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Item Axis-Normalized Ray-Box Intersection(The Eurographics Association and John Wiley & Sons Ltd., 2025) Friederichs, Fabian; Benthin, Carsten; Grogorick, Steve; Eisemann, Elmar; Magnor, Marcus; Eisemann, Martin; Bousseau, Adrien; Day, AngelaRay-axis aligned bounding box intersection tests play a crucial role in the runtime performance of many rendering applications, driven not by complexity but mainly by the volume of tests required. While existing solutions were believed to be pretty much optimal in terms of runtime on current hardware, our paper introduces a new intersection test requiring fewer arithmetic operations compared to all previous methods. By transforming the ray we eliminate the need for one third of the traditional bounding-slab tests and achieve a speed enhancement of approximately 13.8% or 10.9%, depending on the compiler.We present detailed runtime analyses in various scenarios.Item Linearly Transformed Spherical Distributions for Interactive Single Scattering with Area Lights(The Eurographics Association and John Wiley & Sons Ltd., 2025) Kt, Aakash; Shah, Ishaan; Narayanan, P. J.; Bousseau, Adrien; Day, AngelaSingle scattering in scenes with participating media is challenging, especially in the presence of area lights. Considerable variance still remains, in spite of good importance sampling strategies. Analytic methods that render unshadowed surface illumination have recently gained interest since they achieve biased but noise-free plausible renderings while being computationally efficient. In this work, we extend the theory of Linearly Transformed Spherical Distributions (LTSDs) which is a well-known analytic method for surface illumination, to work with phase functions. We show that this is non-trivial, and arrive at a solution with in-depth analysis. This enables us to analytically compute in-scattered radiance, which we build on to semi-analytically render unshadowed single scattering. We ground our derivations and formulations on the Volume Rendering Equation (VRE) which paves the way for realistic renderings despite the biased nature of our method. We also formulate ratio estimators for the VRE to work in conjunction with our formulation, enabling the rendering of shadows. We extensively validate our method, analyze its characteristics and demonstrate better performance compared to Monte Carlo single-scattering.Item Neural Two-Level Monte Carlo Real-Time Rendering(The Eurographics Association and John Wiley & Sons Ltd., 2025) Dereviannykh, Mikhail; Klepikov, Dmitrii; Hanika, Johannes; Dachsbacher, Carsten; Bousseau, Adrien; Day, AngelaWe introduce an efficient Two-Level Monte Carlo (subset of Multi-Level Monte Carlo, MLMC) estimator for real-time rendering of scenes with global illumination. Using MLMC we split the shading integral into two parts: the radiance cache integral and the residual error integral that compensates for the bias of the first one. For the first part, we developed the Neural Incident Radiance Cache (NIRC) leveraging the power of tiny neural networks [MRNK21] as a building block, which is trained on the fly. The cache is designed to provide a fast and reasonable approximation of the incident radiance: an evaluation takes 2-25× less compute time than a path tracing sample. This enables us to estimate the radiance cache integral with a high number of samples and by this achieve faster convergence. For the residual error integral, we compute the difference between the NIRC predictions and the unbiased path tracing simulation. Our method makes no assumptions about the geometry, materials, or lighting of a scene and has only few intuitive hyper-parameters. We provide a comprehensive comparative analysis in different experimental scenarios. Since the algorithm is trained in an on-line fashion, it demonstrates significant noise level reduction even for dynamic scenes and can easily be combined with other noise reduction techniques.Item Adaptive Multi-view Radiance Caching for Heterogeneous Participating Media(The Eurographics Association and John Wiley & Sons Ltd., 2025) Stadlbauer, Pascal; Tatzgern, Wolfgang; Mueller, Joerg H.; Winter, Martin; Stojanovic, Robert; Weinrauch, Alexander; Steinberger, Markus; Bousseau, Adrien; Day, AngelaAchieving lifelike atmospheric effects, such as fog, is essential in creating immersive environments and poses a formidable challenge in real-time rendering. Highly realistic rendering of complex lighting interacting with dynamic fog can be very resourceintensive, due to light bouncing through a complex participating media multiple times. We propose an approach that uses a multi-layered spherical harmonics probe grid to share computations temporarily. In addition, this world-space storage enables the sharing of radiance data between multiple viewers. In the context of cloud rendering this means faster rendering and a significant enhancement in overall rendering quality with efficient resource utilization.Item Inverse Simulation of Radiative Thermal Transport(The Eurographics Association and John Wiley & Sons Ltd., 2025) Freude, Christian; Lipp, Lukas; Zezulka, Matthias; Rist, Florian; Wimmer, Michael; Hahn, David; Bousseau, Adrien; Day, AngelaThe early phase of urban planning and architectural design has a great impact on the thermal loads and characteristics of constructed buildings. It is, therefore, important to efficiently simulate thermal effects early on and rectify possible problems. In this paper, we present an inverse simulation of radiative heat transport and a differentiable photon-tracing approach. Our method utilizes GPU-accelerated ray tracing