Research

17 pages, 15243 KiB

Open AccessArticle

Reconstruction of Sparse-View X-ray Computed Tomography Based on Adaptive Total Variation Minimization

by Zhengshan Yu, Xingya Wen and Yan Yang

Micromachines 2023, 14(12), 2245; https://doi.org/10.3390/mi14122245 - 15 Dec 2023

Viewed by 758

Sparse-view reconstruction has garnered significant interest in X-ray computed tomography (CT) imaging owing to its ability to lower radiation doses and enhance detection efficiency. Among current methods for sparse-view CT reconstruction, an algorithm utilizing iterative reconstruction based on full variational regularization demonstrates good [...] Read more.

Sparse-view reconstruction has garnered significant interest in X-ray computed tomography (CT) imaging owing to its ability to lower radiation doses and enhance detection efficiency. Among current methods for sparse-view CT reconstruction, an algorithm utilizing iterative reconstruction based on full variational regularization demonstrates good performance. The optimized direction and number of computations for the gradient operator of the regularization term play a crucial role in determining not only the reconstructed image quality but also the convergence speed of the iteration process. The conventional TV approach solely accounts for the vertical and horizontal directions of the two-dimensional plane in the gradient direction. When projection data decrease, the edges of the reconstructed image become blurred. Exploring too many gradient directions for TV terms often comes at the expense of more computational costs. To enhance the balance of computational cost and reconstruction quality, this study suggests a novel TV computation model that is founded on a four-direction gradient operator. In addition, selecting appropriate iteration parameters significantly impacts the quality of the reconstructed image. We propose a nonparametric control method utilizing the improved TV approach as a solution to the tedious manual parameter optimization issue. The relaxation parameters of projection onto convex sets (POCS) are determined according to the scanning number and numerical proportion of the projection data; according to the image error before and after iterations, the gradient descent step of the TV item is adaptively adjusted. Compared with several representative iterative reconstruction algorithms, the experimental results show that the algorithm can effectively preserve edges and suppress noise in sparse-view CT reconstruction. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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11 pages, 3901 KiB

Open AccessArticle

Fast Hologram Calculation Method Based on Wavefront Precise Diffraction

by Zimu Wang, Yilong Li, Zhenyan Tang, Zhaosong Li and Di Wang

Micromachines 2023, 14(9), 1690; https://doi.org/10.3390/mi14091690 - 29 Aug 2023

Viewed by 933

Abstract

In this paper, a fast hologram calculation method based on wavefront precise diffraction is proposed. By analyzing the diffraction characteristics of the object point on the 3D object, the effective viewing area of the reproduced image is analyzed. Based on the effective viewing [...] Read more.

In this paper, a fast hologram calculation method based on wavefront precise diffraction is proposed. By analyzing the diffraction characteristics of the object point on the 3D object, the effective viewing area of the reproduced image is analyzed. Based on the effective viewing area, the effective hologram size of the object point is obtained, and then the accurate diffraction calculation from the object point to the wavefront recording plane (WRP) is performed. By calculating all the object points on the recorded object, the optimized WRP of the whole 3D object can be obtained. The final hologram is obtained by calculating the diffraction light field from the WRP to the holographic plane. Compared with the traditional method, the proposed method can improve the calculation speed by more than 55%, while the image quality of the holographic 3D display is not affected. The proposed calculation method provides an idea for fast calculation of holograms and is expected to contribute to the development of dynamic holographic displays. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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16 pages, 23299 KiB

Open AccessArticle

Directional and Eye-Tracking Light Field Display with Efficient Rendering and Illumination

by Guangyong Zhang, Yong He, Haowen Liang, Xuehao Chen, Dongyan Deng and Jianying Zhou

Micromachines 2023, 14(7), 1465; https://doi.org/10.3390/mi14071465 - 21 Jul 2023

Cited by 2 | Viewed by 1209

Abstract

Current efforts with light field displays are mainly concentrated on the widest possible viewing angle, while a single viewer only needs to view the display in a specific viewing direction. To make the light field display a practical practice, a super multi-view light [...] Read more.

