Realistic fabric rendering is still a significant challenge due to their complex structures and varying fiber properties. We present a new fabric shading technique, which models both reflection and transmission using a hybrid of ray and wave optics methods, grounded in simulation data. We target fabrics woven from yarns, each formed by twisting together one or more plies, which further contain twisted fibers. Our model is based on simulations that predict the scattering of a narrow Gaussian beam by a single ply. Comparing results from full-wave simulations and path tracing, we found that ray optics can accurately simulate the average far field scattering from an ensemble of plies, but not the variation among individual ply instances, and ray tracing overlooks important diffraction effects. Following these observations, our model is built from ray simulations performed for many ply instances, with simulation data fitted by Gaussian mixtures to be used during rendering. Wave simulations are used to calibrate noise functions that account for instance-to-instance variation, and an aperture diffraction model is used to handle light passing between plies and yarns. The result is a hybrid model capable of producing realistic appearance and highlight structure in fabrics, while capturing spatial break-ups and irregularities and simulating the subtle color shifts and blurriness that occur in transmission. We validate our results by comparing rendered images with photographs, demonstrating the effectiveness of our approach in achieving realistic cloth rendering.

Abstract We present a practical analytic BRDF that approximates scattering from a generalized microfacet volume with a von Mises‐Fischer NDF. Our BRDF seamlessly blends from smooth Lambertian, through moderately rough height fields with Beckmann‐like statistics and into highly rough/porous behaviours that have been lacking from prior models. At maximum roughness, our model reduces to the recent Lambert‐sphere BRDF. We validate our model by comparing to simulations of scattering from geometries with randomly‐placed Lambertian spheres and show an improvement relative to a rough Beckmann BRDF with very high roughness.

Many animals exhibit structural colors, which are often iridescent, meaning that the perceived colors change with illumination conditions and viewing perspectives. Biological iridescence is usually caused by multilayers or other periodic structures in animal tissues, which selectively reflect light of certain wavelengths and often result in a shiny appearance---which almost always comes with spatially varying highlights, thanks to randomness and irregularities in the structures. Previous models for biological iridescence tend to each target one specific structure, and most models only compute large-area averages, overlooking spatial variation in iridescent appearance. In this work, we build appearance models for biological iridescence using bird feathers as our case study, investigating different types of feathers with a variety of structural coloration mechanisms. We propose an approximate wave simulation method that takes advantage of quasi-regular structures while efficiently modeling the effects of natural structural irregularities. We further propose a method to distill our simulation results into distributions of BRDFs, generated using noise functions, that preserve relevant statistical properties of the simulated BRDFs. This allows us to model the spatially varying, glittery appearance commonly seen on feathers. Our BRDFs are practical and efficient, and we present renderings of multiple types of iridescent feathers with comparisons to photographic images.

We present a complete system for real-time rendering of scenes with complex appearance previously reserved for offline use. This is achieved with a combination of algorithmic and system level innovations. Our appearance model utilizes learned hierarchical textures that are interpreted using neural decoders, which produce reflectance values and importance-sampled directions. To best utilize the modeling capacity of the decoders, we equip the decoders with two graphics priors. The first prior—transformation of directions into learned shading frames—facilitates accurate reconstruction of mesoscale effects. The second prior—a microfacet sampling distribution—allows the neural decoder to perform importance sampling efficiently. The resulting appearance model supports anisotropic sampling and level-of-detail rendering, and allows baking deeply layered material graphs into a compact unified neural representation. By exposing hardware accelerated tensor operations to ray tracing shaders, we show that it is possible to inline and execute the neural decoders efficiently inside a real-time path tracer. We analyze scalability with increasing number of neural materials and propose to improve performance using code optimized for coherent and divergent execution. Our neural material shaders can be over an order of magnitude faster than non-neural layered materials. This opens up the door for using film-quality visuals in real-time applications such as games and live previews.

Abstract Realistic cloth rendering is a longstanding challenge in computer graphics due to the intricate geometry and hierarchical structure of cloth: Fibers form plies which in turn are combined into yarns which then are woven or knitted into fabrics. Previous fiber‐based models have achieved high‐quality close‐up rendering, but they suffer from high computational cost, which limits their practicality. In this paper, we propose a novel hierarchical model that analytically aggregates light simulation on the fiber level by building on dual‐scattering theory. Based on this, we can perform an efficient simulation of ply and yarn shading. Compared to previous methods, our approach is faster and uses less memory while preserving a similar accuracy. We demonstrate both through comparison with existing fiber‐based shading models. Our yarn shading model can be applied to curves or surfaces, making it highly versatile for cloth shading. This duality paired with its simplicity and flexibility makes the model particularly useful for film and games production.

