Method For Computer-generated Holograms Research Articles

Improving image quality of a phase-added stereogram using deep learning

The disadvantages of computer-generated holograms (CGHs) using the direct integral method are the high computational requirements with increased object points and hologram size. This can be addressed by a phase-added stereogram (PAS), a fast calculation method for CGHs. PAS divides the hologram into small blocks and calculates the point-spread functions (PSFs) of the object points in the Fourier domain of each block. The PSF can be approximated using sparse spectra, which accelerate calculations. However, this approximation degrades the image quality. In this study, we improved the image quality of the PAS using deep learning while maintaining high computational speed.

Efficient Encoding Method for Computer-Generated Hologram

Efficient Encoding Method for Computer-Generated Hologram

Fast 3D Content Update for Wide-Angle Holographic Near-Eye Display

Near-eye holographic displays are the holy grail of wear-on 3D display devices because they are intended to project realistic wide-angle virtual scenes with parameters matching human vision. One of the key features of a realistic perspective is the ability to move freely around the virtual scene. This can be achieved by addressing the display with wide-angle computer-generated holograms (CGHs) that enable continuous viewpoint change. However, to the best of our knowledge there is no technique able to generate these types of content. Thus, in this work we propose an accurate and non-paraxial hologram update method for wide-angle CGHs that supports continuous viewpoint change around the scene. This method is based on the assumption that with a small change in perspective, two consecutive holograms share overlapping data. This enables reusing the corresponding part of the information from the previous view, eliminating the need to generate an entirely new hologram. Holographic information for the next viewpoint is calculated in two steps: first, a tool approximating the Angular Spectrum Propagation is proposed to generate the hologram data from previous viewpoint; and second, the efficient Phase Added Stereogram algorithm is utilized for generating the missing hologram content. This methodology offers fast and accurate calculations at the same time. Numerical and optical experiments are carried out to support the results of the proposed method.

High-efficiency fabrication of computer-generated holograms in silica glass using a femtosecond Bessel beam

We demonstrated a method for computer-generated holograms fabricated by using a femtosecond Bessel beam. A high-efficiency point-by-point process was used to fabricate a longitudinal modified array with a period of 10 μm and a modified area with diameter of approximately 1.5 μm on transparent glass. Tuning the modified length of each area by changing the relative positions of the Bessel beam and sample allowed the phase profile of the incident light to be flexibly adjusted to generate an arbitrary far-field pattern. The approach was verified by inscribing a designed phase hologram in borosilicate glass. Twin images were eliminated by high-level modulation.

Computer-Generated Holograms for 3D Imaging

Holography is usually considered as the ultimate way to visually reproduce a three-dimensional scene. Computer-generated holography constitutes an important branch of holography, which enables visualization of artificially generated scenes as well as real three-dimensional scenes recorded under white-light illumination. In this article, we present a comprehensive survey of methods for synthesis of computer-generated holograms, classifying them into two broad categories: wavefront-based methods and ray-based methods. We examine their modern implementations in terms of the quality of reconstruction and computational efficiency. As it is an integral part of computer-generated holography, we devote a special section to speckle suppression, which is also discussed under two categories following the classification of underlying computer-generated hologram methods.

Production and reconstruction of binary Fourier transform computer-generated holograms based on MATLAB

Based on MATLAB, the production and reconstruction of binary Fourier transform computer-generated holograms (CGHs) were realized using Roman III roundabout phase encoding method. The principle, method and step of CGHs were discussed. The digital reconstruction of holograms was completed using CGHs technology, and the simulation of whole holographic recording and reconstruction process was realized by a computer. Additionally, effects of the width of a sampling unit s and the threshold coefficient ef on the production and reconstruction of CGHs were studied, which can provide a theoretical reference for the improvement of the quality of the reconstruction image. Compared with experiments, the result of the CGHs technology is more intuitive and obvious. It is inferred that CGHs have the potential to be used to image encryptions and communications.

Simple and effective calculation method for computer-generated hologram based on non-uniform sampling using look-up-table.

