Research - Illumination and Shading
Detail Highlighting using a Shadow Edge Histogram
A.. Gkaravelis, G. Papaioannou, proc. Eurograhpics (short paper), 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.
Downloads: author-prepared version of the paper
DIRT: Deferred Image-based Ray Tracing
K. Vardis, A. Vasilakis, G. Papaioannou, proc. High Performance Graphics, 2016.
Abstract. We introduce a novel approach to image-space ray tracing ideally suited for the photorealistic synthesis of fully dynamic environments at interactive frame rates. Our method, designed entirely on the rasterization pipeline, alters the acceleration data structure construction from a per-fragment to a per-primitive basis in order to simultaneously support three important, generally conflicting in prior art, objectives: fast construction times, analytic intersection tests and reduced memory requirements. In every frame, our algorithm operates in two stages: A compact representation of the scene geometry is built based on primitive linked-lists, followed by a traversal step that decouples the ray-primitive intersection tests from the illumination calculations; a process inspired by deferred rendering and the path integral formulation of light transport. Efficient empty space skipping is achieved by exploiting several culling optimizations both in xy- and z-space, such as pixel frustum clipping, depth subdivision and lossless buffer down-scaling. An extensive experimental study is finally offered showing that our method advances the area of image-based ray tracing under the constraints posed by arbitrarily complex and animated scenarios.
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.
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
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.
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
Real-time Radiance Caching using Chrominance Compression
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/
Multi-view Ambient Occlusion with Importance Sampling
K. Vardis, G. Papaioannou, A. Gaitatzes, Proc. ACM SIGGRAPH Symposium on Interactive 3D Graphics and Games (i3D 2013), pp. 111-118.
Abstract. Screen-space ambient occlusion and obscurance (AO) techniques have become de-facto methods for ambient light attenuation and contact shadows in real-time rendering. Although extensive research has been conducted to improve the quality and performance of AO techniques, view-dependent artifacts remain a major issue. This paper introduces Multi-view Ambient Occlusion, a generic per-fragment view weighting scheme for evaluating screen-space occlusion or obscurance using multiple, arbitrary views, such as the readily available shadow maps. Additionally, it exploits the resulting weights to perform adaptive sampling, based on the importance of each view to reduce the total number of samples, while maintaining the image quality. Multi-view Ambient Occlusion improves and stabilizes the screen-space AO estimation without overestimating the results and can be combined with a variety of existing screen-space AO techniques. We demonstrate the results of our sampling method with both open volume- and solid angle-based AO algorithms.
Real-Time Diffuse Global Illumination Using Radiance Hints
G. Papaioannou, Proc. High Performance Graphics 2011, pp. 15-24, 2011.
Abstract. GPU-based interactive global illumination techniques are receiving an increasing interest from both the research and the industrial community as real-time graphics applications strive for visually rich and realistic dynamic three-dimensional environments. This paper presents a fast new diffuse global illumination method that generates a sparse set of low-cost radiance field evaluation points (radiance hints) and computes an arbitrary number of diffuse inter-reflections within a given volume. The proposed approximate technique combines ideas from exiting grid-based radiance caching techniques with reflective shadow maps as well as a stochastic scheme for visibility calculations, in order to achieve high frame rates for multiple light bounces.
Global Illumination Using Imperfect Volumes
P. Mavridis, G. Papaioannou, Proc. GRAPP 2011 (Int. Conf. on Computer Graphics Theory and Applications), pp. 160-165.
Abstract. This paper introduces the concept of imperfect volumes, a fast one-pass point-based voxelization algorithm, and presents its applications to the global illumination problem. As often noted, diffuse indirect illumination has the characteristics of a low frequency function, consisting of smooth gradations. We exploit this by performing the indirect lighting computations on a rough approximation of the scene, the imperfect volume. The scene is converted on the fly to a dense point cloud, and each point is directly rendered to a volume texture, marking the corresponding voxel as occupied. A framebuffer reprojection scheme ensures that voxels visible to the main camera will get more points. Ray-marching is then used to compute the ambient occlusion or the indirect illumination of each voxel, and the results are stored using spherical harmonics. We demonstrate that the errors introduced by the imperfections in the volume are small and that our method maintains a high frame rate on scenes with high geometric complexity.
Real-Time Volume-Based Ambient Occlusion
G. Papaioannou, M. L. Menexi, C. Papadopoulos, Real-Time Volume-Based Ambient Occlusion, IEEE Transactions on Visualization and Computer Graphics, pp. 752-762, September/October, 2010.
