Magnification of 3D reconstructed images in integral imaging using an intermediate-view reconstruction technique

A new integral imaging (II) system that can magnify 3D reconstructed images by employing an intermediate-view reconstruction technique (IVRT) is proposed in which the number of the elemental images obtained from a one-step pickup process can be computationally increased by use of an IVRT without mechanical movement and a long multistep pickup process. To show the feasibility of the proposed II system, some optical experiments on the magnification of 3D reconstructed images with a real 3D object have been carried out and results are presented.     

Depth extraction of three-dimensional objects in space by the computational integral imaging reconstruction technique

A novel approach to extract the depth data of 3D objects in space by using the computational integral imaging reconstruction (CIIR) technique is proposed. With elemental images of 3D objects captured by the CCD camera through a pinhole array, depth-dependent object images can be reconstructed on the output plane by the CIIR technique. Only the images reconstructed on the output planes where 3D objects were located are clearly focused; so the depth data of 3D objects in space can be extracted by discriminating these focused output images from the others by using an image separation technique. A feasibility test of the proposed CIIR-based depth extraction method is carried out, and its results are discussed as well.     

Iterative algorithm with a constraint condition for numerical reconstruction of a three-dimensional object from its hologram

A novel method to obtain the three-dimensional mathematical model of a transparent object from its hologram is presented. The proposed method can numerically extract the object information from the fringe pattern of the hologram. Then an iterative algorithm is used to imitate an imaging system by focusing on different layers of the object; and by operating in both the spatial domain and the frequency domain, the algorithm produces a series of two-dimensional layer images. The object is finally reconstructed layer by layer. A constraint condition should be satisfied, and the noise distribution can be rearranged in different reconstruction cycles so as to get better reconstruction quality. Numerical simulations have proved the effectiveness of the proposed method.     

Blur-metric-based resolution enhancement of computationally reconstructed integral images

A computational integral imaging reconstruction technique can reconstruct a set of plane images of three-dimensional (3-D) objects along the output plane, in which only the plane object image (POI) reconstructed on the right planes where the objects were positioned is highly focused, whereas the other POIs reconstructed away from these planes are unfocused and blurred. In fact, these blurred POIs act as additional noises to other object images reconstructed on different output planes, so that the resolution of reconstructed object images should be considerably deteriorated. In this paper, a novel approach is proposed to effectively reduce the blurred images occurring in the focused POIs by employing a blur metric. From the estimated blur metric of each reconstructed POI, the right output planes where the objects were located can be detected. In addition, with an estimated blur metric, focused POIs can be adaptively eroded by a simple gray level erosion operation because it reduces regional expansion caused by the blur effect. The gray values of the eroded POIs are then finally remapped by referencing the original POIs. Some experiments revealed an average increase of 1.95 dB in the peak signal-to-noise ratio in the remapped POIs compared with that of the originally reconstructed POIs, and that the original forms of the object images in the remapped POIs could be preserved even after they had gone through an erosion operation. This feasibility test of the proposed scheme finally suggests a possibility of its application to robust detection and recognition of 3-D objects in a scene.     

Three-dimensional Displacement Analysis by Windowed Phase-shifting Digital Holographic Interferometry

Abstract:  Phase-shifting digital holography is a new method for measuring the displacement distribution on the surface of an object. The authors previously proposed a windowed phase-shifting digital holographic interferometry (windowed PSDHI). This method provides accurate displacement distributions by decreasing the effect of speckle patterns. In this study, the method is extended to analyse three-dimensional displacement components in a microscope. Three object laser beams in the optical system are used. Four phase-shifted holograms are recorded for each object laser beam. The complex amplitude of each reconstructed light at the object is calculated by the Fresnel diffraction integral of the complex amplitude of the hologram. The reconstructed distance is obtained at the point with the maximum of the standard deviation of the intensities of the object reconstructed with changing the reconstruction distance. The three phase-difference values between before and after deformation provide the three-dimensional displacement components. Theoretical treatment and experimental results of three-dimensional displacement measurement using this method are shown.     

