Publications
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Inverse Lighting Design using a Coverage Optimization Strategy
A. Gkaravelis, G. Papaioannou, the Visual Computer (proc. CGI 2016), DOI 10.1007/s00371-016-1237-9, 2016.
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.
Downloads: author-prepared version of the paper
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A Generic Physically-based Approach to the Opening Design Problem
K. Kalampokis, G. Papaioannou, A. Gkaravelis, Proc. Eurographics 2016 (short paper), 2016.
Abstract. Today architectural design harnesses photorealistic rendering to accurately assess energy transport for the design of energyefficient buildings. In this context, we present an automatic physically-based solution to the opening design problem, i.e. the goal-driven process of defining openings on the input geometry given a set of lighting constraints, to better exploit natural daylight. Based on a hierarchical approach that combines a linear optimization strategy and a genetic algorithm, our method computes the optimal number, position, size and shape of openings, using a path tracing-based estimator to precisely model the light transport for arbitrary materials and geometry. The method quickly converges to an opening configuration that optimally approximates the desired illumination, with no special geometry editing requirements and the ability to trade quality for performance for interactive applications. We validate our results against ground truth experiments for various scenes and time-of-day intervals.
Downloads: author-prepared version of the paper
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A Multiview and Multilayer Approach for Interactive Ray Tracing
K. Vardis, A. Vasilakis, G. Papaioannou, Proc. ACM SIGGRAPH Symposium on Interactive 3D Graphics and Games (i3D 2016), pp. 171-178, 2016.
Abstract. We introduce a generic method for interactive ray tracing, able to support complex and dynamic environments, without the need for precomputations or the maintenance of additional spatial data structures. Our method, which relies entirely on the rasterization pipeline, stores fragment information for the entire scene on a multiview and multilayer structure and marches through depth layers to capture both near and distant information for illumination computations. Ray tracing is efficiently achieved by concurrently traversing a novel cube-mapped A-buffer variant in image space that exploits GPU-accelerated double linked lists, decoupled storage, uniform depth subdivision and empty space skipping on a per-fragment basis. We illustrate the effectiveness and quality of our approach on path tracing and ambient occlusion implementations in scenarios, where full scene coverage is of major importance. Finally, we report on the performance and memory usage of our pipeline and compare it against GPGPU ray tracing approaches.
Downloads: author-prepared version of the paper video YouTube video shader source code demo with shader source (~329mb) presentation
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k+-buffer: An Efficient, Memory-Friendly and Dynamic k-buffer Framework
A. A. Vasilakis, G. Papaioannou, I. Fudos, IEEE Transactions on Visualization and Computer Graphics. 2015.
Abstract. Depth-sorted fragment determination is fundamental for a host of image-based techniques which simulates complex rendering effects. It is also a challenging task in terms of time and space required when rasterizing scenes with high depth complexity. When low graphics memory requirements are of utmost importance, k-buffer can objectively be considered as the most preferred framework which advantageously ensures the correct depth order on a subset of all generated fragments. Although various alternatives have been introduced to partially or completely alleviate the noticeable quality artifacts produced by the initial k-buffer algorithm in the expense of memory increase or performance downgrade, appropriate tools to automatically and dynamically compute the most suitable value of k are still missing. To this end, we introduce k+-buffer, a fast framework that accurately simulates the behavior of k-buffer in a single rendering pass. Two memory-bounded data structures: (i) the max-array and (ii) the max-heap are developed on the GPU to concurrently maintain the k-foremost fragments per pixel by exploring pixel synchronization and fragment culling. Memory-friendly strategies are further introduced to dynamically (a) lessen the wasteful memory allocation of individual pixels with low depth complexity frequencies, (b) minimize the allocated size of k-buffer according to different application goals and hardware limitations via a straightforward depth histogram analysis and (c) manage local GPU cache with a fixed-memory depth-sorting mechanism. Finally, an extensive experimental evaluation is provided demonstrating the advantages of our work over all prior k-buffer variants in terms of memory usage, performance cost and image quality.
Downloads: author-prepared version of the paper shader source code
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Improving k-buffer methods via Occupancy Maps
A. A. Vasilakis, G. Papaioannou, proc. Eurographics Conference (short paper). 2015.
Abstract. In this work, we investigate an efficient approach to treat fragment racing when computing k-nearest fragments. Based on the observation that knowing the depth position of the k-th fragment we can optimally find the k-closest fragments, we introduce a novel fragment culling component by employing occupancy maps.Without any softwareredesign, the proposed scheme can easily be attached at any k-buffer pipeline to efficiently perform early-z culling. Finally, we report on the efficiency, memory space, and robustness of the upgraded k-buffer alternatives providing comprehensive comparison results.
Downloads: author-prepared version of the paper shader source code
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Inverse Light 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.
Downloads:author-prepared version of the paper
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Accelerating k+ buffer using efficient fragment culling
A. A. Vasilakis, G. Papaioannou, proc. 19th Symposium on Interactive 3D Graphics and Games (Poster). ACM, San Francisco, California, pp. 129, 2015.
Abstract. In this work, we investigate an efficient approach to treat fragment racing when computing k-nearest fragments. Based on the observation that knowing the depth position of the k-th fragment we can optimally find the k-closest ones, we introduce a novel orderindependent fragment culling component, easily attached to the k+ buffer pipeline. An additional rendering pass of the scene’s geometry is initially employed to construct a per pixel binary fragment occupancy discretization. Then, the nearest depth of the k-th per pixel fragment is concurrently computed by performing bit counting operations and subsequently utilized to perform early-z rejection for the k+ buffer construction process that follows. Any fragment with depth larger than this value will fail the depth test, avoiding the cost of its pixel shading execution. Note that no software modifications are required to the actual k+ buffer implementation.
Downloads: poster summary poster fast-forward presentation
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Real-time Radiance Caching using Chrominance Compressions
Kostas Vardis, Georgios Papaioannou, and Anastasios 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.
Online paper: http://jcgt.org/published/0003/04/06/
Downloads: video demo and shader source code
Reference: BibTex
