Digital watermarking technology is an important means to effectively protect three-dimensional (3D) model data. Among them, "blind detection" and "robustness" are key and difficult points in the current research of digital watermarking technology based on 3D models. In order to realize the blind detection of a watermark and improve its robustness against various common attacks at the same time, this paper proposes a dual blind watermarking method based on the normal feature of the centroid of first-ring neighboring points. The local spherical coordinate system is constructed by calculating two different normal vectors, and the first pattern watermark and the second random binary sequence watermark are embedded, respectively. The experimental results show that this method can not only realize the blind detection of dual watermarks, but also have the ability to resist common attacks such as translation, rotation, scaling, cropping, simplification, smoothing, noise, and vertex reordering to a certain extent.
This Letter presents a ray phase mapping model (RPM) for fringe projection profilometry (FPP) that avoids calibrating intrinsic parameters. The novelty of the RPM, to the best of our knowledge, is the ability to characterize the imaging system with independent rays for each pixel, and to associate the rays with the projected phase in the illumination field for efficient 3D mapping, which avoids complex imaging-specific modeling about lens layout and distortion. Two loss functions are constructed to flexibly optimize camera ray parameters and mapping coefficients, respectively. As a universal approach, it has the potential to calibrate different types of FPP systems with high accuracy. Experiments on wide-angle lens FPP, telecentric lens FPP, and micro-electromechanical system (MEMS)-based FPP are carried out to verify the feasibility of the proposed method.
Fourier ptychography (FP) provides an alternative scheme for improving the spatial bandwidth product with limited device. However, an angle-variation illumination is necessary to realize scanning imaging in the Fourier plane, which dramatically restricts the imaging speed and detection efficiency. In this paper, we propose a multiplexing and compressible FP scheme based on the structured illuminations and compressive sensing technique. Half of the LEDs are lighted together to reduce the exposure time; meanwhile, a learned dictionary is employed to reduce the sampling times. In addition, spectral independent illumination is proposed to retrieve color information from monochrome samplings. We experimentally verify that the proposed method can effectively reduce the sampling time with limited resolution loss.
Single-pixel imaging (SPI) technique has been studied intensively due to its minimum requirement for the detector resolution and the equipment costs. In this work, we proposed a new strategy of the SPI to explore its capability in high-dimensional imaging, which is the first comprehensive scheme as we know to achieve calibration, color texture and viewpoint expansion of single-pixel three-dimensional imaging. We realized a low-cost single-pixel three-dimensional imaging scheme which employ a raster scanner to provide the structured illumination and a grating to encode the height information. In order to reduce the blocking area, we introduce two single-pixel detectors (SPDs) to detect from two detection angles, a modified total variation based criterion is proposed to fuse the height information from two SPDs and reduce the error of shape fusion. To acquire the information of higher dimension, we introduce the third SPD aims to gain the color texture, three bandpass filter is placed in front of three SPDs, respectively, to collect different color information. Meanwhile a viewpoint switching method inspired by the shape from shading theory is presented to improve the color fidelity. Our study is expected to provide a demonstration for SPI in acquisition, reconstruction, and fusion of high-dimensional image data.
We present a single-shot wavelength-multiplexing technique for off-axis digital holography based on a spectral filter. Only a spectral filter is inserted between beam splitter and mirror in reflection off-axis digital holography (RODH). The spectral filter can transmit a well-defined wavelength band of light, while reject other unwanted radiation. By adjusting the filter and mirror separately, the propagation orientation of different reference beams of two wavelengths can be separated, and thus two off- axis holograms with different fringe directions are simultaneously captured by a monochrome camera. The wavefront interference analysis of using a spectral filter is discussed in detail. Our scheme is available for real-time wavelength-multiplexing but requires fewer optical elements and system modifications. Numerical simulation and experiment results of different types of spectral filters demonstrate the validity of proposed method.
Micro-Electro-Mechanical System (MEMS) scanning is increasingly popular in 3D surface measurement with the merits of the compact structure and high frame-rate. In this paper, we achieve real-time fringe structured 3D reconstruction by using a uniaxial MEMS-based projector. To overcome the limitations on uniaxial MEMS-based projector of lensless structure and unidirectional fringe projection, a novel isophase plane model is proposed, in which the laser line from MEMS-based projector is regarded as an isophase plane. Our model directly establishes the mapping relationship between phase and spatial 3D coordinates through the intersection point of camera back-projection light ray and isophase plane. Furthermore, a flexible calibration strategy to obtain 3D mapping coefficients is introduced with a specially designed planar target. Experiments demonstrated that our method can achieve high-accuracy and real-time 3D reconstruction.
Ambiguity caused by a wrapped phase is an intrinsic problem in fringe projection-based 3D shape measurement. Among traditional methods for avoiding phase ambiguity, spatial phase unwrapping is sensitive to sensor noise and depth discontinuity, and temporal phase unwrapping requires additional encoding information that leads to an increase of image sequence acquisition time or a reduction of fringe contrast. Here, to the best of our knowledge, we report a novel method of absolute phase unwrapping based on light field imaging. In a recorded light field under structured illumination, i.e., a structured light field, a wrapped phase-encoded field can be retrieved and resampled in diverse image planes associated with several possible fringe orders in a measurement volume. Then, by leveraging phase consistency constraint in the resampled wrapped phase-encoded field, correct fringe orders can be determined to unwrap the wrapped phase without any additional encoding information. Experimental results demonstrated that the proposed method was suitable for accurate and robust absolute phase unwrapping.
This Letter reports a novel method to establish the metric relationship of depth value between object space and image space for unfocused plenoptic cameras. A three-dimensional (3D) measurement system was introduced to precisely construct benchmarks and matching features to compute the metric depths in the object space and the corresponding depth values in the image space for metric calibration. After metric calibration, precise measurement of the depth dimension was possible. Furthermore, with the aid of metric spatio-angular parameters determined via light field ray calibration, transverse dimensions were computed from the measured depth, realizing light field 3D measurement for unfocused plenoptic cameras. Finally, we experimentally performed accuracy analysis of the proposed method with depth measurement precision of 0.5 mm in a depth range of 300 mm, which illuminated potential applications of unfocused plenoptic cameras in the field of 3D measurement.
Two major methods for 3D reconstruction in fringe projection profilometry, phase-height mapping and stereovision, have their respective problems: the former has low-flexibility in practical application due to system restrictions and the latter requires time-consuming homogenous points searching. Given these limitations, we propose a phase-3D mapping method developed from back-projection stereovision model to achieve flexible and high-efficient 3D reconstruction for fringe projection profilometry. We showed that all dimensional coordinates (X, Y, and Z), but not just the height coordinate (Z), of a measured point can be mapped from phase through corresponding rational functions directly and independently. To determine the phase-3D mapping coefficients, we designed a flexible two-step calibration strategy. The first step, ray reprojection calibration, is to determine the stereovision system parameters; the second step, sampling-mapping calibration, is to fit the mapping coefficients using the calibrated stereovision system parameters. Experimental results demonstrated that the proposed method was suitable for flexible and high-efficient 3D reconstruction that eliminates practical restrictions and dispenses with the time-consuming homogenous point searching.
