Path Tracing Research Articles

Efficient Caustic Rendering with Lightweight Photon Mapping

AbstractRobust and efficient rendering of complex lighting effects, such as caustics, remains a challenging task. While algorithms like vertex connection and merging can render such effects robustly, their significant overhead over a simple path tracer is not always justified and – as we show in this paper ‐ also not necessary. In current rendering solutions, caustics often require the user to enable a specialized algorithm, usually a photon mapper, and hand‐tune its parameters. But even with carefully chosen parameters, photon mapping may still trace many photons that the path tracer could sample well enough, or, even worse, that are not visible at all.Our goal is robust, yet lightweight, caustics rendering. To that end, we propose a technique to identify and focus computation on the photon paths that offer significant variance reduction over samples from a path tracer. We apply this technique in a rendering solution combining path tracing and photon mapping. The photon emission is automatically guided towards regions where the photons are useful, i.e., provide substantial variance reduction for the currently rendered image. Our method achieves better photon densities with fewer light paths (and thus photons) than emission guiding approaches based on visual importance. In addition, we automatically determine an appropriate number of photons for a given scene, and the algorithm gracefully degenerates to pure path tracing for scenes that do not benefit from photon mapping.

Spectral Gradient Sampling for Path Tracing

AbstractSpectral Monte‐Carlo methods are currently the most powerful techniques for simulating light transport with wavelength‐dependent phenomena (e.g., dispersion, colored particle scattering, or diffraction gratings). Compared to trichromatic rendering, sampling the spectral domain requires significantly more samples for noise‐free images. Inspired by gradient‐domain rendering, which estimates image gradients, we propose spectral gradient sampling to estimate the gradients of the spectral distribution inside a pixel. These gradients can be sampled with a significantly lower variance by carefully correlating the path samples of a pixel in the spectral domain, and we introduce a mapping function that shifts paths with wavelength‐dependent interactions. We compute the result of each pixel by integrating the estimated gradients over the spectral domain using a one‐dimensional screened Poisson reconstruction. Our method improves convergence and reduces chromatic noise from spectral sampling, as demonstrated by our implementation within a conventional path tracer.

RenderMan

Pixar’s RenderMan renderer is used to render all of Pixar’s films and by many film studios to render visual effects for live-action movies. RenderMan started as a scanline renderer based on the Reyes algorithm, and it was extended over the years with ray tracing and several global illumination algorithms. This article describes the modern version of RenderMan, a new architecture for an extensible and programmable path tracer with many features that are essential to handle the fiercely complex scenes in movie production. Users can write their own materials using a bxdf interface and their own light transport algorithms using an integrator interface—or they can use the materials and light transport algorithms provided with RenderMan. Complex geometry and textures are handled with efficient multi-resolution representations, with resolution chosen using path differentials. We trace rays and shade ray hit points in medium-sized groups, which provides the benefits of SIMD execution without excessive memory overhead or data streaming. The path-tracing architecture handles surface, subsurface, and volume scattering. We show examples of the use of path tracing, bidirectional path tracing, VCM, and UPBP light transport algorithms. We also describe our progressive rendering for interactive use and our adaptation of denoising techniques.

Arnold

Arnold is a physically based renderer for feature-length animation and visual effects. Conceived in an era of complex multi-pass rasterization-based workflows struggling to keep up with growing demands for complexity and realism, Arnold was created to take on the challenge of making the simple and elegant approach of brute-force Monte Carlo path tracing practical for production rendering. Achieving this required building a robust piece of ray-tracing software that can ingest large amounts of geometry with detailed shading and lighting and produce images with high fidelity, while scaling well with the available memory and processing power. Arnold’s guiding principles are to expose as few controls as possible, provide rapid feedback to artists, and adapt to various production workflows. In this article, we describe its architecture with a focus on the design and implementation choices made during its evolutionary development to meet the aforementioned requirements and goals. Arnold’s workhorse is a unidirectional path tracer that avoids the use of hard-to-manage and artifact-prone caching and sits on top of a ray-tracing engine optimized to shoot and shade billions of spatially incoherent rays throughout a scene. A comprehensive API provides the means to configure and extend the system’s functionality, to describe a scene, render it, and save the results.

Manuka

The Manuka rendering architecture has been designed in the spirit of the classic reyes rendering architecture: to enable the creation of visually rich computer generated imagery for visual effects in movie production. Following in the footsteps of reyes over the past 30 years, this means supporting extremely complex geometry, texturing, and shading. In the current generation of renderers, it is essential to support very accurate global illumination as a means to naturally tie together different assets in a picture. This is commonly achieved with Monte Carlo path tracing, using a paradigm often called shade on hit , in which the renderer alternates tracing rays with running shaders on the various ray hits. The shaders take the role of generating the inputs of the local material structure, which is then used by path-sampling logic to evaluate contributions and to inform what further rays to cast through the scene. We propose a shade before hit paradigm instead and minimise I/O strain on the system, leveraging locality of reference by running pattern generation shaders before we execute light transport simulation by path sampling. We describe a full architecture built around this approach, featuring spectral light transport and a flexible implementation of multiple importance sampling ( mis ), resulting in a system able to support a comparable amount of extensibility to what made the reyes rendering architecture successful over many decades.

