What are the key structures existing in a large real-world MMORPG (Massively Multiplayer Online Role-Playing Game) graph? How can we compactly summarize an MMORPG graph with hierarchical node labels, considering substructures at different levels of hierarchy? Recent MMORPGs generate complex interactions between entities inducing a heterogeneous graph where each entity has hierarchical labels. Succinctly summarizing a heterogeneous MMORPG graph is crucial to better understand its structure; however it is a challenging task since it needs to handle complex interactions and hierarchical labels efficiently. Although there exist few methods to summarize a large-scale graph, they do not deal with heterogeneous graphs with hierarchical node labels.We propose GSHL , a novel method that summarizes a heterogeneous graph with hierarchical labels. We formulate the encoding cost of hierarchical labels using MDL (Minimum Description Length). GSHL exploits the formulation to identify and segment subgraphs, and discovers compact and consistent structures in the graph. Experiments on a large real-world MMORPG graph with multi-million edges show that GSHL is a useful and scalable tool for summarizing the graph, finding important structures in the graph, and finding similar users.

How can we classify graph-structured data only with positive labels? Graph-based positive-unlabeled (PU) learning is to train a binary classifier given only the positive labels when the relationship between examples is given as a graph. The problem is of great importance for various tasks such as detecting malicious accounts in a social network, which are difficult to be modeled by supervised learning when the true negative labels are absent. Previous works for graph-based PU learning assume that the prior distribution of positive nodes is known in advance, which is not true in many real-world cases. In this work, we propose GRAB (Graph-based Risk minimization with iterAtive Belief propagation), a novel end-to-end approach for graph-based PU learning that requires no class prior. GRAB models a given graph as a Markov network and runs the marginalization and update steps iteratively. The marginalization step estimates the marginals of latent variables, while the update step trains a classifier network utilizing the computed priors in the objective function. Extensive experiments on five datasets show that GRAB achieves state-of-the-art accuracy, even compared with previous methods that are given the true prior.

Holographic optical elements (HOEs) have a wide range of applications, including their emerging use in virtual and augmented reality displays, but their design and fabrication have remained largely limited to configurations using simple wavefronts. In this paper, we present a pipeline for the design, optimization, and fabrication of complex, customized HOEs that enhances their imaging performance and enables new applications. In particular, we propose an optimization method for grating vector fields that accounts for the unique selectivity properties of HOEs. We further show how our pipeline can be applied to two distinct HOE fabrication methods. The first uses a pair of freeform refractive elements to manufacture HOEs with high optical quality and precision. The second uses a holographic printer with two wavefront-modulating arms, enabling rapid prototyping. We propose a unified wavefront decomposition framework suitable for both fabrication approaches. To demonstrate the versatility of these methods, we fabricate and characterize a series of specialized HOEs, including an aspheric lens, a head-up display lens, a lens array, and, for the first time, a full-color caustic projection element.

In this paper, we present a concept for speckle reduction using optical superposition instead of temporal superposition. The proposed speckle reduction method does not contain any mechanical movement nor time-multiplexing technique. Therefore, the system is stable and free from the frame rate degradation or additional computation load. In addition, the angle diversity method is adopted with the proposed method to intensify the speckle reduction. In order to apply the angle diversity method, the required minimum angle difference is analyzed theoretically. To realize the proposed speckle reduction concept, the system is implemented with practical optical components. The system includes an echelon, which is a stair-shaped glass and a static diffuser. The echelon is employed to decrease the spatial coherence of the incident beam. The static diffuser imposes the band-limited random phases on each of the split beams that have passed through the echelon. Also, the static diffuser is used to combine mutually incoherent split beams into a single speckle-reduced beam. The proposed speckle-reduced light source is applied to the holographic display to demonstrate the speckle reduction effect. Experimental results show that the speckle contrast of the reconstructed image is reduced by 55%.

