Jan Novák

Scalable Realistic Rendering with Many-Light Methods

Recent years have seen increasing attention and significant progress in many‐light rendering, a class of methods for efficient computation of global illumination. The many‐light formulation offers a unified mathematical framework for the problem reducing the full lighting transport simulation to the calculation of the direct illumination from many virtual light sources. These methods are unrivaled in their scalability: they are able to produce plausible images in a fraction of a second but also converge to the full solution over time. In this state‐of‐the‐art report, we give an easy‐to‐follow, introductory tutorial of the many‐light theory; provide a comprehensive, unified survey of the topic with a comparison of the main algorithms; discuss limitations regarding materials and light transport phenomena and present a vision to motivate and guide future research. We will cover both the fundamental concepts as well as improvements, extensions and applications of many‐light rendering.

Virtual ray lights for rendering scenes with participating media

We present an efficient many-light algorithm for simulating indirect illumination in, and from, participating media. Instead of creating discrete virtual point lights (VPLs) at vertices of random-walk paths, we present a continuous generalization that places virtual ray lights (VRLs) along each path segment in the medium. Furthermore, instead of evaluating the lighting independently at discrete points in the medium, we calculate the contribution of each VRL to entire camera rays through the medium using an efficient Monte Carlo product sampling technique. We prove that by spreading the energy of virtual lights along both light and camera rays, the singularities that typically plague VPL methods are significantly diminished. This greatly reduces the need to clamp energy contributions in the medium, leading to robust and unbiased volumetric lighting not possible with current many-light techniques. Furthermore, by acting as a form of final gather, we obtain higher-quality multiple-scattering than existing density estimation techniques like progressive photon beams.

Progressive Virtual Beam Lights

A recent technique that forms virtual ray lights (VRLs) from path segments in media, reduces the artifacts common to VPL approaches in participating media, however, distracting singularities still remain. We present Virtual Beam Lights (VBLs), a progressive many‐lights algorithm for rendering complex indirect transport paths in, from, and to media. VBLs are efficient and can handle heterogeneous media, anisotropic scattering, and moderately glossy surfaces, while provably converging to ground truth. We inflate ray lights into beam lights with finite thicknesses to eliminate the remaining singularities. Furthermore, we devise several practical schemes for importance sampling the various transport contributions between camera rays, light rays, and surface points. VBLs produce artifact‐free images faster than VRLs, especially when glossy surfaces and/or anisotropic phase functions are present. Lastly, we employ a progressive thickness reduction scheme for VBLs in order to render results that converge to ground truth.

Path Regeneration for Interactive Path Tracing

Rendering of photo-realistic images at interactive frame rates is currently a n extensively researched area of computer graphics. Many of these approaches attempt to utilize the computation al power of modern graphics hardware for ray tracing based methods. When using path tracing algorithms the ray p aths are highly incoherent, hence we propose an efficient technique that minimizes the divergence in executionflow and ensures full utilization by intelligently regenerating the paths. We analyze the conditions under which our improvements provide the highest speedup, and demonstrate the performance of the overall system by re ndering interactive previews of global illumination solutions using (bidirectional) path tracing with progressive refinem ent.

Path-space manipulation of physically-based light transport

Industry-quality content creation relies on tools for lighting artists to quickly prototype, iterate, and refine final renders. As industry-leading studios quickly adopt physically-based rendering (PBR) across their art generation pipelines, many existing tools have become unsuitable as they address only simple effects without considering underlying PBR concepts and constraints. We present a novel light transport manipulation technique that operates directly on path-space solutions of the rendering equation. We expose intuitive direct and indirect manipulation approaches to edit complex effects such as (multi-refracted) caustics, diffuse and glossy indirect bounces, and direct/indirect shadows. With our sketch- and object-space selection, all built atop a parameterized regular expression engine, artists can search and isolate shading effects to inspect and edit. We classify and filter paths on the fly and visualize the selected transport phenomena. We survey artists who used our tool to manipulate complex phenomena on both static and animated scenes.