Heavy computational complexity and imprecise reconstruction of objects are crucial problems in computer-generated holograms. In this paper, we propose a non-uniform sampling based on novel compressed look up table method to generate holograms. The method consists of two steps: in the first step, the non-uniform basic modulation factors are precalculated and stored in look-up-table. Secondly, fringe patterns for other points are obtained by simply shifting and multiplying the pre-calculated non-uniform basic modulation factors, and the final computer-generated hologram is obtained by adding them all together. The proposed method eliminates the redundant information properly and modulates the reconstructed images precisely. Numerical simulation results show proposed method reduces the memory usage, speeds up computation time and the quality of reconstructed images do not degrade evidently compared with uniform sampling method. Optical experiments results are in good agreement with numerical simulation results. The proposed method is simple, effective and could be applied in the holographic field in the future.

Fast calculation method for computer-generated holograms using saccade suppression by lowering the resolution based on Fresnel zone plate reduction

Fast calculation method for computer-generated holograms using saccade suppression by lowering the resolution based on Fresnel zone plate reduction

Calculation Reduction Method for Computer-Generated Hologram using Angular Redundancy and Color Space Conversion

Calculation Reduction Method for Computer-Generated Hologram using Angular Redundancy and Color Space Conversion

Scaling of Three-Dimensional Computer-Generated Holograms with Layer-Based Shifted Fresnel Diffraction

Holographic three-dimensional (3D) displays can reconstruct a whole wavefront of a 3D scene and provide rich depth information for the human eyes. Computer-generated holographic techniques offer an efficient way for reconstructing holograms without complicated interference recording systems. In this work, we present a technique for generating 3D computer-generated holograms (CGHs) with scalable samplings, by using layer-based diffraction calculations. The 3D scene is partitioned into multiple layers according to its depth image. Shifted Fresnel diffraction is used for calculating the wave diffractions from the partitioned layers to the CGH plane with adjustable sampling rates, while maintaining the depth information. The algorithm provides an effective method for scaling 3D CGHs without an optical zoom module in the holographic display system. Experiments have been performed, demonstrating that the proposed method can reconstruct quality 3D images at different scale factors.

Geometrically invariant watermarking for medical images based on digital holography and three-level DWT-SVD.

Digital imaging and communications in medicine are being applied widely to improve health care for people, and their security attracted our attention when they are transmitted through the Internet. The paper proposes a digital watermarking method for medical images based on digital holography and three-level discrete wavelength transform (DWT) singular value decomposition (SVD), where a live code including privacy information is transformed into digital holography by an improved computer-generated hologram method. The holography then is embedded into the singular values of the LL3 subband wavelet of the medical image. Experimental results show that the proposed method provides sufficient security results against various attacks, especially geometrical attacks.

Fast calculation method of computer-generated hologram using a depth camera with point cloud gridding

We propose a fast calculation method for a computer-generated hologram (CGH) of real objects that uses a point cloud gridding method. The depth information of the scene is acquired using a depth camera and the point cloud model is reconstructed virtually. Because each point of the point cloud is distributed precisely to the exact coordinates of each layer, each point of the point cloud can be classified into grids according to its depth. A diffraction calculation is performed on the grids using a fast Fourier transform (FFT) to obtain a CGH. The computational complexity is reduced dramatically in comparison with conventional methods. The feasibility of the proposed method was confirmed by numerical and optical experiments.

High-efficiency photorealistic computer-generated holograms based on the backward ray-tracing technique

Holographic displays can provide the complete optical wave field of a three-dimensional (3D) scene, including the depth perception. However, it often takes a long computation time to produce traditional computer-generated holograms (CGHs) without more complex and photorealistic rendering. The backward ray-tracing technique is able to render photorealistic high-quality images, which noticeably reduce the computation time achieved from the high-degree parallelism. Here, a high-efficiency photorealistic computer-generated hologram method is presented based on the ray-tracing technique. Rays are parallelly launched and traced under different illuminations and circumstances. Experimental results demonstrate the effectiveness of the proposed method. Compared with the traditional point cloud CGH, the computation time is decreased to 24 s to reconstruct a 3D object of 100×100 rays with continuous depth change.

