KEYWORDS: Holography, Cameras, Stereo holograms, 3D displays, Image fusion, Printing, 3D modeling, Augmented reality, 3D image reconstruction, 3D image processing
Holographic stereogram opens up a new way for holographic 3D display of objects, and has high research value and significance in commercial, military and other aspects. In order to write real and virtual three-dimensional (3D) scenes in holographic stereogram at the same time, so as to achieve the display effect of augmented reality, a light field fusion method is proposed. The basic principle of light field fusion is introduced. The light field sampling is introduced and completed, and depth image based rendering (DIBR) algorithm is applied for regularization and densification of sampled images. The projection relationship of the light rays are analyzed, and the pixel level light field fusion is accomplished. Effective perspectives segmentation and mosaicking (EPISM) holographic stereogram printing method is used to write and reproduce the fused light field which further verifies the effectiveness of the proposed method.
In this paper, the principle as well as the implementation of EPISM method are introduced firstly. In order to evaluate the reconstruction quality better, the imaging process of EPISM based holographic stereogram is regarded as a general optical system imaging, and modeling and optimization of EPISM method are proposed from two different aspects of angular spectrum and spatial domain. In the analysis of angle spectrum theory, the exit pupil function model is simplified firstly and the optical transfer function (OTF) with defocusing aberrations was established. In spatial domain analysis, the modulation characteristics of the hogel based holographic stereogram is constructed and validated while a diffraction-limited imaging model of the hogel based holographic stereogram is established, and the effective resolvable size of the reconstructed image point is simulated. The theories show that there is an optimal hogel size existed for the certain depth of scene. Optical experiments demonstrate the validity of our analysis, and the optimized parameters of hogel sizes can improve the imaging quality of full parallax holographic stereogram effectively.
Different light-fields information collection methods will cause different light-fields information cannot be fused and recorded into the holographic stereogram. In order to print the holographic stereogram recording information of virtualreal fusion, the depth-image-based rendering (DIBR) technology is applied to the holographic stereogram fabricated by effective perspective images’ segmentation and mosaicking method (EPISM). This method firstly obtains matching real scene sampling parameters according to the virtual scene, and combines with the improved DIBR technology to draw the initial perspective images without holes or crack problems. After EPISM processing, the synthetic perspective images of real and virtual scenes with the same parameters are fused. Finally, the fused perspective images are recorded into holographic stereogram. The experimental results demonstrate this proposed method obtain high quality virtual-real fusion display.
In this paper, image processing technology is used to simulate the reconstructed images of holographic stereogram in different depth planes. The quality of the reconstructed image of a single hogel at different depths is analyzed firstly. The size of the hogel, the distance between the holographic stereogram and the LCD and the distance between the holographic stereogram and the imaging plane are all factors affecting the quality of the reconstructed image. By dividing the hogel into many sub-parts, the reconstructed images of each sub-parts are analyzed one by one, and by superimposing the reconstructed images of each sub-parts, the reconstructed images of the hogel at different depths can be obtained. Then, the relationship between the reconstructed images of each hogel is analyzed, and the reconstructed images of all hogels are translated and superimposed to obtain the reconstructed images of the holographic stereogram in different depth. The synthetic parallax images generated by using the effective perspective image segmentation and mosaicking (EPISM) algorithm proposed by our group are used to print the holographic stereogram. The experiment results show that the reconstructed images of the stereogram in different depth planes are consistent with the simulation results, which verifies the validation of the simulation.
The problem of the reconstructed of different scene depth was analyzed in single-step holographic stereogram printing based on effective perspective images’ segmentation and mosaicking (EPISM). The reconstructed quality of short scene depth is bad. the causes of flipping effect in holographic stereogram are studied in detail, and the influence of flipping effect on image quality is alleviated by reducing the size of holographic element (Hogel). The curvature distortion of holographic stereogram is analyzed. The effect of curvature distortion on the reconstructed quality of holographic stereogram is verified by changing the distance of object protruding sampling plane. The theoretical analysis was verified experimentally with different scene depth. The reconstructed image of high quality and short scene depth was obtained, and the practicability of EPISM was improved.
