Computer-generated Holograms Research Articles

High-brightness holographic projection.

We propose a holographic projection system that achieves high image quality, brightness, and light efficiency. Using a novel, to the best of our knowledge, light-efficiency loss function, we are able to concentrate more light on the projection region and improve display brightness compared with conventional projectors. Leveraging emerging artificial intelligence-driven computer-generated holography and camera-in-the-loop calibration techniques, we learn a holographic wave propagation model using experimentally captured holographic images and demonstrate state-of-the-art light reallocation performance with high image quality.

Holographic display using layered computer-generated volume hologram.

The spatial frequency of the reconstructed image of planar computer-generated hologram(CGH) is limited by the sampling interval and the lack of thickness. To break through this limitation of planar CGH, we propose a new computer-generated volume hologram(CGVH) for full-color dynamic holographic three-dimensional(3D) display, and an iteration-free layered CGVH generation method. The proposed CGVH is equivalent to a volume hologram sampled discretely in three directions. The generation method employs the layered angular spectral diffraction to calculate the light field in the layered CGVH, and then encodes it into a CGVH. Numerical simulation results show that the CGVH can accurately reconstruct full-color 3D objects, where better imaging quality, more concentrated diffraction energy, denser reconstructed spatial frequency information, and larger viewing angle are achieved. The proposed CGVH is expected to be applied to realize dynamic modulation, wavelength multiplexing, and angle multiplexing in various optical fields in the future.

Efficient rendering by parallelogram-approximation for full analytical polygon-based computer-generated holography using planar texture mapping.

We have developed a full analytical method with texture mapping for polygon-based computer-generated holography. A parallel planar projection mapping for holographic rendering along with affine transformation and self-similar segmentation is derived. Based on this method, we further propose a parallelogram-approximation to reduce the number of polygons used in the polygon-based technique. We demonstrate that the overall method can reduce the computational effort by 50% as compared to an existing method without sacrificing the reconstruction quality based on high precision rendering of complex textures. Numerical and optical reconstructions have shown the effectiveness of the overall scheme.

DGE-CNN: 2D-to-3D holographic display based on a depth gradient extracting module and ZCNN network.

Holography is a crucial technique for the ultimate three-dimensional (3D) display, because it renders all optical cues from the human visual system. However, the shortage of 3D contents strictly restricts the extensive application of holographic 3D displays. In this paper, a 2D-to-3D-display system by deep learning-based monocular depth estimation is proposed. By feeding a single RGB image of a 3D scene into our designed DGE-CNN network, a corresponding display-oriented 3D depth map can be accurately generated for layer-based computer-generated holography. With simple parameter adjustment, our system can adapt the distance range of holographic display according to specific requirements. The high-quality and flexible holographic 3D display can be achieved based on a single RGB image without 3D rendering devices, permitting potential human-display interactive applications such as remote education, navigation, and medical treatment.

Polarimetric calibrated robust dual-SLM complex-amplitude computer-generated holography.

Liquid crystal on silicon (LCoS) is a widely used spatial light modulator (SLM) in computer-generated holography (CGH). However, the phase-modulating profile of LCoS is often not ideally uniform in application, bringing about undesired intensity fringes. In this study, we overcome this problem by proposing a highly robust dual-SLM complex-amplitude CGH technique, which incorporates a polarimetric mode and a diffractive mode. The polarimetric mode linearizes the general phase modulations of the two SLMs separately, while the diffractive mode uses camera-in-the-loop optimization to achieve improved holographic display. Experimental results show the effectiveness of our proposal in improving reconstructing accuracy by 21.12% in peak signal-to-noise ratio (PSNR) and 50.74% in structure similarity index measure (SSIM), using LCoS SLMs with originally non-uniform phase-modulating profiles.

Color multilayer holographic near-eye augmented reality display

This study demonstrates a full-color near-eye holographic display capable of superimposing color virtual scenes with 2D, 3D, and multiple objects with extended depth upon a real scene, which also has the ability to present different 3D information depending on the focus of the user’s eyes using a single computer-generated hologram per color channel. Our setup makes use of a hologram generation method based on two-step propagation and the singular value decomposition of the Fresnel transform impulse response function to efficiently generate the holograms of the target scene. Then, we test our proposal by implementing a holographic display that makes use of a phase-only spatial light modulator and time-division multiplexing for color reproduction. We demonstrate the superior quality and computation speed of this approach compared with other hologram generation techniques with both numerical and experimental results.

Polarization-controlled nonlinear computer-generated holography

Dynamic phase-only beam shaping with a liquid crystal spatial light modulator is a powerful technique for tailoring the intensity profile or wave front of a beam. While shaping and controlling the light field is a highly researched topic, dynamic nonlinear beam shaping has hardly been explored so far. One potential reason is that generating the second harmonic is a degenerate process as it mixes two fields at the same frequency. To overcome this problem, we propose the use of type II phase matching as a control mechanism to distinguish between the two fields. Our experiments demonstrate that distributions of arbitrary intensity can be shaped in the frequency-converted field at the same quality as for linear beam shaping and with conversion efficiencies similar to without beam shaping. We envision this method as a milestone toward beam shaping beyond the physical limits of liquid crystal displays by facilitating dynamic phase-only beam shaping in the ultraviolet spectral range.