Screen-space bias compensation for interactive high-quality global illumination with virtual point lights

In this paper we present a method that targets high-quality global illumination at interactive frame rates. As many techniques in this context, our method is based on instant radiosity, which represents the indirect illumination in a scene with a set of virtual point lights, and therefore enables efficient GPU rendering. Instant radiosity captures light transport over larger distances well, but it requires clamping of the point lights contribution to avoid bright splotches on nearby surfaces. By bounding the short distance light transport, the algorithm removes some energy and thus introduces bias, that is visible as incorrect darkening near edges and corners. Our method improves the quality and correctness of the rendered images by removing bias using a hierarchical screen space approach. We show that the bias compensation can be formulated as a post-processing step and demonstrate renderings comparable to results from offline algorithms at interactive speed.

Rasterized Bounding Volume Hierarchies

We present the rasterized bounding volume hierarchy (RBVH), a compact data structure that accelerates approximate ray casting of complex meshes and provides adjustable level of detail. During construction, we identify subtrees of BVHs containing surfaces that can be represented by height fields. For these subtrees the conventional ray‐surface intersection, which possibly involves a large number of triangles, is replaced by a simple ray marching procedure to find the intersection with the surface. We describe GPU algorithms for construction, ray casting, and data querying of the RBVH that achieve comparable or higher performance than state of the art acceleration structures for triangle meshes. Moreover, RBVHs provide an inherent surface parameterization for storing data on the surfaces and natively handle triangle and point‐based surface representations. We also show that RBVHs support adaptive level‐of‐detail and can be combined with traditional BVHs to handle complex scenes.

Approximate Bias Compensation for Rendering Scenes with Heterogeneous Participating Media

In this paper we present a novel method for high‐quality rendering of scenes with participating media. Our technique is based on instant radiosity, which is used to approximate indirect illumination between surfaces by gathering light from a set of virtual point lights (VPLs). It has been shown that this principle can be applied to participating media as well, so that the combined single scattering contribution of VPLs within the medium yields full multiple scattering. As in the surface case, VPL methods for participating media are prone to singularities, which appear as bright “splotches” in the image. These artifacts are usually countered by clamping the VPLs contribution, but this leads to energy loss within the short‐distance light transport. Bias compensation recovers the missing energy, but previous approaches are prohibitively costly. We investigate VPL‐based methods for rendering scenes with participating media, and propose a novel and efficient approximate bias compensation technique. We evaluate our technique using various test scenes, showing it to be visually indistinguishable from ground truth.

Path Regeneration for Random Walks

Publisher Summary This chapter presents a method for efficiently generating random walks on the GPU. It analyzes the main drawback of naive random walk generators resulting in a low GPU utilization over time, and proposes an intuitive scheme for keeping all the processing units busy during the entire computation. A random walk is a Markov chain describing a trajectory of a walker that takes a number of successive random steps, thus forming a path through the domain. To sufficiently explore the domain, vast number of such paths needs to be computed. Even worse, the length of the walk is generally not known prior to execution because the termination criteria are probabilistic. A well-known method for rendering photo-realistic images that suffers from this problem is path tracing. The algorithm traces a high number of random walks from the camera in order to determine the color of each pixel in the image. The algorithm does not require interthread communication, collective operations, or intricate handling of work queues. Instead, the improved utilization is achieved by intelligently regenerating terminated walks. Optimization in the context of rendering global illumination images where random walks are used to compute the propagation of energy between light sources and cameras are discussed. Algorithms such as (bidirectional) path tracing, photon mapping, and irradiance caching directly benefit from the higher throughput; however, our technique is also applicable to nongraphical problems that explore the domain of interest by random walks.

Progressive Visibility Caching for Fast Indirect Illumination

Rendering realistic images requires exploring the vast space of all possible paths that light can take between emitters and receivers. Thanks to the advances in rendering we can tackle this problem using different algorithms; but in general, we will likely be evaluating many expensive visibility queries. In this paper, we leverage the observation that certain kinds of visibility calculations do not need to be resolved exactly and results can be shared efficiently among similar queries. We present a visibility caching algorithm that significantly accelerates computation of diffuse and glossy inter-reflections. By estimating the visibility correlation between surface points, the cache automatically adapts to the scene geometry, placing more cache records in areas with rapidly changing visibility. We demonstrate that our approach is most suitable for progressive algorithms delivering approximate but fast previews as well as high quality converged results.