Molecular Dynamics Simulations on the Thermal Decomposition of Meta-Aramid Fibers.

The thermal decomposition mechanism of a meta-aramid fiber was simulated at the atomic level using the ReaxFF reactive force field. The simulation results indicated that the main initial decomposition positions of the meta-aramid fiber elements were Caromatic ring–N and C=O, which could be used as targets for the modification of meta-aramid fibers. The meta-aramid fiber elements first decomposed into C6–C13 and then into smaller segments and micromolecular gases. The temperature was shown to be the key factor affecting the thermal decomposition of the meta-aramid fibers. More complex compositions and stable gases were produced at high temperatures than at lower temperatures. HCN was a decomposition product at high temperature, suggesting that its presence could be used for detecting thermal faults in meta-aramid fibers. Generation path tracing of the thermal decomposition products NH3 and H2O was also performed. NH3 was produced when the NH2 group captured an H atom adjacent to the system. H2O was formed after a carbonyl group captured an H atom, became a hydroxyl group, with subsequent intramolecular dehydration or intermolecular hydrogen abstraction.

Multiple importance sampling characterization by weighted mean invariance

In this paper, we examine the linear combination of techniques and multiple importance sampling for Monte Carlo integration from a new perspective of quasi-arithmetic weighted means. The invariance property of these means allows us to define a new family of heuristics. We illustrate our results with several rendering examples, including environment mapping and path tracing.

Specialized Computer Systems for Environment Visualization

The need for real time image generation of landscapes arises in various fields as part of tasks solved by virtual and augmented reality systems, as well as geographic information systems. Such systems provide opportunities for collecting, storing, analyzing and graphically visualizing geographic data. Algorithmic and hardware software tools for increasing the realism and efficiency of the environment visualization in 3D visualization systems are proposed. This paper discusses a modified path tracing algorithm with a two-level hierarchy of bounding volumes and finding intersections with Axis-Aligned Bounding Box. The proposed algorithm eliminates the branching and hence makes the algorithm more suitable to be implemented on the multi-threaded CPU and GPU. A modified ROAM algorithm is used to solve the qualitative visualization of reliefs’ problems and landscapes. The algorithm is implemented on parallel systems—cluster and Compute Unified Device Architecture-networks. Results show that the implementation on MPI clusters is more efficient than Graphics Processing Unit/Graphics Processing Clusters and allows real-time synthesis. The organization and algorithms of the parallel GPU system for the 3D pseudo stereo image/video synthesis are proposed. With realizing possibility analysis on a parallel GPU-architecture of each stage, 3D pseudo stereo synthesis is performed. An experimental prototype of a specialized hardware-software system 3D pseudo stereo imaging and video was developed on the CPU/GPU. The experimental results show that the proposed adaptation of 3D pseudo stereo imaging to the architecture of GPU-systems is efficient. Also it accelerates the computational procedures of 3D pseudo-stereo synthesis for the anaglyph and anamorphic formats of the 3D stereo frame without performing optimization procedures. The acceleration is on average 11 and 54 times for test GPUs.

A detailed study of ray tracing performance: render time and energy cost

Optimizations for ray tracing have typically focused on decreasing the time taken to render each frame. However, in modern computer systems it may actually be more important to minimize the energy used, or some combination of energy and render time. Understanding the time and energy costs per ray can enable the user to make conscious trade-offs between image quality and time/energy budget in a complete system. To facilitate this, in this paper we present a detailed study of per-ray time and energy costs for ray tracing. Specifically, we use path tracing, broken down into distinct kernels, to carry out an extensive study of the fine-grained contributions in time and energy for each ray over multiple bounces. As expected, we have observed that both the time and energy costs are highly correlated with data movement. Especially in large scenes that do not mostly fit in on-chip caches, accesses to DRAM not only account for the majority of the energy use, but also the corresponding stalls dominate the render time.

Bidirectional Rendering of Vector Light Transport

AbstractOn the foundations of many rendering algorithms it is the symmetry between the path traversed by light and its adjoint path starting from the camera. However, several effects, including polarization or fluorescence, break that symmetry, and are defined only on the direction of light propagation. This reduces the applicability of bidirectional methods that exploit this symmetry for simulating effectively light transport. In this work, we focus on how to include these non‐symmetric effects within a bidirectional rendering algorithm. We generalize the path integral to support the constraints imposed by non‐symmetric light transport. Based on this theoretical framework, we propose modifications on two bidirectional methods, namely bidirectional path tracing and photon mapping, extending them to support polarization and fluorescence, in both steady and transient state.