Current efforts with light field displays are mainly concentrated on the widest possible viewing angle, while a single viewer only needs to view the display in a specific viewing direction. To make the light field display a practical practice, a super multi-view light field display is proposed to compress the information in the viewing zone of a single user by reducing the redundant viewpoints. A quasi-directional backlight is proposed, and a lenticular lens array is applied to achieve the restricted viewing zone. The eye-tracking technique is applied to extend the viewing area. Experimental results show that the proposed scheme can present a vivid 3D scene with smooth motion parallax. Only 16.7% conventional light field display data are required to achieve 3D display. Furthermore, an illumination power of 3.5 watt is sufficient to lighten a 31.5-inch light field display, which takes up 1.5% of the illumination power required for planar display of similar configuration. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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13 pages, 3959 KiB

Open AccessArticle

Saturation-Induced Phase Error Compensation Method Using Complementary Phase

by Yingying Wan, Yiping Cao, Min Xu and Tao Tang

Micromachines 2023, 14(6), 1258; https://doi.org/10.3390/mi14061258 - 16 Jun 2023

Cited by 2 | Viewed by 1031

Abstract

Intensity saturation can induce phase error and, thus, measurement error in fringe projection profilometry. To reduce saturation-induced phase errors, a compensation method is developed. The mathematical model of saturation-induced phase errors is analyzed for N-step phase-shifting profilometry, and the phase error is [...] Read more.

Intensity saturation can induce phase error and, thus, measurement error in fringe projection profilometry. To reduce saturation-induced phase errors, a compensation method is developed. The mathematical model of saturation-induced phase errors is analyzed for N-step phase-shifting profilometry, and the phase error is approximately N-folder of the frequency of the projected fringe. Additional N-step phase-shifting fringe patterns with initial phase-shift π/N are projected for generating a complementary phase map. The final phase map is obtained by averaging the original phase map extracted from the original fringe patterns and the complementary phase map, and then the phase error can be canceled out. Both simulations and experiments demonstrated that the proposed method can substantially reduce the saturation-induced phase error and realize accurate measurements for a highly dynamic range of scenarios. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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10 pages, 3085 KiB

Open AccessArticle

Depth-Enhanced Holographic Super Multi-View Maxwellian Display Based on Variable Filter Aperture

by Kefeng Tu, Qiyang Chen, Zi Wang, Guoqiang Lv and Qibin Feng

Micromachines 2023, 14(6), 1167; https://doi.org/10.3390/mi14061167 - 31 May 2023

Cited by 2 | Viewed by 1214

Abstract

The super multi-view (SMV) near-eye display (NED) effectively provides depth cues for three-dimensional (3D) displays by projecting multiple viewpoint images or parallax images onto the retina simultaneously. Previous SMV NED suffers from a limited depth of field (DOF) due to the fixed image [...] Read more.

The super multi-view (SMV) near-eye display (NED) effectively provides depth cues for three-dimensional (3D) displays by projecting multiple viewpoint images or parallax images onto the retina simultaneously. Previous SMV NED suffers from a limited depth of field (DOF) due to the fixed image plane. Aperture filtering is widely used to enhance the DOF; however, an invariably sized aperture may have opposite effects on objects with different reconstruction depths. In this paper, a holographic SMV display based on the variable filter aperture is proposed to enhance the DOF. In parallax image acquisition, multiple groups of parallax images, each group recording a part of the 3D scene on a fixed depth range, are captured first. In the hologram calculation, each group of wavefronts at the image recording plane (IRP) is calculated by multiplying the parallax images with the corresponding spherical wave phase. Then, they are propagated to the pupil plane and multiplied by the corresponding aperture filter function. The size of the filter aperture is variable which is determined by the depth of the object. Finally, the complex amplitudes at the pupil plane are back-propagated to the holographic plane and added together to form the DOF-enhanced hologram. Simulation and experimental results verify the proposed method could improve the DOF of holographic SMV display, which will contribute to the application of 3D NED. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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12 pages, 5697 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Multi-Depth Computer-Generated Hologram Based on Stochastic Gradient Descent Algorithm with Weighted Complex Loss Function and Masked Diffraction

by Jiale Quan, Binbin Yan, Xinzhu Sang, Chongli Zhong, Hui Li, Xiujuan Qin, Rui Xiao, Zhi Sun, Yu Dong and Huming Zhang

Micromachines 2023, 14(3), 605; https://doi.org/10.3390/mi14030605 - 06 Mar 2023

Viewed by 1596

Abstract

In this paper, we propose a method to generate multi-depth phase-only holograms using stochastic gradient descent (SGD) algorithm with weighted complex loss function and masked multi-layer diffraction. The 3D scene can be represented by a combination of layers in different depths. In the [...] Read more.