Abstract. Real-time rendering can benefit from global illumination methods to make the 3D environments look more convincing and lifelike. On the other hand, the conventional global illumination algorithms for the estimation of the diffuse surface interreflection make heavy usage of intra- and interobject visibility calculations, so they are time-consuming, and using them in real-time graphics applications can be prohibitive for complex scenes. Modern illumination approximations, such as ambient occlusion variants, use precalculated or frame-dependent data to reduce the problem to a local shading one. This paper presents a fast real-time method for visibility sampling using volumetric data in order to produce accurate inter- and intraobject ambient occlusion. The proposed volume sampling technique disassociates surface representation data from the visibility calculations, and therefore, makes the method suitable for both primitive-order or screen-order rendering, such as deferred rendering. The sampling mechanism can be used in any application that performs visibility queries or ray marching.
Volume-based Diffuse Glocal Illumination
P. Mavridis, A. Gaitatzes, G. Papaioannou, Proc. CGVCVIP ’10: Proceedings of Computer Graphics, Visualization, Computer Vision and Image Processing 2010.
Extended version: Gaitatzes, P. Mavridis, G Papaioannou, Interactive Volume-based Indirect Illumination of Dynamic Scenes, Proc. 3IA ’10: Proceedings of the 2010 International Conference on Computer Graphics and Artificial Intelligence, May 2010 (Studies in Computational Intelligence, Vol. 321 Plemenos, Dimitri; Miaoulis, Georgios (Eds.) ).
Abstract. In this paper we present a novel real-time algorithm to compute the global illumination of scenes with dynamic geometry and arbitrarily complex dynamic illumination. We use a virtual point light (VPL) illumination model on the volume representation of the scene. Light is propagated in void space using an iterative diffusion approach. Unlike other dynamic VPL-based real-time approaches, our method handles occlusion (shadowing and masking) caused by the interference of geometry and is able to estimate diffuse inter-reflections from multiple light bounces.
Fast Approximate Visibility on the GPU Using Precomputed 4D Visibility Fields
A. Gaitatzes, A. Andreadis, G. Papaioannou, Y. Chrysanthou, Fast Approximate Visibility on the GPU Using Precomputed 4D Visibility Fields, Proc. WSCG 2010, pp. 131-138, 2010.
Abstract. We present a novel GPU-based method for accelerating the visibility function computation of the lighting equation in dynamic scenes composed of rigid objects. The method pre-computes, for each object in the scene, the visibility and normal information, as seen from the environment, onto the bounding sphere surrounding the object and encodes it into maps. The visibility function is encoded by a four-dimensional visibility field that describes the distance of the object in each direction for all positional samples on a sphere around the object. In addition, the normal vectors of each object are computed and stored in corresponding fields for the same positional samples for use in the computation of reflection in ray-tracing. Thus we are able to speed up the calculation of most algorithms that trace rays to real-time frame rates. The pre-computation time of our method is relatively small. The space requirements amount to 1 byte per ray direction for the computation of ambient occlusion and soft shadows and 4 bytes per ray direction for the computation of reflection in ray-tracing. We present the acceleration results of our method and show its application to two different intersection intensive domains, ambient occlusion computation and stochastic ray tracing on the GPU.
Downloads: the paper
Realistic Real-time Underwater Caustics and Godrays
C. Papadopoulos, G. Papaioannou, Proc. GraphiCon '09, pp. 89-95, 2009.
Abstract. Realistic rendering of underwater scenes has been a subject of increasing importance in modern real-time 3D applications, such as open-world 3D games, which constantly present the user with opportunities to submerge oneself in an underwater environment. Crucial to the accurate recreation of these environments are the effects of caustics and godrays. In this paper, we shall present a novel algorithm, for physically inspired real-time simulation of these phenomena, on commodity 3D graphics hardware, which can easily be integrated in a modern 3D engine.
Presampled Visibility for Ambient Occlusion
A. Gaitatzes, Y. Chrysanthou, G. Papaioannou, Proc. WSCG 2008, Journal of WSCG, 16(1-3), pp. 17-24, 2008.
We present a novel method to accelerate the computation of the visibility function of the lighting equation, in dynamic scenes composed of rigid, non-penetrating objects. The main idea of the technique is to pre-compute for each object in the scene its associated four-dimensional field that describes the visibility in each direction for all positional samples on a sphere around the object, we call this a displacement field. We are able to speed up the calculation of algorithms that trace visibility rays to near real time frame rates. The storage requirements of the technique, amounts from one byte to one bit per ray direction making it particularly attractive to scenes with multiple instances of the same object, as the same cached data can be reused, regardless of the geometric transformation applied to each instance. We suggest an acceleration technique and identify the sampling method that gives the best results based on experimentation.