Publication List

Rayground: An Online Educational Tool for Ray Tracing
N. Vitsas, A. Gkaravelis, A. A. Vasilakis, K. Vardis, G. Papaioannou, Eurographics 2020 Educational track, May, 2020
Abstract. In 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.

Illumination-Guided Furniture Layout Optimization
N. Vitsas, G. Papaioannou, A. Gkaravelis, A. A. Vasilakis, Computer Graphics Forum (proc. Eurographics 2020), 39(2), pp. XXX-XXX, May, 2020
Abstract. Lighting plays a very important role in interior design. However, in the specific problem of furniture layout recommendation, illumination has been either neglected or addressed with empirical or very simplified solutions. The effectiveness of a particular layout in its expected task performance can be greatly affected by daylighting and artificial illumination in a non-trivial manner. In this paper, we introduce a robust method for furniture layout optimization guided by illumination constraints. The method takes into account all dominant light sources, such as sun light, skylighting and fixtures, while also being able to handle movable light emitters. For this task, the method introduces multiple generic illumination constraints and physically-based light transport estimators, operating alongside typical geometric design guidelines, in a unified manner. We demonstrate how to produce furniture arrangements that comply with important safety, comfort and efficiency illumination criteria, such as glare suppression, under complex light-environment interactions, which are very hard to handle using empirical or simplified models.

Light Optimization for Detail Highlighting
A. Gkaravelis, G. Papaioannou, Computer Graphics Forum (proc. Pacific Graphics 2018), 37(7), pp. 37-44, October, 2018
Abstract. In this paper we propose an effective technique for the automatic arrangement of spot lights and other luminaires on or near user-provided arbitrary mounting surfaces in order to highlight the geometric details of complex objects. Since potential applications include the lighting design for exhibitions and similar installations, the method takes into account obstructing geometry and potential occlusion from visitors and other non-permanent blocking geometry. Our technique generates the most appropriate position and orientation for light sources based on a local contrast maximization near salient geometric features and a clustering mechanism, producing consistent and view-independent results, with minimal user intervention. We validate our method with realistic test cases including multiple and disjoint exhibits as well as high occlusion scenarios.

Detail Highlighting using a Shadow Edge Histogram
A. Gkaravelis, G. Papaioannou, Proc. Eurographics 2017 short paper track, 2017
Abstract. In this paper we propose a simple and effective technique for setting up a configuration of directional light sources to accentuate the prominent geometric features of complex objects by increasing the local shadow contrast near them. Practical applications of such a task are encountered among others in professional photography, and cinematography. The method itself, which is based on a voting mechanism, quickly produces consistent and view-independent results, with minimal user intervention.

Inverse Lighting Design using a Coverage Optimization Strategy
A. Gkaravelis, G. Papaioannou, Visual Computer (proc. CGI 2016), Springer
Abstract. Lighting design is an essential process in computer cinematography, games, architectural design and various other applications for correctly illuminating or highlighting parts of a scene and enhancing storytelling. When targeting specific illumination goals and constraints, this process can be tedious and counterintuitive, even for experienced users and thus automatic, goal-driven methods have emerged for the estimation of a lighting configuration to match the desired result. We present a general automatic approach to such an inverse lighting design problem, where the number of light sources along with their position and emittance are computed given a set of user-specified lighting goals. To this end, we employ a special hierarchical light clustering that operates in the lighting goal coverage domain and overcomes limitations of previous approaches in environments with high occlusion or structural complexity. Our approach is independent of the underlying light transport model and can quickly converge to usable solutions. We validate our results and provide comparative evaluation with the current state of the art.

Inverse Lighting Design for High-Occlusion Environments
A. Gkaravelis, G. Papaioannou, K. Kalampokis, proc. Grapp 2015
Abstract. Lighting design is a demanding but very important task in computer cinematography, games and architectural design. Computer-assisted lighting design aims at providing the designers with tools to describe the desired outcome and derive a suitable lighting configuration to match their goal. In this paper, we present an automatic approach to the inverse light source emittance and positioning problem, based on a layered linear / non-linear optimization strategy and the introduction of a special light source indexing according to the compatibility of each individual luminary position with the desired illumination. Our approach is independent of a particular light transport model and can quickly converge to an appropriate and plausible light configuration that approximates the desired illumination and can handle environments with high occlusion.

Real-time Radiance Caching using Chrominance Compression
K.Vardis, G. Papaioannou, A. Gkaravelis, Journal of Computer Graphics Techniques (JCGT), 3(4), pp. 111-131, 2014
Abstract. This paper introduces the idea of expressing the radiance field in luminance/chrominance values and encoding the directional chrominance in lower detail. We exploit this alternative radiance representation in a low-cost real-time volume-based radiance caching method. Reducing the spherical harmonics coefficients for the chrominance components allows the finer representation of luminance transitions, stored in higher order spherical harmonics and the support for arbitrary light bounces and view-independent indirect occlusion. We combine the radiance field chrominance compression with an optimized cache population scheme, where cache points are generated only at locations, which are guaranteed to contribute to the reconstructed surface irradiance. These computation and storage savings allow the use of third-order spherical harmonics representation to sufficiently capture and reconstruct the directionality of diffuse irradiance, while maintaining fast and customizable performance. Our method performs well in highly complex and dynamic environments and is mainly aimed at real-time applications, although our general qualitative evaluation indicates benefits for offline rendering as well.

Phd Thesis

Efficient Algorithms for Inverse Lighting Design
A. Gkaravelis, G. Papaioannou
Description: A dissertation submitted to the Department of Informatics of Athens University of Economics & Business.

Technical Reports

State of the Art Report on Interactive Global Illumination Techniques and Inverse Lighting Problems
A. A. Vasilakis, K. Vardis, A. Gkaravelis, G. Papaioannou, K. Kalampokis
Description: This report presents a thorough investigation of the complex and active research area in both interactive global illumination and inverse lighting problems, with a focus on interactive applications and dynamic environments.