Improving the Efficiency of GPGPU Work-Queue Through Data Awareness

The architecture and programming model of current GPGPUs are best suited for applications that are dominated by structured control and data flows across large regular datasets. Parallel workloads with irregular control and data structures cannot easily harness the processing power of the GPGPU. One approach for mapping these irregular-parallel workloads to GPGPUs is using work-queues. The work-queue approach improves the utilization of SIMD units by only processing useful works that are dynamically generated during execution. As current GPGPUs lack necessary supports for work-queues, a software-based work-queue implementation often suffers from memory contention and load balancing issues. In this article, we present a novel hardware work-queue design named DaQueue , which incorporates three data-aware features to improve the efficiency of work-queues on GPGPUs. We evaluate our proposal on the irregular-parallel workloads and carry out a case study on a path tracing pipeline with a cycle-level simulator. Experimental results show that for the tested workloads, DaQueue improves performance by 1.53× on average and up to 1.91×. Compared to a hardware worklist approach that is the state-of-the-art prior work, DaQueue can achieve an average of 33.92% extra speedup with less hardware area cost.

Fast rendering of fabric micro-appearance models under directional and spherical gaussian lights

Rendering fabrics using micro-appearance models---fiber-level microgeometry coupled with a fiber scattering model---can take hours per frame. We present a fast, precomputation-based algorithm for rendering both single and multiple scattering in fabrics with repeating structure illuminated by directional and spherical Gaussian lights. Precomputed light transport (PRT) is well established but challenging to apply directly to cloth. This paper shows how to decompose the problem and pick the right approximations to achieve very high accuracy, with significant performance gains over path tracing. We treat single and multiple scattering separately and approximate local multiple scattering using precomputed transfer functions represented in spherical harmonics. We handle shadowing between fibers with precomputed per-fiber-segment visibility functions, using two different representations to separately deal with low and high frequency spherical Gaussian lights. Our algorithm is designed for GPU performance and high visual quality. Compared to existing PRT methods, it is more accurate. In tens of seconds on a commodity GPU, it renders high-quality supersampled images that take path tracing tens of minutes on a compute cluster.

A BSSRDF model for efficient rendering of fur with global illumination

Physically-based hair and fur rendering is crucial for visual realism. One of the key effects is global illumination, involving light bouncing between different fibers. This is very time-consuming to simulate with methods like path tracing. Efficient approximate global illumination techniques such as dual scattering are in widespread use, but are limited to human hair only, and cannot handle color bleeding, transparency and hair-object inter-reflection. We present the first global illumination model, based on dipole diffusion for subsurface scattering, to approximate light bouncing between individual fur fibers. We model complex light and fur interactions as subsurface scattering, and use a simple neural network to convert from fur fibers' properties to scattering parameters. Our network is trained on only a single scene with different parameters, but applies to general scenes and produces visually accurate appearance, supporting color bleeding and further inter-reflections.

Microfacet-based normal mapping for robust Monte Carlo path tracing

Normal mapping enhances the amount of visual detail of surfaces by using shading normals that deviate from the geometric normal. However, the resulting surface model is geometrically impossible and normal mapping is thus often considered a fundamentally flawed approach with unavoidable problems for Monte Carlo path tracing, such as asymmetry, back-facing normals, and energy loss arising from this incoherence. These problems are usually sidestepped in real-time renderers, but they cannot be fixed robustly in a path tracer: normal mapping breaks either the appearance (black fringes, energy loss) or the integrator (different forward and backward light transport); in practice, workarounds and tweaked normal maps are often required to hide artifacts. We present microfacet-based normal mapping, an alternative way of faking geometric details without corrupting the robustness of Monte Carlo path tracing. It takes the same input data as classic normal mapping and works with any input BRDF. Our idea is to construct a geometrically valid microfacet surface made of two facets per shading point: the one given by the normal map at the shading point and an additional facet that compensates for it such that the average normal of the microsurface equals the geometric normal. We derive the resulting microfacet BRDF and show that it mimics geometric detail in a plausible way, although it does not replicate the appearance of classic normal mapping. However, our microfacet-based normal mapping model is well-defined, symmetric, and energy conserving, and thus yields identical results with any path tracing algorithm (forward, backward, or bidirectional).

