One of the main goals of many augmented reality applications is to provide a seamless integration of a real scene with additional virtual data. To fully achieve that goal, such applications must typically provide high-quality real-world tracking, support real-time performance and handle the mutual occlusion problem, estimating the position of the virtual data into the real scene and rendering the virtual content accordingly. In this survey, we focus on the occlusion handling problem in augmented reality applications and provide a detailed review of 161 articles published in this field between January 1992 and August 2020. To do so, we present a historical overview of the most common strategies employed to determine the depth order between real and virtual objects, to visualize hidden objects in a real scene, and to build occlusion-capable visual displays. Moreover, we look at the state-of-the-art techniques, highlight the recent research trends, discuss the current open problems of occlusion handling in augmented reality, and suggest future directions for research.

Virtual reality experiences are frequently created using game engines, yet they are not simple for novices and unskilled professionals who do not have programming and 3D modeling skills. Concurrently, there is a knowledge gap in software project design for intuitive virtual reality authoring tools, which were supposed to be easier to use. This study compiles design guidelines derived from a systematic literature review to contribute to the development of more intuitive virtual reality authoring tools. We searched the Scopus and Web of Science knowledge databases for studies published between 2018 and 2021 and discovered fourteen articles. We compiled fourteen requirement and feature design guidelines, such as Visual Programming, Immersive Authoring, Reutilization, Sharing and Collaboration, Metaphors, and Movement Freedom, among others. The gathered guidelines have the potential to either guide the development of new authoring tools or to evaluate the intuitiveness of existing tools. Furthermore, they can also support the development of the metaverse since virtual content creation is one of its bases.

Virtual reality software might be challenging to utilize for beginners and unskilled professionals who do not have a programming or 3D modeling background. Concurrently, there is a knowledge gap in software project design for intuitive virtual reality authoring tools, which were supposed to be easier to use. These tools are frequently insufficient due to a lack of support and standard operating procedures. This study evaluates the validity of fourteen design guidelines for the development of intuitive virtual reality authoring tools. Adopting the Design Science Research approach, a previous study completed the first steps of the protocol by identifying problems, defining solution objectives, and developing and demonstrating the design guidelines. This work evaluates their application in a experiment. A group of engineering students with no prior experience in creating virtual worlds were tasked with examining the design guidelines while using the NVIDIA Omniverse Enterprise as an exemplary use case by following the tutorials in the platform. Previously, the students answered a Likert-scale questionnaire and a focus group interview, with eighteen questions about how they perceived these guidelines. An average score value for the tool was estimated through the questionnaire answers and a correlation analysis and, using the Pearson Correlation Coefficient (PCC), it was confirmed that most guidelines scores behaved as expected and were ranked according to the use-case functionality. The participants understood the guidelines’ definition and could decide if they agreed or disagreed with their presence during the experiment. We evaluated that, in accordance with the Design Science Research, the proposed artifact is useful, i.e., the design guidelines for virtual reality authoring tools perform what they are designed to do and are operationally reliable in accomplishing their goals.

To improve the human perception of an augmented reality scene, its virtual and real entities can be rendered according to the focus+context visualization. This paradigm is specially important in the field of on-patient medical data visualization, as it provides insight to the physicians about the spatial relations between the patient's anatomy (focus region) and his entire body (context region). However, the current existing methods proposed in this field do not give special treatment to the effect of volume clipping, which can open new ways for physicians to explore and understand the entire scene. In this paper we introduce an on-patient focus+context medical data visualization based on volume clipping. It is proposed in a markerless augmented reality environment. From the estimated camera pose, the volumetric medical data can be displayed to a physician inside the patient's anatomy at the location of the real anatomy. To improve the visual quality of the final scene, three methods based on volume clipping are proposed to allow new focus+context visualizations. Moreover, the whole solution supports occlusion handling. From the evaluation of the proposed techniques, the results obtained highlight that these methods improve the visual quality of the final rendering. Furthermore, the application still runs in realtime.

The underwater world has gained significant attention in research in recent years, particularly in the context of ocean exploration. Images serve as a valuable data source for underwater tasks, but they face several issues related to light behavior in this environment. Given the complexity of capturing data from the sea and the large variability of environmental components (depth, distance, suspended particles, turbidity, etc.), synthesized underwater scenes can provide relevant data to improve image processing algorithms and computer vision tasks. The main goal of this survey is to summarize techniques to underwater image synthesis, their contributions and correlations, and to highlight further directions and opportunities in this research domain.