a fully automatic high-speed digital fringe projection technique is presented to profile 3D facial characteristics. The structured light with fringe pattern is used to be the light source in the measurement system and is projected by a digital light processing projector. The distorted fringe patterns from facial surface are captured by the digital camera. The absolute phase maps are calculated by using phase-shifting and quality guided path unwrapping algorithm. A complete, 3D facial feature is obtained by our measurement. We achieved simultaneous phase acquisition, reconstruction and three-dimensional (3D) exhibition at a speed of 0.5[Formula: see text]s. This technique may provide a high accuracy and real-time 3D facial measurement for biometric verification.

High-speed and high-accuracy 3D surface measurement using a mechanical projector

This paper presents a method to achieve high-speed and high-accuracy 3D surface measurement using a custom-designed mechanical projector and two high-speed cameras. We developed a computational framework that can achieve absolute shape measurement in sub-pixel accuracy through: 1) capturing precisely phase-shifted fringe patterns by synchronizing the cameras with the projector; 2) generating a rough disparity map between two cameras by employing a standard stereo-vision method using texture images with encoded statistical patterns; and 3) utilizing the wrapped phase as a constraint to refine the disparity map. The projector can project binary patterns at a speed of up to 10,000 Hz, and the camera can capture the required number of phase-shifted fringe patterns with 1/10,000 second, and thus 3D shape measurement can be realized as high as 10,000 Hz regardless the number of phase-shifted fringe patterns required for one 3D reconstruction. Experimental results demonstrated the success of our proposed method.

Absolute three-dimensional shape measurement with a known object

This paper presents a novel method for absolute three-dimensional (3D) shape measurement that does not require conventional temporal phase unwrapping. Our proposed method uses a known object (i.e., a ping-pong ball) to provide cues for absolute phase unwrapping. During the measurement, the ping-pong ball is positioned to be close to the nearest point from the scene to the camera. We first segment ping-pong ball and spatially unwrap its phase, and then determine the integer multiple of 2π to be added such that the recovered shape matches its actual geometry. The nearest point of the ball provides zmin to generate the minimum phase Φmin that is then used to unwrap phase of the entire scene pixel by pixel. Experiments demonstrated that only three phase-shifted fringe patterns are required to measure absolute shapes of objects moving along depth z direction.