Browsing by Author "Vardis, Konstantinos"
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Item Efficient Illumination Algorithms for Global Illumination in Interactive and Real-Time Rendering(National Archive of PhD Theses, 2017-11-22) Vardis, KonstantinosThe synthesis of photorealistic digital imagery has long been considered as one of the most fascinating domains in the field of computer graphics. Its main goal is the generation of visually stunning images, mimicking as close as possible the appearance of objects in the physical world. The endeavor for visual realism has directed a large amount of research interest in the investigation of the interactions of light and matter, resulting in an established mathematical framework and the striking beauty of the generated images on today’s production level renderers. While the theoretical concepts of light transport are well understood and applied in offline rendering, the interactive reproduction of the underlying physical processes remains a highly challenging topic due to the various constraints involved in the process. Furthermore, the increased need for the delivery of highly dynamic interactive content in today’s vast virtual environments, that can potentially change in every frame, has undoubtedly increased the necessity for highly efficient, interactive illumination algorithms. In this thesis, we investigate such methods, in the field of photorealistic image synthesis. Our contributions focus exclusively on the development of robust algorithms for real-time and interactive global illumination, under the considerations of fully dynamic content. Regarding real-time rendering, the majority of algorithms are based on the rasterization pipeline, where the support of dynamic content is inherently provided. However, the strict time restrictions of real-time applications pose significant constraints in the operation of the computationally demanding global illumination algorithms, severely impacting the resulting quality of the rendered images. There, the most common approximation is imposed on the algorithmic input where, typically, the highly-detailed geometric information is replaced by either (i) a partial (layered), view-dependent and discretized representation, or a (ii) view-independent but, crude, regular subdivision of the environment in image- and volume-space methods, respectively. We contribute to the domain of real-time rendering by proposing two novel techniques, focusing particularly on the improvement of the visual instability of prior approaches as well as on the efficiency of the underlying representations. First, we propose a generic method to efficiently address the view-dependent inconsistencies of image-domain methods, demonstrated on screen-space ambient occlusion. This is accomplished by taking advantage of buffers containing geometric information from other view points, already generated as part of the rendering process, such as the shadow maps. Second, we improve the efficiency as well as on the visual stability of volume-based global illumination methods, by introducing the idea of directional chrominance subsampling for radiance field compression, an optimized cache point population scheme and a view-independent approximate indirect shadowing technique. In order to support dynamic content in interactive applications, the effort of the research community has been heavily focused on the improvement of the efficiency of the ray tracing pipeline, which has been traditionally employed for production rendering. However, the computational overhead of the required complex acceleration structures is still restricting these approaches to partially static content. Alternatively, a recent category of techniques have attempted to exploit the rasterization pipeline, which inherently supports dynamic environments, to achieve quality identical to the ray tracing pipeline. Still, the proposed solutions are not yet able to support a full global illumination algorithm without posing any restrictions on the geometric representation or on the effects that can be reproduced. In the domain of interactive rendering, we present two methods that investigate the ability of the modern rasterization pipeline to provide a viable alternative to the costly construction stages of spatial acceleration structures. The proposed methods are able to perform high-quality interactive ray tracing in arbitrarily complex and dynamic environments, thus lifting the limitations of prior rasterization-based methods. Our first method ii employs multifragment rendering techniques to effectively capture, for the first time, a highly-detailed representation of the entire environment where a full global illumination algorithm, such as path tracing, can be elegantly supported. Ray tracing is efficiently achieved by exploiting image-space empty space skipping and approximate ray-fragment intersection tests. The presented solution advances the field of image-space ray tracing and provides small construction times as well as scalable traversal performance. However, the resulting quality is approximate and can suffer from high memory overhead due to its fragment-based data structure. Our second approach, which completes our investigation, applies