Abstract We present a Monte Carlo path tracing technique to sample extended next event estimation contributions in participating media: we consider one additional scattering vertex on the way to the next event, accounting for focused blur, resulting in visually interesting image features. Our technique is tailored to thin homogeneous media with strongly forward scattering phase functions, such as water or atmospheric haze. Previous methods put emphasis on sampling transmittances or geometric factors, and are either limited to isotropic scattering, or used tabulation or polynomial approximation to account for some specific phase functions. We will show how to jointly importance sample the product of an arbitrary phase function with analytic sampling in the solid angle domain and the two reciprocal squared distance terms of the adjacent edges of the transport path. The technique is fast and simple to implement in an existing rendering system. Our estimator is designed specifically for forward scattering, so the new technique has to be combined with other estimators to cover the backward scattering contributions.

Abstract We present Optimised Path Space Regularisation (OPSR), a novel regularisation technique for forward path tracing algorithms. Our regularisation controls the amount of roughness added to materials depending on the type of sampled paths and trades a small error in the estimator for a drastic reduction of variance in difficult paths, including indirectly visible caustics. We formulate the problem as a joint bias‐variance minimisation problem and use differentiable rendering to optimise our model. The learnt parameters generalise to a large variety of scenes irrespective of their geometric complexity. The regularisation added to the underlying light transport algorithm naturally allows us to handle the problem of near‐specular and glossy path chains robustly. Our method consistently improves the convergence of path tracing estimators, including state‐of‐the‐art path guiding techniques where it enables finding otherwise hard‐to‐sample paths and thus, in turn, can significantly speed up the learning of guiding distributions.

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.

Abstract We present a spectral rendering technique that offers a compelling set of advantages over existing approaches. The key idea is to propagate energy along paths for a small, constant number of changing wavelengths. The first of these, the hero wavelength, is randomly sampled for each path, and all directional sampling is solely based on it. The additional wavelengths are placed at equal distances from the hero wavelength, so that all path wavelengths together always evenly cover the visible range. A related technique, spectral multiple importance sampling, was already introduced a few years ago. We propose a simplified and optimised version of this approach which is easier to implement, has good performance characteristics, and is actually more powerful than the original method. Our proposed method is also superior to techniques which use a static spectral representation, as it does not suffer from any inherent representation bias. We demonstrate the performance of our method in several application areas that are of critical importance for production work, such as fidelity of colour reproduction, sub‐surface scattering, dispersion and volumetric effects. We also discuss how to couple our proposed approach with several technologies that are important in current production systems, such as photon maps, bidirectional path tracing, environment maps, and participating media.

In Computer Graphics, the polarisation properties of light currently play a role in several contexts: in certain forms of highly realistic ray-based image synthesis (sometimes colloquially referred to as Polarisation Ray Tracing), in some 3D display systems, and in some material acquisition technologies. The properties of light that are behind all of these applications are basically the same, although the technologies for which this property of light is being used differ considerably. Also, the notations and mathematical formalisms used in these application areas differ to some degree as well. This course aims to provide a unified resource for those areas of computer graphics which require a working knowledge of light polarisation: rendering and material acquisition. Consequently, the course is structured into three main parts: I - Background, II - Polarisation Ray Tracing, and III - Polarised Light in Acquisition Technology. Care is taken so that the information provided in Part I is applicable to both Part II and III of the course, and is formulated in a way that emphasises the underlying similarities.

This course serves as a guide to the considerable potential of layered surface models that are available in many commercial products. The key advantage of using such layered materials over traditional shading language constructs is that the end result is highly physically plausible because it simulates real materials more precisely. However, this does not mean that these models cannot be used for artistic purposes. Using simple layered surface models, we demonstrate how a surprisingly large number of interesting and important surface types can be efficiently represented. We also show how handy such an approach is for the end-user, whose main concern is the ease with which one can describe object appearance based only on a few intuitive parameters. We first discuss layered surface models in general and the constraints of modeling object appearance in a physically plausible fashion by explaining basic material properties. We then demonstrate the techniques that are used to analyze such materials, both for high quality offline rendering as well as in a realtime setting. We then give examples of the surface types that can be described in this way and demonstrate how we create them in our company.