Gradient-domain path reusing

Monte-Carlo rendering algorithms have traditionally a high computational cost, because they rely on tracing up to billions of light paths through a scene to physically simulate light transport. Traditional path reusing amortizes the cost of path sampling over multiple pixels, but introduces visually unpleasant correlation artifacts and cannot handle scenes with specular light transport. We present gradient-domain path reusing , a novel unbiased Monte-Carlo rendering technique, which merges the concept of path reusing with the recently introduced idea of gradient-domain rendering. Since correlation is a key element in gradient sampling, it is a natural fit to be performed together with path reusing and we show that the typical artifacts of path reusing are significantly reduced by exploiting the gradient domain. Further, by employing the tools for shifting paths that were designed in the context of gradient-domain rendering over the last years, we can generalize path reusing to support arbitrary scenes including specular light transport. Our method is unbiased and currently the fastest converging unidirectional rendering technique outperforming conventional and gradient-domain path tracing by up to almost an order of magnitude.

Composite power system reliability evaluation using modified minimal cut set approach

The composite power system reliability analysis is generally based on minimal path or cut enumeration, tracing of power flow paths from which the related reliability indices are calculated. The minimal cut set is a popular method in the reliability analysis for simple and complex configurations. Average availability of power supply at the consumer end is one of the reliability assessment parameter. This paper is concerned about the evaluation of this reliability index. A step by step procedure for a modified minimal cut set method is explained in this paper using IEEE 6 bus, 14 bus and Single area IEEE RTS 96 system. The proposed algorithm is easy to program and can be applicable to any system. The proposed algorithm is validated with the Classical Node Elimination method, Step by Step algorithm using Conditional Probability and Monte Carlo Simulation method. The proposed technique is tested with a practical example taken from Roy Billinton paper (Reliability evaluation in distribution and transmission systems).

Gateway selection and clustering in multi‐interface wireless mesh networks considering network reliability and traffic

AbstractThis work considers the gateway selection and clustering problem in a multi‐interface wireless mesh network. The evolved reliability and traffic‐aware gateway selection scheme is here introduced to increase the performance in terms of throughput. There are 2 main phases in the proposed idea. In the first step, some Internet Gateway Candidates are selected from the mesh nodes in the network, based on the network traffic. Then, in the second step, using the path tracing method, the best of these candidates are selected as Internet gateways. Moreover, to decrease the network energy consumption, a refined evolved reliability and traffic‐aware gateway selection scheme is also proposed whose effect in simulation is shown. A clustering method is later proposed exploiting the genetic algorithm to give priority to the nodes with the shortest hop to connect to the cluster head. Simulation results demonstrate how our gateway selection and clustering scheme outperforms 2 successful approaches in terms of throughput and network energy consumption.

Aether

Implementing Monte Carlo integration requires significant domain expertise. While simple samplers, such as unidirectional path tracing, are relatively forgiving, more complex algorithms, such as bidirectional path tracing or Metropolis methods, are notoriously difficult to implement correctly. We propose Aether, an embedded domain specific language for Monte Carlo integration, which offers primitives for writing concise and correct-by-construction sampling and probability code. The user is tasked with writing sampling code, while our compiler automatically generates the code necessary for evaluating PDFs as well as the book keeping and combination of multiple sampling strategies. Our language focuses on ease of implementation for rapid exploration, at the cost of run time performance. We demonstrate the effectiveness of the language by implementing several challenging rendering algorithms as well as a new algorithm, which would otherwise be prohibitively difficult.

Lighting grid hierarchy for self-illuminating explosions

Rendering explosions with self-illumination is a challenging problem. Explosions contain animated volumetric light sources immersed in animated smoke that cast volumetric shadows, which play an essential role and are expensive to compute. We propose an efficient solution that redefines this problem as rendering with many animated lights by converting the volumetric lighting data into a large number of point lights. Focusing on temporal coherency to avoid flickering in animations, we introduce lighting grid hierarchy for approximating the volumetric illumination at different resolutions. Using this structure we can efficiently approximate the lighting at any point inside or outside of the explosion volume as a mixture of lighting contributions from all levels of the hierarchy. As a result, we are able to capture high-frequency details of local illumination, as well as the potentially strong impact of distant illumination. Most importantly, this hierarchical structure allows us to efficiently precompute volumetric shadows, which substantially accelerates the lighting computation. Finally, we provide a scalable approach for computing the multiple scattering of light within the smoke volume using our lighting grid hierarchy. Temporal coherency is achieved by relying on continuous formulations at all stages of the lighting approximation. We show that our method is efficient and effective approximating the self-illumination of explosions with visually indistinguishable results, as compared to path tracing. We also show that our method can be applied to other problems involving a large number of (animated) point lights.

Point-to-source path tracing Monte Carlo to compute the Clausing and distribution functions in high-vacuum systems

A numerical method for the Boltzmann transport equation in ultrahigh-vacuum systems is presented in which molecular collisions are neglected, which improves accuracy and convergence time. The method is used to compute particle density in a conical segment for which a density drop is observed at the entrance.