Since 1960s, digital holography (DH) has been developed significantly as an exquisite optical sensing technique with its computational interpretation. Recent advances in the image sensor and the coherent light source have suggested the possibility of using DH in portable form that can be utilized in industrial fields. Field of view (FOV) should be sufficiently wide and the system be simple for successful landing of DH on the industrial market. To embody the possibility, in this paper, we present a method for implementing off-axis reflective DH using a single diffraction grating. Beam splitter and mirror in the conventional off-axis DH are replaced with a single diffraction grating. This replacement enables the implementation of simple interferometer while maintaining the performance of the holography. In addition, we apply the multiplexed illumination method to enhance space-bandwidth product of off-axis hologram. Using the wavelength dependence of the diffraction grating, the FOV can be doubled without additional optical element. We demonstrate the proposed method by three holographic imaging experiments with two laser diodes. The prototype provides lateral resolution of 19.69$\mu$m with the FOV of 6.85 mm × 8.72 mm. The FOV is 1.9 times wider than the image sensor area of 6.85 mm × 4.59 mm. Overall size of the prototype is 12 cm × 17 cm × 5 cm, showing the suitability of implementation to the various industrial fields.

Holographic displays have great potential to realize mixed reality by modulating the wavefront of light in a fundamental manner. As a computational display, holographic displays offer a large degree of freedom, such as focus cue generation and vision correction. However, the limited bandwidth of spatial light modulator imposes an inherent trade-off relationship between the field of view and eye-box size. Thus, we demonstrate the first practical eye-box expansion method for a holographic near-eye display. Instead of providing an intrinsic large exit-pupil, we shift the optical system's exit-pupil to cover the expanded eye-box area with pupil-tracking. For compact implementation, a pupil-shifting holographic optical element (PSHOE) is proposed that can reduce the form factor for exit-pupil shifting. A thorough analysis of the design parameters and display performance are provided. In particular, we provide a comprehensive analysis of the incorporation of the holographic optical element into a holographic display system. The influence of holographic optical elements on the intrinsic exit-pupil and pupil switching is revealed by numerical simulation and Wigner distribution function analysis.

We introduce an augmented reality near-eye display dubbed "Retinal 3D." Key features of the proposed display system are as follows: Focus cues are provided by generating the pupil-tracked light field that can be directly projected onto the retina. Generated focus cues are valid over a large depth range since laser beams are shaped for a large depth of field (DOF). Pupil-tracked light field generation significantly reduces the needed information/computation load. Also, it provides "dynamic eye-box" which can be a break-through that overcome the drawbacks of retinal projection-type displays. For implementation, we utilized a holographic optical element (HOE) as an image combiner, which allowed high transparency with a thin structure. Compared with current augmented reality displays, the proposed system shows competitive performances of a large field of view (FOV), high transparency, high contrast, high resolution, as well as focus cues in a large depth range. Two prototypes are presented along with experimental results and assessments. Analysis on the DOF of light rays and validity of focus cue generation are presented as well. Combination of pupil tracking and advanced near-eye display technique opens new possibilities of the future augmented reality.

We propose a see-through additive light field display as a novel type of compressive light field display. We utilize holographic optical elements (HOEs) as transparent additive layers. The HOE layers are almost free from diffraction unlike spatial light modulator layers, which makes this additive light field display more advantageous when modifying the number of layers, thickness, and pixel density compared with conventional compressive displays. Meanwhile, the additive light field display maintains advantages of compressive light field displays. The proposed additive light field display shows bright and full-color volumetric images in high definition. In addition, users can view real-world scenes beyond the displays. Hence, we expect that our method can contribute to the realization of augmented reality. Here, we describe implementation of a prototype additive light field display with two additive layers, evaluate the performance of transparent HOE layers, describe several results of display experiments, discuss the diffraction effect of spatial light modulators, and analyze the ability of the additive light field display to express uncorrelated light fields.

Recently, various 3D imaging devices and applications have been developed and also commercialized. In this paper, we specially discuss lens array imaging and incoherent digital holography. In lens array imaging, it is possible to capture the light field information object through the lens array, and the depth information can be extracted by the captured light field information. Also the captured information can be visually reconstructed by the 3D images in the manner of integral imaging or light field display. Incoherent digital holography is capable of recording the wavefront information from incoherent light, which was impossible in conventional digital holography. There are various remarkable applications including the natural light holographic camera or the holographic fluorescence microscopy using incoherent digital holography, and also acquired hologram can be used to implement holographic display since the original wavefront information also includes the depth information of the object. We expect that the integral imaging and incoherent digital holography can be applied to various fields, such as photography, video systems, biomedical area, and industrial measurement, according to the characteristics of the applications.