Synthesis of computer-generated spherical hologram of real object with 360° field of view using a depth camera

Reconstructed quality improvement with a stochastic gradient descent optimization algorithm for a spherical hologram

The spherical holography is a promising technology to realize a true three-dimensional (3D) display. Compared to plane holography and cylindrical holography, it has an unlimited field of view, which can be observed from all perspectives. However, so far, the reconstructed images from computer-generated spherical holograms (CGSHs) are not of high quality, especially phase-only holograms, which will seriously affect its application. In this paper, an optimization algorithm for a CGSH based on stochastic gradient descent (SGD) is proposed to improve the quality of the reconstructed image. First, a new, to the best of our knowledge, diffraction model used in the process of optimization is proposed by considering the obliquity factor and occlusion culling. Based on our proposed diffraction model, the optimization process includes diffracting the initial random phase to another sphere, calculating the loss between the reconstructed image and the original image, and optimizing the initial phase through the SGD optimization algorithm. Both the correctness of the proposed diffraction model and the effectiveness of the SGD optimization for spherical holograms are verified well by numerical simulations. Through SGD, a high-quality reconstructed image can be achieved, which is 18 dB higher in the PSNR than that of spherical self-diffraction iteration. Meaningfully, our method has broad application prospects in 3D and omnidirectional displays. The SGD optimization algorithm is brought into the CGSH, and remarkable results have been achieved.

Conical holographic display to expand the vertical field of view

Recently, cylindrical holographic display technology as a 360-degree display technology has attracted much attention. However, all the studies are based on the field of view (FOV) in the azimuth direction, and the issue of FOV in the vertical direction has never been discussed. In this paper, a new holographic display is proposed to expand the vertical FOV by a conical holographic diffraction model, in which the object plane is the outer cylinder and the observation plane is a part of the cone. In this proposed method, the proposed diffraction model is firstly established by the Rayleigh-Sommerfeld diffraction formula, and then the convolution and FFT are used for a fast diffraction calculation. The correctness and effectiveness of our proposed method are verified by the simulation of Young's interference and the numerical reconstructions from the complex amplitude and encoded holograms, respectively. In addition, an accurate relationship between the conical inclination angle and the vertical FOV expansion is analyzed, and the simulation results show that our proposed method can significantly expand the vertical FOV by 0.4 of the original object. Therefore, the issue of the vertical FOV in cylindrical holography is deeply discussed and successfully addressed for the first time.

Occlusion culling for computer-generated cylindrical holograms based on a horizontal optical-path-limit function

Recently, great progress has been made in the research of cylindrical holography as a promising technique of 360° display. However, there is an unsolved issue of occlusion culling, which is critical to cylindrical holography and degrades the reconstructed images due to overlapping. To our knowledge, the occlusion issue in cylindrical holography has never been deeply discussed. In this paper, a method of occlusion culling is proposed for computer-generated cylindrical holograms based on a horizontal optical-path-limit function. In cylindrical diffraction, the propagation characteristics of light waves can be described by the point spread function, which is mainly obtained by analyzing the meaning of the obliquity factor in the concentric cylinder model. Different from the planar diffraction, the diffraction area of each source point is limited within the tangents in cylindrical diffraction. Therefore, a horizontal optical path limit function that acts directly on the point spread function for occlusion culling is established. Besides, the proposed method can be applied to the three-dimensional object by using the layer-oriented method. Moreover, the effectiveness of the proposed occlusion culling method is verified by the numerical simulation results and error analysis of the reconstructed images.

Performance enhancement of integral imaging based Fresnel hologram capturing by the intermediate view reconstruction

A method aiming at improving the performance of integral imaging (II) based Fresnel hologram is proposed, which is generated by using the intermediate view reconstruction (IVR). The conventional integral holograms are generally generated through Fourier transforming the elemental images (EI) of II into hogels. However, a trade-off between the angular resolution and the spatial resolution of II is inevitable within the generation of integral hologram. The IVR is introduced to enhance the angular spectrum of II-based Fresnel hologram while keeping a compact image size and being free from moving the lenslet array. Multiple elemental image array (EIA) sequences are generated with the IVR and transformed to the corresponding holograms. All the generated hologram sequences shift depending on the relative position of the virtual lens array and are added together to synthesize the Fresnel hologram with a high angular spectrum. The synthesized hologram can reconstruct the 3D image with the combined light fields of all the integral hologram sequences. Finally, both the simulation with multiple objects and experiments of real 3D object are numerically and optically conducted. The high matching results among them confirm this work a better performance over the conventional methods.