Producing of conventional optical reflection hologram can be classified into one-step method and two-step method. In one-step method, only the diverging light of the object could be recorded, and the reconstructed scene is a virtual one behind the recording medium. In two-step method, the diverging light or the converging light could be recorded alternatively. However, the process is complicated considering double exposures. We propose a novel method of one-step reflection hologram. The object is first imaged by a 4f optical system, then the interference fringes are recorded by single exposure. The reconstructed image can be either a virtual image behind the recording medium, or a real image in front of the recording medium. The ideal imaging properties of 4f optical system have been demonstrated theoretically and the proposed method has been verified experimentally.
A spatial frequency index method is proposed to cull the occlusion in computer generated hologram. The object points with the same spatial frequency are put into a set, and only the point closest to the hologram is contributed to the hologram because of their mutual occlusion. The phases of corresponded spatial frequency are precomputed and stored in a table. The phases on the hologram are obtained from the table according to the spatial frequency of object point. Experiments are performed and the results demonstrate that the proposed method can cull the hidden surfaces of 3-D scene correctly. The occlusion effect can be well reproduced along with the speeding up of the calculation.
The effect of the combined guiding, the thermal induced guiding and gain guiding, on the beam quality in Nd:YVO4 MOPA laser is studied experimentally and theoretically. The beam quality enhancement is achieved during the power-amplification process in a 4-amplifier-stage Nd:YVO4 MOPA laser. 195 W TEM00 mode laser is achieved while its beam quality enhanced from M2=3.0 to 1.2 after power amplification. This is an interesting phenomenon since the beam quality is always deteriorated during the laser amplification. To explain the phenomenon of beam quality improvement, a theoretical model is established with the thermal induced guiding effect and gain guiding effect is considered, and a nonlinear dynamical equation of the laser mode in laser amplifier is derived. The wave equation is a Schrödinger-like two order nonlinear partial differential equation, and a symmetrical split-step Fourier mothed is developed to analyze the wave equation. Both experimental and theoretical results show that the combined guiding effect dominates the evolution of beam quality in the insensitive pumping Nd:YVO4 MOPA lasers, and the combined guiding effect can be employed to improve the performance of beam quality in MOPA lasers.
KEYWORDS: 3D displays, Integral imaging, Spatial resolution, LCDs, 3D image processing, 3D modeling, Image resolution, Imaging arrays, Cameras, Holography
Integral imaging is known as a promising 3D display method for its ability to reconstruct the light field of the scene. However, integral imaging suffers from low spatial resolution and narrow viewing angle due to the limited spatial bandwidth product of LCD, which prevents its commercial application. In conventional integral imaging display, spatial resolution and viewing angle are two vital factors that should be considered, and many previous research focuses on the two factors. A novel integral imaging 3D display system with large viewing angle about 35° and high spatial resolution for HVS is presented. The method is composed of a high definition 5K LCD panel, a macro lens array and a light shaping diffusor. One point of the method different from conventional integral imaging in which micro lens array is used, a macro lens array with elemental lens diameter 1cm is employed in our method to ensure a large viewing angle, however, this may result in low spatial resolution for HVS. And the other point is a light shaping diffusor is placed in front of the lens array with proper distance, and lifelike 3D reconstruction is obtained. Experimental results with full parallax, large viewing angle and high resolution 3D images are shown to verify the validity of the proposed system.
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.
Holographic stereogram display technology is one of the research focuses in three-dimensional (3D) holographic display. Holographic stereogram is essentially to infinitely approach the true 3D object using a sequence of 2D perspective images. In this paper, several holographic printing methods are summarized firstly, including two-steps transfer hologram printing technique, horizontal parallax only (HPO) stereogram with laser direct writing and holographic elements based full-parallax holographic stereogram method. Also the principles of printing, method of recording, and the various factors that affect the printing quality of the holographic stereogram are discussed and analyzed in detail. Then, after designing the matching relationship between full-parallax holographic stereogram and holographic elements, a new method to print holographic stereogram is proposed. With this method, image distortions can be decreased to improve the printing quality because this method has less impact factors and easier operations in experiment. Finally, it points out the recent development state of holographic stereogram.