Enhanced reconstruction of three-dimensional shape and texture from integral photography images

A method for the reconstruction of 3D shape and texture from integral photography (IP) images is presented. Sharing the same principles with stereoscopic-based object reconstruction, it offers increased robustness to noise and occlusions due to the unique characteristics of IP images. A coarse-to-fine approach is used, employing what we believe to be a novel grid refinement step in order to increase the quality of the reconstructed objects. The proposed method's properties include configurable depth accuracy and direct and seamless triangulation. We evaluate our method using synthetic data from a computer-simulated IP setup as well as real data from a simple yet effective digital IP setup. Experiments show reconstructed objects of high-quality indicating that IP can be a competitive modality for 3D object reconstruction.     

双视三维重建的高精度运动参数估计方法

从两幅透视图像恢复被摄目标的三维结构是计算机视觉最基本的任务之一,其中,运动估计算法的性能决定了最终的三维重建精度。首先讨论了双视成像的基本数学模型,并介绍了几种现有运动参数估计方法的基本原理和不足。随后,基于投影误差最小判决函数,提出了用于双像运动估计的改进非线性迭代优化方法。数值仿真结果表明,在大平移小旋转角及小平移大旋转角2种运动条件下,采用文中提出的方法,运动估计精度均有所提高。此外,根据运动参数的估计值对真实目标进行三维重建实验,结果表明尺度重建误差小于2%且角度误差在3°以内。     

Wide-viewing-angle integral three-dimensional imaging system by curving a screen and a lens array

A wide-viewing-angle integral three-dimensional imaging system made by curving a screen and a lens array is described. A flexible screen and a curved lens array are incorporated into an integral imaging system in place of a conventional flat display panel and a flat lens array. One can effectively eliminate flipped images by adopting barriers. As a result, the implemented system permits the limitation of viewing angle to be overcome and the viewing angle to be expanded remarkably. Using the proposed method, we were able to achieve a viewing angle of 33 degrees (one side) for real integral imaging and 40 degrees (one side) for virtual integral imaging, which is four times wider than that of the currently used conventional techniques. The principle of the implemented system is explained, and some experimental results are presented.     

Visibility-enhanced reconstruction of three-dimensional objects under a heavily scattering medium through combined use of intermediate view reconstruction, multipixel extraction, and histogram equalization methods in the conventional integral imaging system

In this paper, we propose an effective approach for reconstructing visibility-enhanced three-dimensional (3D) objects under the heavily scattering medium of dense fog in the conventional integral imaging system through the combined use of the intermediate view reconstruction (IVR), multipixel extraction (MPE), and histogram equalization (HE) methods. In the proposed system, the limited number of elemental images (EIs) picked up from the 3D objects under the dense fog is increased by as many as required by using the IVR technique. The increased number of EIs is transformed into the subimages (SIs) in which the resolution of the transformed SIs has been also improved as much as possible with the MPE method. Subsequently, by using the HE algorithm, the histogram of the resolution-enhanced SIs is uniformly redistributed over the entire range of discrete pixel levels of the image in a way that the subimage contrast can be much enhanced. Then, these equalized SIs are converted back into the newly modified EIs, and consequently a visibility-enhanced 3D object image can be reconstructed. Successful experimental results with the test object confirmed the feasibility of the proposed method.     

Image quality enhancement of computational integral imaging reconstruction for partially occluded objects using binary weighting mask on occlusion areas

This paper presents an image quality enhancement of computational integral imaging reconstruction (CIIR) method by using a binary weighting mask on occlusion areas in elemental images. The proposed method utilizes a block-matching algorithm to estimate the occlusion areas in elemental images. Then, a binary weighting mask generated from the estimated occlusion area is applied to our CIIR method. This minimizes the overlapping effect of occluding objects in the reconstructed plane images and thus improves visual quality dramatically. To show the usefulness of our proposed scheme, we conduct several experiments and present the results. The experimental results indicate that our method is superior to the existing methods.     