the deferred nature of the traditional ray-tracing pipeline in a rasterization-based framework. Thus, we are able to exploit a primitive-based acceleration data structure and support three, conflicting in prior work, objectives: (i) dynamic environments through fast construction times, (ii) quality identical to the ray tracing pipeline via primitive-based intersection tests, and (iii) reduced memory requirements. Additionally, the presented method further generalizes on the field of image-space ray tracing by exploiting various empty space skipping optimization strategies in order to efficiently support accurate ray-primitive intersection queries.Item Illumination-driven Light Probe Placement(The Eurographics Association, 2021) Vardis, Konstantinos; Vasilakis, Andreas Alexandros; Papaioannou, Georgios; Bittner, Jirí and Waldner, ManuelaWe introduce a simplification method for light probe configurations that preserves the indirect illumination distribution in scenes with diverse lighting conditions. An iterative graph simplification algorithm discards the probes that, according to a set of evaluation points, have the least impact on the global light field. Our approach is simple, generic and aims at improving the repetitive and often non-intuitive and tedious task of placing light probes on complex virtual environments.Item Rayground: An Online Educational Tool for Ray Tracing(The Eurographics Association, 2020) Vitsas, Nick; Gkaravelis, Anastasios; Vasilakis, Andreas-Alexandros; Vardis, Konstantinos; Papaioannou, Georgios; Romero, Mario and Sousa Santos, BeatriceIn this paper, we present Rayground; an online, interactive education tool for richer in-class teaching and gradual self-study, which provides a convenient introduction into practical ray tracing through a standard shader-based programming interface. Setting up a basic ray tracing framework via modern graphics APIs, such as DirectX 12 and Vulkan, results in complex and verbose code that can be intimidating even for very competent students. On the other hand, Rayground aims to demystify ray tracing fundamentals, by providing a well-defined WebGL-based programmable graphics pipeline of configurable distinct ray tracing stages coupled with a simple scene description format. An extensive discussion is further offered describing how both undergraduate and postgraduate computer graphics theoretical lectures and laboratory sessions can be enhanced by our work, to achieve a broad understanding of the underlying concepts. Rayground is open, cross-platform, and available to everyone.Item Sampling Clear Sky Models using Truncated Gaussian Mixtures(The Eurographics Association, 2021) Vitsas, Nick; Vardis, Konstantinos; Papaioannou, Georgios; Bousseau, Adrien and McGuire, MorganParametric clear sky models are often represented by simple analytic expressions that can efficiently generate plausible, natural radiance maps of the sky, taking into account expensive and hard to simulate atmospheric phenomena. In this work, we show how such models can be complemented by an equally simple, elegant and generic analytic continuous probability density function (PDF) that provides a very good approximation to the radiance-based distribution of the sky. We describe a fitting process that is used to properly parameterise a truncated Gaussian mixture model, which allows for exact, constant-time and minimal-memory sampling and evaluation of this PDF, without rejection sampling, an important property for practical applications in offline and real-time rendering. We present experiments in a standard importance sampling framework that showcase variance reduction approaching that of a more expensive inversion sampling method using Summed Area Tables.Item A Survey of Multifragment Rendering(The Eurographics Association and John Wiley & Sons Ltd., 2020) Vasilakis, Andreas Alexandros; Vardis, Konstantinos; Papaioannou, Georgios; Mantiuk, Rafal and Sundstedt, VeronicaIn the past few years, advances in graphics hardware have fuelled an explosion of research and development in the field of interactive and real-time rendering in screen space. Following this trend, a rapidly increasing number of applications rely on multifragment rendering solutions to develop visually convincing graphics applications with dynamic content. The main advantage of these approaches is that they encompass additional rasterised geometry, by retaining more information from the fragment sampling domain, thus augmenting the visibility determination stage. With this survey, we provide an overview of and insight into the extensive, yet active research and respective literature on multifragment rendering. We formally present the multifragment rendering pipeline, clearly identifying the construction strategies, the core image operation categories and their mapping to the respective applications. We describe features and trade-offs for each class of techniques, pointing out GPU optimisations and limitations and provide practical recommendations for choosing an appropriate method for each application. Finally, we offer fruitful context for discussion by outlining some existing problems and challenges as well as by presenting opportunities for impactful future research directions.