to speed up both the forward and adjoint simulation. Moreover, we incorporate matrix compression to further increase the efficiency of our thermal solver and support larger scenes. In addition to our differentiable photon-tracing approach, we introduce a novel approximate edge sampling scheme that re-uses primary samples instead of relying on explicit edge samples or auxiliary rays to resolve visibility discontinuities. Our inverse simulation system enables designers to not only predict the temperature distribution, but also automatically optimize the design to improve thermal comfort and avoid problematic configurations. We showcase our approach using several examples in which we optimize the placement of buildings or their facade geometry. Our approach can be used to optimize arbitrary geometric parameterizations and supports steady-state, as well as transient simulations.Item UBVH: Unified Bounding Volume and Scene Geometry Representation for Ray Tracing(The Eurographics Association and John Wiley & Sons Ltd., 2025) Kácerik, Martin; Bittner, Jirí; Knoll, Aaron; Peters, ChristophBounding volume hierarchies (BVHs) are currently the most common data structure used to accelerate ray tracing. The existing BVH methods distinguish between the bounding volume representation associated with the interior BVH nodes and the scene geometry representation associated with leaf nodes. We propose a new method that unifies the representation of bounding volumes and triangular scene geometry. Our unified representation builds on skewed oriented bounding boxes (SOBB) that yield tight bounds for interior nodes and precise representation for triangles in the leaf nodes. This innovation allows to streamline the conventional massively parallel BVH traversal, as there is no need to switch between testing for ray intersection in interior nodes and leaf nodes. The results show that the proposed method accelerates ray tracing of incoherent rays between 1.2x-11.8x over the AABB BVH, 1.4x-4.2x over the 14-DOP BVH, 1.1x-2.0x over the OBB BVH, and by 1.1x-1.7x over the SOBB BVH.Item Fused Collapsing for Wide BVH Construction(The Eurographics Association and John Wiley & Sons Ltd., 2025) Barbier, Wilhem; Paulin, Mathias; Knoll, Aaron; Peters, ChristophWe propose a novel approach for constructing wide bounding volume hierarchies on the GPU by integrating a simple bottom-up collapsing procedure within an existing binary bottom-up BVH builder. Our approach directly constructs a wide BVH without traversing a temporary binary BVH as done by previous approaches and achieves 1.4−1.6× lower build times. We demonstrate the ability of our algorithm to output compressed wide BVHs using existing compressed representations. We analyze the impact of our method on software raytracing performance and show that it reduces the overall frame time on complex dynamic scenes where rebuilding the BVH every frame is the limiting factor on rendering performance.Item SOBB: Skewed Oriented Bounding Boxes for Ray Tracing(The Eurographics Association and John Wiley & Sons Ltd., 2025) Kácerik, Martin; Bittner, Jirí; Bousseau, Adrien; Day, AngelaWe propose skewed oriented bounding boxes (SOBB) as a novel bounding primitive for accelerating the calculation of rayscene intersections. SOBBs have the same memory footprint as the well-known oriented bounding boxes (OBB) and can be used with a similar ray intersection algorithm. We propose an efficient algorithm for constructing a BVH with SOBBs, using a transformation from a standard BVH built for axis-aligned bounding boxes (AABB). We use discrete orientation polytopes as a temporary bounding representation to find tightly fitting SOBBs. Additionally, we propose a compression scheme for SOBBs that makes their memory requirements comparable to those of AABBs. For secondary rays, the SOBB BVH provides a ray tracing speedup of 1.0-11.0x over the AABB BVH and it is 1.1x faster than the OBB BVH on average. The transformation of AABB BVH to SOBB BVH is, on average, 2.6x faster than the ditetrahedron-based AABB BVH to OBB BVH transformation.Item A Data-Driven Approach to Analytical Dwivedi Guiding(The Eurographics Association and John Wiley & Sons Ltd., 2025) Gouder, Darryl; Vorba, Jirí; Droske, Marc; Wilkie, Alexander; Wang, Beibei; Wilkie, AlexanderPath tracing remains the gold standard for high-fidelity subsurface scattering despite requiring numerous paths for noisefree estimates. We introduce a novel variance-reduction method based on two complementary zero-variance-theory-based approaches. The first one, analytical Dwivedi sampling, is lightweight but struggles with complex lighting. The second one, surface path guiding, learns incident illumination at boundaries to guide sampled paths, but it does not reduce variance from subsurface scattering. In our novel method, we enhance Dwivedi sampling by incorporating the radiance field learned only at the volume boundary. We use the average normal of points on an illuminated boundary region or directions sampled from distributions of incident light at the boundary as our analytical Dwivedi slab normals. Unlike previous methods based on Dwivedi sampling, our method is efficient even in scenes with complex light rigs typical for movie production and under indirect illumination. We achieve comparable noise reduction and even slightly improved estimates in some scenes compared to volume path guiding, and our method can be easily added on top of any existing surface path guiding system. Our method is particularly effective for homogeneous, isotropic media, bypassing the extensive training and caching inside the 3D volume that volume path guiding requires.