High topographical accuracy by optical shot noise reduction in digital holographic microscopy

In this work, we present a new method to reduce the shot noise in phase imaging of digital holograms. A spatial averaging process of phase images reconstructed at different reconstruction distances is performed, with the reconstruction distance range being specified by the numerical focus depth of the optical system. An improved phase image is attained with a 50% shot noise reduction. We use the integral of the angular spectrum as a reconstruction method to obtain a single-object complex amplitude that is needed to perform our proposal. We also show the corresponding simulations and experimental results. The topography of a homemade TiO2 stepwise of 100 nm high was measured and compared with the atomic force microscope results.     

Speckle-reduced three-dimensional volume holographic display by use of integral imaging

We propose a method to implement a speckle-reduced coherent three-dimensional (3D) display system by a combination of integral imaging and photorefractive volume holographic storage. The 3D real object is imaged through the microlens array and stored in the photorefractive crystal. During the reconstruction process a phase conjugate reading beam is used to minimize aberration, and a rotating diffuser located on the imaging plane of the lens array is employed to reduce the speckle noise. The speckle-reduced 3D image with a wide viewing angle can be reconstructed by use of the proposed system. Experimental results are presented and optical parameters of the proposed system are discussed in detail.     

Photoacoustic tomography using a Mach-Zehnder interferometer as an acoustic line detector

A three-dimensional photoacoustic imaging method is presented that uses a Mach-Zehnder interferometer for measurement of acoustic waves generated in an object by irradiation with short laser pulses. The signals acquired with the interferometer correspond to line integrals over the acoustic wave field. An algorithm for reconstruction of a three-dimensional image from such signals measured at multiple positions around the object is shown that is a combination of a frequency-domain technique and the inverse Radon transform. From images of a small source scanning across the interferometer beam it is estimated that the spatial resolution of the imaging system is in the range of 100 to about 300 mum, depending on the interferometer beam width and the size of the aperture formed by the scan length divided by the source-detector distance. By taking an image of a phantom it could be shown that the imaging system in its present configuration is capable of producing three-dimensional images of objects with an overall size in the range of several millimeters to centimeters. Strategies are proposed how the technique can be scaled for imaging of smaller objects with higher resolution.     

基于梯度投影稀疏重建算法的电容层析成像图像重建

为解决两相流中存在中心物体、物体比较小或存在多个物体且相距较近时电容层析成像(ECT)重建图像精度较差的问题,基于稀疏分布的流型其介电常数分布满足稀疏性的先验条件,采用梯度投影稀疏重建(GPSR-BB)算法进行ECT图像重建。仿真及实验测试结果表明:GPSR-BB算法对于流体中小目标以及复杂流型的图像重建质量较好,重建图像的形状保真度高。     

Effect of illumination in an integral imaging system with large depth of focus

We present the characteristics of integral imaging systems with large depth of focus (DOF) by use of two kinds of illumination: plane illumination and diffusing illumination. For each system, we perform ray analysis based on ray optics. To check the visual quality through optical experiments, we use an average image of observed images picked up at various positions within a large DOF. The synthesized elemental images for a three-dimensional (3-D) object with two character patterns were displayed in an optical system and its reconstruction experiments are performed. Experimental results show that use of diffusing illumination can improve visual quality of reconstruction 3-D images in depth-priority integral imaging.     

Multidirectional curved integral imaging with large depth by additional use of a large-aperture lens

We propose a curved integral imaging system with large depth achieved by the additional use of a large-aperture lens in a conventional large-depth integral imaging system. The additional large-aperture lens provides a multidirectional curvature effect and improves the viewing angle. The proposed system has a simple structure due to the use of well-fabricated, unmodified flat devices. To calculate the proper elemental images for the proposed system, we explain a modified computer-generated pickup technique based on an ABCD matrix and analyze an effective viewing zone in the proposed system. From experiments, we show that the proposed system has an improved viewing angle of more than 7 degrees compared with conventional integral imaging.     