In this paper, we propose a method to generate multi-depth phase-only holograms using stochastic gradient descent (SGD) algorithm with weighted complex loss function and masked multi-layer diffraction. The 3D scene can be represented by a combination of layers in different depths. In the wave propagation procedure of multiple layers in different depths, the complex amplitude of layers in different depths will gradually diffuse and produce occlusion at another layer. To solve this occlusion problem, a mask is used in the process of layers diffracting. Whether it is forward wave propagation or backward wave propagation of layers, the mask can reduce the occlusion problem between different layers. Otherwise, weighted complex loss function is implemented in the gradient descent optimization process, which analyzes the real part, the imaginary part, and the amplitude part of the focus region between the reconstructed images of the hologram and the target images. The weight parameter is used to adjust the ratio of the amplitude loss of the focus region in the whole loss function. The weight amplitude loss part in weighted complex loss function can decrease the interference of the focus region from the defocus region. The simulations and experiments have validated the effectiveness of the proposed method. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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15 pages, 3234 KiB

Open AccessArticle

Single-Shot Multi-Frequency 3D Shape Measurement for Discontinuous Surface Object Based on Deep Learning

by Min Xu, Yu Zhang, Yingying Wan, Lin Luo and Jianping Peng

Micromachines 2023, 14(2), 328; https://doi.org/10.3390/mi14020328 - 27 Jan 2023

Cited by 3 | Viewed by 1391

Abstract

It is challenging to stably and rapidly achieve accurate absolute phase retrieval for isolated objects with a single-shot pattern in fringe projection profilometry (FPP). In this context, a single-shot multi-frequency absolute phase retrieval (SAPR-DL) method based on deep learning is proposed, which only [...] Read more.

It is challenging to stably and rapidly achieve accurate absolute phase retrieval for isolated objects with a single-shot pattern in fringe projection profilometry (FPP). In this context, a single-shot multi-frequency absolute phase retrieval (SAPR-DL) method based on deep learning is proposed, which only needs to capture one fringe image to obtain the full-field precise absolute phase. Specifically, a low-frequency deformed fringe image is loaded into the trained one-to-two deep learning framework (DLFT) to predict unit-frequency and high-frequency deformed fringe images. Then, three fringe images with different frequencies are loaded into the trained deep learning phase retrieval framework (DLPR) to calculate the corresponding absolute phase. The experimental results prove that the proposed SAPR-DL method can obtain the three-dimensional (3D) shape measurement of multiple complex objects by collecting a single-shot fringe image, showing great prospects in advancing scientific and engineering applications. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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10 pages, 4420 KiB

Open AccessArticle

Wide-Viewing-Angle Integral Imaging System with Full-Effective-Pixels Elemental Image Array

by Zesheng Liu, Dahai Li and Huan Deng

Micromachines 2023, 14(1), 225; https://doi.org/10.3390/mi14010225 - 15 Jan 2023

Cited by 2 | Viewed by 1347

Abstract

There exists a defect of the narrow viewing angle in the conventional integral imaging system. One reason for this is that only partial pixels of each elemental image contribute to the viewing angle and the others cause image flips. In this paper, a [...] Read more.

There exists a defect of the narrow viewing angle in the conventional integral imaging system. One reason for this is that only partial pixels of each elemental image contribute to the viewing angle and the others cause image flips. In this paper, a wide-viewing-angle integral imaging system with a full-effective-pixels elemental image array (FEP-EIA) was proposed. The correspondence between viewpoints and pixel coordinates within the elemental image array was built up, and effective pixel blocks and pixels leading to flipping images were deduced. Then, a pixel replacement method was proposed to generate the FEP-EIAs, which adapt to different viewing distances. As a result, the viewing angle of the proposed integral imaging system was effectively extended through the replacement of the pixels, which caused the image flips. Experiment results demonstrated that wide viewing angles are available for the proposed integral imaging system regardless of the viewing distances. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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11 pages, 3596 KiB

Open AccessArticle

Tabletop 360-Degree Three-Dimensional Light-Field Display Based on Viewpoint-Fitting Encoding Algorithm for Reducing Facet Braiding

by Peiren Wang, Jinqiang Bi, Zilong Li, Binbin Yan, Zhengyang Li, Xiaozheng Wang and Li Liu

Micromachines 2023, 14(1), 178; https://doi.org/10.3390/mi14010178 - 10 Jan 2023

Viewed by 1310

Abstract

Since the effect of the facet braiding phenomenon, the display quality of reconstructed image degrades with increasing depth of field in tabletop three-dimensional light-field display. Here, to analysis the facet braiding, the imaging process of the tabletop 360-degree three-dimensional light-field display based on [...] Read more.