Based on Gaussian mixture model, an improved detection algorithm, which aimed at updating the real-time character and accuracy of the moving target detection in intelligent video surveillance systems effectively, is elaborated in this paper. It combines the advantages of GMM and improved maximum between class variance method. The algorithm not only improves the speed of detecting targets in the intelligent systems, but also solves the inherent problems efficiently in poor real-time performance and error detection problem. In conclusion, the experiment results demonstrated that the algorithm has an excellent adaptability and anti-interference performance to fit the complicated situation and changing environment.
A pure phase-modulated computer-generated hologram (CGH) method is presented to generate a full parallax holographic stereogram. The holographic stereogram plane is divided into several two-dimensional holographic elements (hogels). The spectra of the hogels are rendered from multiview full parallax images of three-dimensional (3-D) objects. The phase-modulated hogel is calculated by iterative Fourier transform algorithms to improve diffraction efficiency and eliminate conjugate images. A gray calibration technique is introduced to generate the accurate intensity modulation of pure phase hogels. The holographic stereogram that we proposed is reconstructed by an optical system based on a phase only spatial light modulator. The experimental results demonstrate that our proposed method can successfully reconstruct parallax images of 3-D objects.
The vergence-accommodation conflict in holographic stereograms is investigated. The visual distortion and fatigue caused by the conflict are analyzed. A method for generating full-parallax holographic stereograms without vergence-accommodation conflicts is proposed. Two-dimensional spatial and spectral samplings are carried out on both the hologram and the reconstructed planes. The depth cues of three-dimensional object points are introduced in the iterative process of calculating subholograms with different spectral components. The stereogram is a combination of holographic elements (hogels), and each hogel is formed by performing a weighted summation of subholograms, where parallax images and depth information are used to select the constituent subholograms. A proof-of-principle experiment is carried out in an optical system based on a spatial light modulator. The results show that the improved full-parallax holographic stereogram can control the focusing depths of points and guarantee consistency between the vergence and accommodation distances. The influence of the size of the hogels on holographic imaging quality is also investigated.
A phase-searching look-up table (PS-LUT) method is proposed to dramatically reduce the memory size of the new look-up table (N-LUT) method as well as to keep its advantage of fast computational speed. Small quantities of samples are chosen as the basic phase points (BPPs) in the principle fringe pattern (PFP), and phases of the object points for BPPs are precalculated and stored in the table. The phases of object point for the whole PFP could be quickly obtained through a phase-searching method. With proper reference beam phases, PFP could be rapidly generated. The experimental results reveal that the computational speed of the proposed method is about 24 times faster than that of the ray-tracing method, and the required memory size is 1731 times less than that of the N-LUT method. To eliminate the coherent noise of the N-LUT and PS-LUT, the random PFP N-LUT (RN-LUT) and PS-LUT (RPS-LUT), in which the PFP is randomly chosen for every object point, are put forward. The reconstructed images with a spatial light modulator indicate that both the RN-LUT and RPS-LUT methods are extremely effective in improving the quality of the reconstructed images.
Based on the existed ways of 2-D reconstructed images by spatial light modulator (SLM), the methods to enhance the reconstructed quality of 3-D images are investigated in this paper. Based on the diffraction theory, the effects of a lattice SLM with a limited fill factor on the reconstructed images are analyzed. Through adding the phase of the convergent spherical wave, the two focused planes of reconstructed images and the multi-order beams caused by the lattice structure of the SLM can be separated spatially. Therefore, the spatial filter is used to eliminate the influences of higher-orders diffraction beams and zero-order light of reconstructed images, respectively. A holographic optoelectronic display system based on liquid crystal spatial light modulator (LC-SLM) is set up to demonstrate this method.
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