Reflectance models capture many types of visual appearances. The most plausible reflectance models follow the Microfacet theory, which is specifically based on statistical representations, with an analytic visibility term. This visibility term has a significant impact on appearance. Visibility computed with the masking term proposed by Smith (1967), and revisited by Ashikhmin et al. (2000), is nowadays considered as the most plausible in the literature. It is simple and efficient to evaluate for statistical distributions, but it relies on assumptions that are not necessarily respected by real surfaces. This article proposes an in-depth study of masking for meshed height-field surfaces, generated either from measured real-world materials or from functions derived from distributions of surface normals. We experimentally estimate the masking (and shadowing) of surfaces using a ray-casting technique, and compare their measurements with the theoretical model from Smith and Ashikhmin et al. We show that their assumptions are too restrictive for a majority of real-world surfaces. We propose a model capable of predicting how close the theoretical masking term can be from the masking term estimated by a ray-casting approach. Although most surfaces break their assumptions, our results show that the term from Smith and Ashikhmin et al. can still be reasonably employed for a fraction in a set of more than 400 measured surfaces, with low errors compared to a ray-casting masking estimation, much lower computation times, and very similar visual appearances. Our model can be used to predict the incurred error on a physically-based rendering simulation with a microfacet-based BRDF created from real-world surfaces, instead of explicitly calculating the masking term from its height field.

Abstract Man‐made objects commonly exhibit rounded edges and corners generated through their manufacturing processes. The variation of surface normals at these confined locations produces shading details that are visually essential to the realism of synthetic scenes. The more specular the surface, the finer and more prominent its highlights. However, most geometric modellers represent rounded edges and corners with dense polygonal meshes that are limited in terms of smoothness, while tremendously increasing scene complexity. This paper proposes a non‐invasive method (i.e. that does not modify the original geometry) for the modelling and rendering of smooth edges and corners from any input polygonal geometry defined with infinitely sharp edges. At the heart of our contribution is a geometric structure that automatically and accurately defines the geometry of edge and corner rounded areas, as well as the topological relationships at edges and vertices. This structure, called SREC‐RT, is integrated in a ray‐tracing‐based acceleration structure in order to determine the region of interest of each rounded edge and corner. It allows systematic rounding of all edges and vertices without increasing the 3D scene geometric complexity. While the underlying rounded geometry can be of any type, we propose a practical ray‐edge and ray‐corner intersection based on parametric surfaces. We analyse comparisons generated with existing methods. Our results present the advantages of our method, including extreme close‐up views of surfaces with a much higher quality for very little additional memory, and reasonable computation time overhead.

Microfacet distributions are considered nowadays as a reference for physically plausible BSDF representations. Many authors have focused on their physical and mathematical correctness, while introducing means to enlarge the range of possible appearances. This article is dedicated to Normal Distribution Functions (NDFs) and the influence of their shape on the rendered material aspect. We provide a complete framework for studying the impact of NDFs on the observed Bidirectional Scattering Distribution Functions (BSDFs). To explore very general NDFs, manually controlled by the user, and including anisotropic materials, we propose to use a piecewise continuous representation. It is derived with its associated Smith shadowing-masking function and importance sampling formulations for ensuring efficient global illumination computations. A new procedure is also proposed in this article for generating an explicit geometric micro-surface, used to evaluate the validity of analytic models and multiple scattering effects. The results are produced with a computer-generated process using path tracing. They show that this generation procedure is suitable with any NDF model, independently from its shape complexity.

Specular microfacet distributions have been successfully employed by many authors for representing glossiness of materials. They are generally combined with a Lambertian term to account for the colored aspect. These representations make use of the Fresnel reflectance factor at the interface, but the transmission factor at the interface should also be managed. One solution is to employ a multi-layered model with a single layer for the rough interface, which requires a numerical simulation for handling the multiple reflections of light between the substrate and the interface. In this paper, we propose rather to use a representation corresponding to a Fresnel interface lying on a Lambertian substrate, for which the multiple reflections of light between the interface and the substrate can be expressed analytically. With this interfaced Lambertian model, we show how Fresnel transmission affects the material appearance for flat and rough surfaces with isotropic and anisotropic distributions, that produce light backscattering effects. We also propose a methodology for using such materials in any physically based Monte Carlo rendering system, as well as an approximate representation, suitable for GPU applications or measured data fitting. Our approach generalizes several previous models, including flat Lambertian materials as well as specular and Lambertian microfacets. Our results illustrate the wide range of materials that can be rendered with this representation.