Polarisation Ray Tracing is a bit like spectral rendering. You might have heard about it, but the technical details - what exactly it is all about, how it is best done, what the benefits are, where you ought to use it, and where you can safely omit it - are not that well known. At least in principle, information about all this is available already - but it is scattered across many different sources, some of which are outside graphics literature, and consequently can be a bit hard to find at first. Also, a number of these resources - in particular publications in physics literature - were not written with graphics engineering in mind, and can prove hard to directly apply to the engineering problem of getting such a Tenderer to work. This course aims to provide a unified one-stop information resource on this topic, and should enable graphics engineers with a background in physically-based rendering to properly assess whether the phenomenon has to be included in a given simulation, and what has to be done in order to properly allow for handling of the phenomenon in a ray-based Tenderer. However, the contents of the course are also potentially very relevant for other areas of graphics beyond rendering proper: polarised light is currently being used in several other contexts, such as face scanning, highlight removal, or 3D projection technologies. Engineers and researchers in these other areas might benefit from the course insofar as the knowledge needed for an outright simulation of polarised light transport presented here is a superset of the information needed for more specialised tasks. This comprehensive overview would thus be a useful repository of knowledge for other areas as well.

Abstract The emissive properties of glowing solid objects appear to be something that the graphics community has not considered in depth before. While the volumetric emission of plasma, i.e. flames, has been discussed numerous times, and while the emission characteristics of entire luminaires can be handled via IESNA profiles, the exact appearance of glowing solid objects appears to have eluded detailed scrutiny so far. In this paper, we discuss the theoretical background to thermally induced light emission of objects, describe how one can handle this behaviour with very little effort in a physically based rendering system, and provide examples for the visual importance of handling this in a plausible fashion.

Skeletal implicit modelling systems have been used to design models of both organic and man-made structures, however existing systems lack convenient and accurate methods for users to define fillets when building prototype engineering models. In this work we extend the methodology and skeletal primitives found in the BlobTree and introduce an improved method for modelling fillets. This is done by interpolating between hard-edged and soft-edged representations of a primitive, using the field to control the interpolation. Blending planes are introduced as a way of providing user control for this interpolation. The new methods are expressed as blend operators between two objects and can thus be implemented as a new node in the BlobTree. In addition we introduce an efficient methodology for modelling hard edged primitives.

Abstract In coloured media, the index of refraction does not decrease monotonically with increasing wavelength, but behaves in a quite non‐monotonical way. This behaviour is called anomalous dispersion and results from the fact that the absorption of a material influences its index of refraction. So far, this interesting fact has not been widely acknowledged by the graphics community. In this paper, we demonstrate how to calculate the correct refractive index for a material based on its absorption spectrum with the Kramers‐Kronig relation, and we discuss for which types of objects this effect is relevant in practice.

Abstract We propose a method that improves automatic colour correction operations for rendered images. In particular, we propose a robust technique for estimating the visible and pertinent illumination in a given scene. We do this at very low computational cost by mostly re‐using information that is already being computed during the image synthesis process. Conventional illuminant estimations either operate only on 2D image data, or, if they do go beyond pure image analysis, only use information on the luminaires found in the scene. The latter is usually done with little or no regard for how the light sources actually affect the part of the scene that is being viewed. Our technique goes beyond that, and also takes object reflectance into account, as well as the incident light that is actually responsible for the colour of the objects that one sees. It is therefore able to cope with difficult cases, such as scenes with mixed illuminants, complex scenes with many light sources of varying colour, or strongly coloured indirect illumination.

In this article we derive the complete set of formulas needed to generate physically plausible images of uniaxial crystals. So far no computer graphics publication contains all the formulas one needs to compute the interaction of light with such crystals in a form that is useable by a graphics application, especially if a polarization-aware rendering system is being used. This paper contains the complete derivation of the Fresnel coefficients for birefringent transparent materials, as well as for the direction cosines of the extraordinary ray and the Mueller matrices necessary to describe polarization effects. The formulas we derive can be directly used in a ray based renderer, and we demonstrate these capabilities in test scenes.