Point-polygon hybrid method for generating holograms.

Computer-generated holograms (CGHs) are usually calculated from point clouds or polygon meshes. Point-based holograms are good at depicting details of objects, such as continuous depth cues, while polygon-based holograms tend to efficiently render high-density surfaces with accurate occlusions. Herein, we propose a novel point-polygon hybrid method (PPHM) to compute CGHs for the first time (to the best of our knowledge), which takes advantage of both point-based and polygon-based methods, and thus performs better than each of them separately. Reconstructions of 3D object holograms confirm that the proposed PPHM can present continuous depth cues with fewer triangles, implying high computational efficiency without losing quality.

H-Seg: a horizontal reconstruction volume segmentation method for accurate depth estimation in a computer-generated hologram.

In this work, we introduce a novel approach for depth estimation in a computer-generated hologram by employing horizontal segmentation of the reconstruction volume instead of conventional vertical segmentation. The reconstruction volume is divided into horizontal slices and each slice is processed using a residual U-net architecture to identify in-focus lines, enabling determination of the slice's intersection with the 3D scene. The individual slice results are then combined to generate a dense depth map of the scene. Our experiments demonstrate the effectiveness of our method, with improved accuracy, faster processing times, lower graphics processing unit (GPU) utilization, and smoother predicted depth maps than existing state-of-the-art models.

76‐4: Analytical Computer‐Generated Holography for Quadrilateral Meshes

Analytical polygon‐based computer‐generated holography (CGH) is promising because of its fast rendering speed. However, quadrilateral‐based full‐analytical CGH is infeasible. In this paper, we proposed a novel analytical CGH rendering method for quadrilateral meshes, which is compatible with modern modeling software and can simultaneously reduce the rendering load.

Gaze-contingent efficient hologram compression for foveated near-eye holographic displays

Deep learning-based computer-generated holography (CGH) has developed rapidly and yielded remarkable outcomes. However, CGH rendering technologies are confronted with a significant challenge in transmitting massive holograms, which hinders the development of lightweight wearable near-eye holographic displays. Currently, hologram compression frameworks share a resemblance with image compression methods, which fail take into account the human visual system in practical near-eye displays, limiting the improvement of compression efficiency. Herein, we presented an efficient holographic compression framework based on foveated rendering, where we transmitted a high-resolution foveal region at a low compression rate and a low-resolution peripheral region at a high compression rate with dramatically reduced pixel numbers. Our method achieved a compression rate of 40× for a hologram resolution of 1024 × 1024, which represents a twofold increase in compression rate compared to the state-of-the-art (SOTA) method with a PSNR of ∼28.8 dB in the foveal image. Moreover, we further demonstrated the effectiveness of the proposed method in the optical experiment. We believe the proposed approach could be a viable remedy for the evergrowing data issue in near-eye holographic displays.

Diffraction model-driven neural network trained using hybrid domain loss for real-time and high-quality computer-generated holography.

Learning-based computer-generated holography (CGH) has demonstrated great potential in enabling real-time, high-quality holographic displays. However, most existing learning-based algorithms still struggle to produce high-quality holograms, due to the difficulty of convolutional neural networks (CNNs) in learning cross-domain tasks. Here, we present a diffraction model-driven neural network (Res-Holo) using hybrid domain loss for phase-only hologram (POH) generation. Res-Holo utilizes the weights of the pretrained ResNet34 as the initialization during the encoder stage of the initial phase prediction network to extract more generic features and also to help prevent overfitting. Also, frequency domain loss is added to further constrain the information that the spatial domain loss is insensitive. The peak signal-to-noise ratio (PSNR) of the reconstructed image is improved by 6.05 dB using hybrid domain loss compared to using spatial domain loss alone. Simulation results show that the proposed Res-Holo can generate high-fidelity 2 K resolution POHs with an average PSNR of 32.88 dB at 0.014 seconds/frame on the DIV2K validation set. Both monochrome and full-color optical experiments show that the proposed method can effectively improve the quality of reproduced images and suppress image artifacts.

Analysis of reconstruction quality for computer-generated holograms using a model free of circular-convolution error.

Continuous complex-amplitude computer-generated holograms (CGHs) are converted to discrete amplitude-only or phase-only ones in practical applications to cater for the characteristics of spatial light modulators (SLMs). To describe the influence of the discretization correctly, a refined model that eliminates the circular-convolution error is proposed to emulate the propagation of the wavefront during the formation and reconstruction of a CGH. The effects of several significant factors, including quantized amplitude and phase, zero-padding rate, random phase, resolution, reconstruction distance, wavelength, pixel pitch, phase modulation deviation and pixel-to-pixel interaction, are discussed. Based on evaluations, the optimal quantization for both available and future SLM devices is suggested.