This paper focuses on microfacet reflectance models, and more precisely on the definition of a new and more general distribution function, which includes both Beckmann's and GGX distributions widely used in the computer graphics community. Therefore, our model makes use of an additional parameter γ, which controls the distribution function slope and tail height. It actually corresponds to a bivariate Student's t-distribution in slopes space and it is presented with the associated analytical formulation of the geometric attenuation factor derived from Smith representation. We also provide the analytical derivations for importance sampling isotropic and anisotropic materials. As shown in the results, this new representation offers a finer control of a wide range of materials, while extending the capabilities of fitting parameters with captured data.

Abstract This paper presents a topology‐based representation dedicated to complex indoor scenes. It accounts for memory management and performances during modelling, visualization and lighting simulation. We propose to enlarge a topological model (called generalized maps) with multipartition and hierarchy. Multipartition allows the user to group objects together according to semantics. Hierarchy provides a coarse‐to‐fine description of the environment. The topological model we propose has been used for devising a modeller prototype and generating efficient data structure in the context of visualization, global illumination and 1 GHz wave propagation simulation. We presently handle buildings composed of up to one billion triangles.

This paper describes a new scheme for computing out-of-core global illumination in complex indoor scenes using a photon-mapping approach. Our method makes use of a cells-and-portals representation of the environment for preserving memory coherence and storing rays or photons. We have successfully applied our method to various buildings, composed of up to one billion triangles. As shown in the results, our method requires only a few hundred megabytes of memory for tracing more than 1.6 billion photons in large buildings.

The giraffe and its patches, the leopard and its spots, the tiger and its stripes are spectacular examples of the integration of a pattern and a body shape. We present an approach that integrates a biologically-plausible pattern generation model, which can effectively deliver a variety of patterns characteristic of mammalian coats, and a body growth and animation system that uses experimental growth data to produce individual bodies and their associated patterns automatically. We use the example of the giraffe to illustrate how our approach takes us from a canonical embryo to a full adult giraffe in a continuous way, with results that are not only realistic looking, but also objectively validated. The flexibility of the approach is demonstrated by examples of big cat patterns, including an interpolation between patterns. The approach also allows a considerable amount of user control to fine-tune the results and to animate the resulting body with the pattern.

In this paper we propose a SPMD parallel hierarchical radiosity algorithm relying on a novel partitioning method which may apply to any kind of architectural scene. This algorithm is based on MPI (Message Passing Interface), a communication library which allows the use of either a heterogeneous set of concurrent computers or a parallel computer or both. The database is stored on a common directory and accessed by all the processors (through NFS in case of a network of computers). As the objective is to handle complex scenes such as building interiors, to cope with the problem of memory size, only a subset of the database resides in memory of each processor. This subset is determined with the help of a partitioning into 3D cells, clustering and visibility calculations. A graph expressing visibility between the resulting clusters is determined, partitioned (with a new method based on classification of K‐means type) and distributed amongst all the processors. Each processor is responsible for gathering energy (using the Gauss‐Seidel method) only for its subset of clusters. In order to reduce the disk transfers due to downloading these subsets of clusters, we use an ordering strategy based on the traveling salesman algorithm. Dynamic load balancing relies on a task stealing approach while termination is detected by configuring the processors into a ring and moving a token around this ring. The parallel iterative resolution is of group iterative type. Its mathematical convergence is proven in the appendix.

Hierarchical radiosity is a very demanding process in terms of computation time and memory resources even for scenes of moderate complexity. To handle complex environments which don't fit in the memory, new solutions have to be devised. One solution is to partition the scene into subsets of polygons (3D cells or clusters) and to maintain in memory only some of them. The radiosity computation is performed only for this resident subset which changes during the resolution process. This change entails many read and write operations from or onto the disk. These disk transfers must be ordered to make the radiosity algorithms tractable. The authors propose different ordering strategies which can be seen as complementary to those devised by Teller (1994).

Computing global illumination for complex environments in moderate time and walking through them is one of the challenges in computer graphics. To meet this goal, preprocessing is necessary. This preprocessing consists in partitioning the environment into cells and determining visibility between these cells. Most of the existing partitioning methods rely on the binary space partitioning (BSP) technique which can be easily applied to axial environments. However, for non-axial scenes, BSP has a high complexity of O (n3) in time to construct a tree of size at worst O(n2), n being the total number of input polygons. Moreover, this technique entails a large number of cells that do not necessarily fit with the topology of the environment. We propose in this paper a partitioning method which can be applied to non-axial buildings with several floors. It consists of two steps. In the first step each floor is extracted by applying a BSP technique using the most occlusive horizontal polygons for splitting. In the second step each floor is in turn partitioned with a model-based method operating in a dual 2D space. The result is a low number of cells fitting at best with the environment topology. © 1998 John Wiley & Sons, Ltd.