Since the effect of the facet braiding phenomenon, the display quality of reconstructed image degrades with increasing depth of field in tabletop three-dimensional light-field display. Here, to analysis the facet braiding, the imaging process of the tabletop 360-degree three-dimensional light-field display based on conical lens array is mathematically modeled. A viewpoint-fitting encoding algorithm is proposed to reduce the effect of the facet-braiding phenomenon and improve the range of depth of field, which is optimized to form the best synthetic encoded image by fitting the reconstructed image seen by the simulated human eye to the parallax image captured at the corresponding location. The effectiveness of the proposed optimization algorithm is verified by simulation analysis and optical experiments, respectively. In the experiment, the clear depth of field range of the display system is increased from 13 cm to 15 cm, and the visualization effect of the reconstructed three-dimensional image is enhanced. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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13 pages, 7427 KiB

Open AccessArticle

High Resolution Multiview Holographic Display Based on the Holographic Optical Element

by Xiujuan Qin, Xinzhu Sang, Hui Li, Rui Xiao, Chongli Zhong, Binbin Yan, Zhi Sun and Yu Dong

Micromachines 2023, 14(1), 147; https://doi.org/10.3390/mi14010147 - 06 Jan 2023

Cited by 1 | Viewed by 2165

Abstract

Limited by the low space-bandwidth product of the spatial light modulator (SLM), it is difficult to realize multiview holographic three-dimensional (3D) display. To conquer the problem, a method based on the holographic optical element (HOE), which is regarded as a controlled light element, [...] Read more.

Limited by the low space-bandwidth product of the spatial light modulator (SLM), it is difficult to realize multiview holographic three-dimensional (3D) display. To conquer the problem, a method based on the holographic optical element (HOE), which is regarded as a controlled light element, is proposed in the study. The SLM is employed to upload the synthetic phase-only hologram generated by the angular spectrum diffraction theory. Digital grating is introduced in the generation process of the hologram to achieve the splicing of the reconstructions and adjust the position of the reconstructions. The HOE fabricated by the computer-generated hologram printing can redirect the reconstructed images of multiview into multiple viewing zones. Thus, the modulation function of the HOE should be well-designed to avoid crosstalk between perspectives. The experimental results show that the proposed system can achieve multiview holographic augmented reality (AR) 3D display without crosstalk. The resolution of each perspective is 4K, which is higher than that of the existing multiview 3D display system. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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8 pages, 2670 KiB

Open AccessArticle

High-Aperture-Ratio Dual-View Integral Imaging Display

by Bai-Chuan Zhao, Fan Yang and Fei Wu

Micromachines 2022, 13(12), 2213; https://doi.org/10.3390/mi13122213 - 14 Dec 2022

Viewed by 994

Abstract

Low aperture ratio is a problem in the conventional dual-view integral imaging (DVII) display using a point light source array. A high-aperture-ratio DVII display using a gradient width point light source array is reported in this work. The elemental Images 1 and 2, [...] Read more.

Low aperture ratio is a problem in the conventional dual-view integral imaging (DVII) display using a point light source array. A high-aperture-ratio DVII display using a gradient width point light source array is reported in this work. The elemental Images 1 and 2, which are alternatively aligned on a liquid crystal panel, are illuminated by the light rays emitted from an assigned point light source. The optical path is optimized by optimizing the widths of the point light sources. The aperture ratio of the proposed DVII display was demonstrated as 1.88 times the conventional DVII display. Experiments showed that the vertical viewing range is related to the vertical width of the first row point light source, whereas the aperture ratio is related to the vertical widths of all point light sources. By optimizing the widths of the point light sources, the aperture ratio is enhanced without loss of viewing range. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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9 pages, 20432 KiB

Open AccessArticle

Crosstalk Suppressed 3D Light Field Display Based on an Optimized Holographic Function Screen

by Hui Zhang, Xunbo Yu, Xin Gao, Chongli Zhong, Yingying Chen, Xinzhu Sang and Kuiru Wang

Micromachines 2022, 13(12), 2106; https://doi.org/10.3390/mi13122106 - 29 Nov 2022

Cited by 1 | Viewed by 1356

Abstract

A holographic function screen (HFS) can recompose the wavefront and re-modulate the light-field distribution from a three-dimensional (3D) light field display (LFD) system. However, the spread function of existing HFSs does not particularly suit integral imaging (II) 3D LFD systems, which causes crosstalk [...] Read more.

A holographic function screen (HFS) can recompose the wavefront and re-modulate the light-field distribution from a three-dimensional (3D) light field display (LFD) system. However, the spread function of existing HFSs does not particularly suit integral imaging (II) 3D LFD systems, which causes crosstalk and reduces the sharpness of reconstructed 3D images. An optimized holographic function screen with a flat-top rectangular spread function (FRSF) was designed for an II 3D LFD system. A simulation was carried out through ray tracing, which verified that the proposed diffusion function could suppress crosstalk and improve the overall effect. Full article

(This article belongs to the Special Issue Three-Dimensional Display Technologies)

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