Holographic Three-Dimensional Imaging of Terra-Cotta Warrior Model Using Fractional Fourier Transform

Holographic three-dimensional (3D) imaging of Terra-Cotta Warrior model using Fractional Fourier Transform is introduced in this paper. Phase holograms of Terra-Cotta Warrior model are calculated from 60 horizontal viewing-angles by the use of fractional Fourier transform (FRT). Multiple phase holograms are calculated for each angle by adding proper pseudorandom phase to reduce the speckle noise of a reconstructed image. Experimental system based on high-resolution phase-only spatial light modulator (SLM) is built for 3D image reconstruction from the calculated phase holograms. The texture of the Terra-Cotta Warrior model is rough. The calculation of rough texture is optimized in order to show better model details. The effects of computing distance and layer thickness on imaging quality are analyzed finally.

Quality enhancement and GPU acceleration for a full-color holographic system using a relocated point cloud gridding method

The calculation of realistic full-color holographic displays is hindered by the high computational cost. Previously, we suggested a point cloud gridding (PCG) method to calculate monochrome holograms of real objects. In this research, a relocated point cloud gridding (R-PCG) method is proposed to enhance the reconstruction quality and accelerate the calculation speed in GPU for a full-color holographic system. We use a depth camera to acquire depth and color information from the real scene then reconstruct the point cloud model virtually. The R-PCG method allows us to classify groups of object points with the same depth values into grids in the red, green, and blue (RGB) channels. Computer-generated holograms (CGHs) are obtained by applying a fast Fourier transform (FFT) diffraction calculation to the grids. The feasibility of the R-PCG method is confirmed by numerical and optical reconstruction.

Depth-layer weighted prediction method for a full-color polygon-based holographic system with real objects

We propose a full-color polygon-based holographic system for real three-dimensional (3D) objects using a depth-layer weighted prediction method. The proposed system is composed of four main stages: acquisition, preprocessing, hologram generation, and reconstruction. In the preprocessing stage, the point cloud model is separated into red, green, and blue channels with depth-layer weighted prediction. The color component values are characterized based on the depth information of the real object, then color prediction is derived from the measurement data. The computer-generated holograms reconstruct 3D full-color images with a strong sensation of depth resulting from the polygon approach. The feasibility of the proposed method was confirmed by numerical and optical reconstruction.

Holographic display for see-through augmented reality using mirror-lens holographic optical element

A holographic display system for realizing a three-dimensional optical see-through augmented reality (AR) is proposed. A multi-functional holographic optical element (HOE), which simultaneously performs the optical functions of a mirror and a lens, is adopted in the system. In the proposed method, a mirror that is used to guide the light source into a reflection type spatial light modulator (SLM) and a lens that functions as Fourier transforming optics are recorded on a single holographic recording material by utilizing an angular multiplexing technique of volume hologram. The HOE is transparent and performs the optical functions just for Bragg matched condition. Therefore, the real-world scenes that are usually distorted by a Fourier lens or an SLM in the conventional holographic display can be observed without visual disturbance by using the proposed mirror-lens HOE (MLHOE). Furthermore, to achieve an optimized optical recording condition of the MLHOE, the optical characteristics of the holographic material are measured. The proposed holographic AR display system is verified experimentally.

Space bandwidth product enhancement of holographic display using high-order diffraction guided by holographic optical element

A space bandwidth product (SBP) enhancement method for holographic display using high-order diffraction of a spatial light modulator (SLM) is proposed. Among numerous high order diffraction terms, the plus-minus first and the zeroth are adopted and guided by holographic optical elements (HOEs) to an identical direction with the same intensity. By using a set of electro-shutters synchronized with corresponding order component, the system acts as if three SLMs are tiled in the horizontal direction. To confirm the feasibility of using HOE as the guiding optics for the system, several optical characteristics of the recording material are measured before using them. Furthermore, a computer generated hologram algorithm is proposed for compensating the wavefront distortion caused by use of the HOE. The demonstrated system achieves a three-fold increase in SBP of a single SLM. The results are verified experimentally.

Fast calculation method of computer-generated cylindrical hologram using wave-front recording surface

Fast calculation method for a computer-generated cylindrical hologram (CGCH) is proposed. The method consists of two steps: the first step is a calculation of a virtual wave-front recording surface (WRS), which is located between the 3D object and CGCH. In the second step, in order to obtain a CGCH, we execute the diffraction calculation based on the fast Fourier transform (FFT) from the WRS to the CGCH, which are in the same concentric arrangement. The computational complexity is dramatically reduced in comparison with direct integration method. The simulation results confirm that our proposed method is able to improve the computational speed of CGCH.