Efficient Illumination Algorithms for Global Illumination in Interactive and Real-Time Rendering

Abstract

The 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.