Adaptive framework for robust high-resolution image reconstruction in multiplexed computational imaging architectures

In multiplexed computational imaging schemes, high-resolution images are reconstructed by fusing the information in multiple low-resolution images detected by a two-dimensional array of low-resolution image sensors. The reconstruction procedure assumes a mathematical model for the imaging process that could have generated the low-resolution observations from an unknown high-resolution image. In practical settings, the parameters of the mathematical imaging model are known only approximately and are typically estimated before the reconstruction procedure takes place. Violations to the assumed model, such as inaccurate knowledge of the field of view of the imagers, erroneous estimation of the model parameters, and/or accidental scene or environmental changes can be detrimental to the reconstruction quality, even if they are small in number. We present an adaptive algorithm for robust reconstruction of high-resolution images in multiplexed computational imaging architectures. Using robust M-estimators and incorporating a similarity measure, the proposed scheme adopts an adaptive estimation strategy that effectively deals with violations to the assumed imaging model. Comparisons with nonadaptive reconstruction techniques demonstrate the superior performance of the proposed algorithm in terms of reconstruction quality and robustness.     

An efficient Peaceman–Rachford splitting method for constrained TGV-shearlet-based MRI reconstruction

As a fundamental application of compressive sensing, magnetic resonance imaging (MRI) can be efficiently achievable by exploiting fewer k-space measurements. In this paper, we propose a constrained total generalized variation and shearlet transform-based model for MRI reconstruction, which is usually more undemanding and practical to identify appropriate tradeoffs than its unconstrained counterpart. The proposed model can be facilely and efficiently solved by the strictly contractive Peaceman–Rachford splitting method, which generally outperforms some state-of-the-art algorithms when solving separable convex programming. Numerical simulations demonstrate that the sharp edges and grainy details in magnetic resonance images can be well reconstructed from the under-sampling data.     

High-resolution scanning phase retrieval with a probe beam of unknown modulus

A high-resolution phase-retrieval method for an imaging system with scanning illumination that is capable of reconstructing the modulus and phase of an object without a holographic reference wave is proposed. Reconstruction involves the synthesis of the reconstructed objects from the data of zero- and higher-frequency components of two Fourier intensity measurements: the Fourier intensity of the product of the object and a probe beam that is scanned across the object and the Fourier intensity of the product of the object and a probe beam that is modulated with an exponential filter. This method improves on the resolution of a reconstructed object by previous methods that make use of the data of only the zero-frequency components of the two Fourier intensities. In addition, phase retrieval in the scanning-illumination system with a probe of unknown modulus can be treated by use of the synthetic procedure, provided the phase of the probe function is a constant or a known distribution. Computer-simulated examples of the reconstruction of one- and two-dimensional complex objects demonstrate that reconstruction is robust.     

Image reconstruction algorithm based on the extended regularised total least squares method for electrical capacitance tomography

Electrical capacitance tomography (ECT) is a non-invasive imaging technology that aims at the visualisation of the cross-sectional permittivity distribution of a dielectric object based on the measured capacitance data. Successful applications of ECT depend greatly on the precision and speed of the image reconstruction algorithms. ECT image reconstruction is a typical ill-posed problem, and its solution is unstable, that is, the solution is sensitive to noises in the input data. Methods that ensure the stability of a solution while enhancing the quality of the reconstructed images should be used to obtain a meaningful reconstruction result. An image reconstruction algorithm based on the regularised total least squares (TLS) method that considers the errors in both the sensitivity field matrix and the capacitance data for ECT is presented. The regularised TLS method is extended using a combination robust estimation technique and an extended stabilising functional according to the ill-posed characteristics of ECT, which transforms the image reconstruction problem into an optimisation problem. In addition, the Newton algorithm is employed to solve the objective functional. Numerical simulations indicate that the algorithm is feasible and overcomes the numerical instability of ECT image reconstruction; for the cases of the reconstructed objects considered here, the spatial resolution of the reconstructed images obtained using the algorithm is enhanced; as a result, an efficient method for ECT image reconstruction is introduced.