Depth compensating calculation method of computer-generated holograms using symmetry and similarity of zone plates

Computer-generated hologram (CGH) is a promising 3D display technology while it is challenged by heavy computation load and vast memory requirement. To solve these problems, a depth compensating CGH calculation method based on symmetry and similarity of zone plates is proposed and implemented on graphics processing unit (GPU). An improved LUT method is put forward to compute the distances between object points and hologram pixels in the XY direction. The concept of depth compensating factor is defined and used for calculating the holograms of points with different depth positions instead of layer-based methods. The proposed method is suitable for arbitrary sampling objects with lower memory usage and higher computational efficiency compared to other CGH methods. The effectiveness of the proposed method is validated by numerical and optical experiments.

Method of computer-generated hologram compression and transmission using quantum back-propagation neural network

A method for computer-generated hologram (CGH) compression and transmission using a quantum back-propagation neural network (QBPNN) is proposed, with the Fresnel transform technique adopted for image reconstruction of the compressed and transmitted CGH. Experiments of simulation were conducted to compare the reconstructed images from CGHs processed using a QBPNN with those processed using a back-propagation neural network (BPNN) at the optimal learning coefficients. The experimental results show that the method using a QBPNN could produce reconstructed images with a better quality than those obtained using a BPNN despite the use of fewer learning iterations at the same compression ratio.

Encoding method of CGH for highly accurate optical measurement based on non-maxima suppression

The use of a computer-generated hologram (CGH) in interferometric testing provides new methods for highly accurate optical measurement. To fabricate a CGH, polygons are used to approximate the smooth CGH pattern. Because the data size supported by CGH writing machines is limited, the number of polygon vertices must be limited. Therefore, the CGH-encoding method determines the encoding accuracy. To realize a highly accurate optical measurement using CGHs, we propose a CGH-pattern-encoding method based on non-maxima suppression. A self-aligned CGH is designed to verify the accuracy. The experimental result shows that a highly accurate CGH can be fabricated using this method.

Improved look-up table method of computer-generated holograms.

Heavy computation load and vast memory requirements are major bottlenecks of computer-generated holograms (CGHs), which are promising and challenging in three-dimensional displays. To solve these problems, an improved look-up table (LUT) method suitable for arbitrarily sampled object points is proposed and implemented on a graphics processing unit (GPU) whose reconstructed object quality is consistent with that of the coherent ray-trace (CRT) method. The concept of distance factor is defined, and the distance factors are pre-computed off-line and stored in a look-up table. The results show that while reconstruction quality close to that of the CRT method is obtained, the on-line computation time is dramatically reduced compared with the LUT method on the GPU and the memory usage is lower than that of the novel-LUT considerably. Optical experiments are carried out to validate the effectiveness of the proposed method.

Generation of optical vortices with the same topological charges and controllable separation distances using diffraction gratings

In this paper, we first generate optical vortices with different topological charges, using the method of computer-generated holograms. Then, we separate one of the optical vortices from others with a special topological charge and pass it through a diffraction grating with a specified line spacing. It is observed that the vortex beam, after passing through the grating, converts to several separated vortices with the same topological charge whose value is similar to the topological charge of the input vortex. Finally, we show that the distance between generated vortices can be controlled with the variation of spacing between grating lines. So, the proposed setup in this paper can be exploited as an optical vortex divider which is useful in communication and trapping systems.

Spatial frequency sampling look-up table method for computer-generated hologram

A spatial frequency sampling look-up table method is proposed to generate a hologram. The three-dimensional (3-D) scene is sampled as several intensity images by computer rendering. Each object point on the rendered images has a defined spatial frequency. The basis terms for calculating fringe patterns are precomputed and stored in a table to improve the calculation speed. Both numerical simulations and optical experiments are performed. The results show that the proposed approach can easily realize color reconstructions of a 3-D scene with a low computation cost. The occlusion effects and depth information are all provided accurately.

Fabrication of computer-generated holograms using femtosecond laser direct writing.

We demonstrate a single-step fabrication method for computer-generated holograms based on femtosecond laser direct writing. Therefore, a tightly arranged longitudinal waveguide array is directly inscribed into a transparent material. By tailoring the individual waveguide length, the phase profile of an incident laser beam can be arbitrarily adapted. The approach is verified in common borosilicate glass by inscribing a designed phase hologram, which forms the desired intensity pattern in its far field. The resulting performance is analyzed, and the potential as well as limitations of the method are discussed.