Frugal techniques for optimization of CGH generation code

The numerically calculated and digitally stored full complex wavefront emerging from an object or scene is called a computer generated hologram (CGH). The CGH generation involves intensive calculations resulting in a huge amount of computation time. This makes it hard to display the object’s holographic information in real time. Therefore, the optimization of a CGH generation code is required to reduce the involved computational cost. The present work is an effort to optimize the CGH generation code using basic frugal techniques. Different techniques of optimization viz. analytical solution of mathematical equations, reduction in redundancy, utilization of all available resources etc. are implemented. Results infer that our optimization techniques have the potential to reduce the execution times significantly. The optimization of analytical solution reduced the execution time by more than 55% in the case of phase calculations and by around 98% in the case of calculations of the Fresnel zone plate function. Similarly, the graphics processing unit cores completed the calculations in around 2% of the time that is required by central processing unit cores for the same calculation.

Diffraction-based dual path multispectral imaging

Abstract Diffraction based multispectral imaging is a method for obtaining snapshot multispectral image information by using a structured diffraction grating or computer-generated hologram in an intermediate image plane of an imaging system. Without the need of complicated filter manufacturing it becomes possible to realize application specific spatio-spectral sampling of an image. The spectral resolution is coupled with the spatial resolution because the central stop of the imaging system serves also as the main spectral filtering component. By using two image sensors and the zeroth order of the computer-generated holograms it becomes possible to obtain high resolution spatial information as well as spectral information on a coarser sampling grid.

Fast transmission of computer-generated hologram with compressed sensing and quantum-inspired neural network

A two-stage compression framework for computer-generated hologram (CGH) with compressed sensing (CS) and quantum-inspired neural network (QINN) is proposed. A deep learning-based CS model is applied to sample the CGH at sub-Nyquist rates to generate measurements, and its more compact and representative information using interblock information is further extracted by QINN. With two-stage compression framework, the CGH is greatly compressed and suitable for storage and transmission. Experimental results demonstrate that our proposed two-stage compression framework can preserve more important holographic information during the compression, thus significantly improving the overall quality of reconstructed images compared to only using CS or QINN.

Applying multi-focus filtering plane in double-step Fresnel diffraction method for enhanced holographic projection region

Computer-generated holography has been anticipated in augmented reality (AR) since it can fully provide multi-depth 3D information to users, but speckle noises are often accompanied as one of the most significant degradation factors. Since complex amplitude encoding can suppress speckle noises, applying double-phase encoding to a lensless holographic projection system using a double-step Fresnel diffraction (DSF) algorithm has been reported to reduce those noises. However, a single filtering focus at a virtual focal plane might significantly reduce the projection region compared to the maximum diffraction bandwidth of the spatial light modulator. In this paper, we applied multiple foci for the focal plane of the DSF method with off-axis phase encoding, which can expand the projection region and is applicable to display multi-depth images with DSF. Numerical analysis conducted for image quality compared with conventional single focus DSF algorithm shows that the speckle performance does not degrade so much as the number of foci increase. We expect the proposed method to be useful for holographic projection optical systems such as AR head-up display.

Fast shadow casting algorithm in analytical polygon-based computer-generated holography.

Shadow casting is essential in computer graphics, which can significantly enhance the reality of rendered images. However, shadow casting is rarely studied in polygon-based computer-generated holography (CGH) because state-of-art triangle-based occlusion handling methods are too complicated for shadow casting and unfeasible for complex mutual occlusion handling. We proposed a novel drawing method based on the analytical polygon-based CGH framework and achieved Z-buffer-based occlusion handling instead of the traditional Painter's algorithm. We also achieved shadow casting for parallel and point light sources. Our framework can be generalized to N-edge polygon (N-gon) rendering and accelerated using CUDA hardware, by which the rendering speed can be significantly enhanced.

Double-sided computer-generated holography.

We present a method for computer-generated holography (CGH) in which different images are reproduced on both sides of a hologram with a single illumination source. In the proposed method, we use a transmissive spatial light modulator (SLM) and a half mirror (HM) located downstream of the SLM. The light modulated by the SLM is partially reflected by the HM, and the reflected light is modulated again by the SLM for the double-sided image reproduction. We derive an algorithm for double-sided CGH and experimentally demonstrate it.

Ultrahigh-density 3D holographic projection by scattering-assisted dynamic holography

Computer-generated holography offers a promising route to three-dimensional (3D) video displays. To realize a realistic-looking 3D display, the critical challenge is to create a 3D hologram that enables high-density multi-plane projection with full depth control. However, two long-existing issues in current digital holographic techniques, low axial resolution and high inter-plane crosstalk, prevent fine depth control and therefore limit the ultimate quality. Here, we report 3D scattering-assisted dynamic holography (3D-SDH) that further breaks the depth-control limit of the state-of-the-art method. Our approach achieves orders of magnitude improvement in axial resolution and greatly suppresses crosstalk, enabling ultrahigh-density 3D holographic projection. Moreover, 3D-SDH enables dynamic 3D vectorial projections via phase-only holograms. The concept is validated through both simulations and experiments, where dynamic projections of 3D point-cloud objects onto high-density successive planes are demonstrated. Our work opens perspectives for 3D holographic technology with ultra-fine depth control, dynamic projection, and polarization multiplexing functionalities.