Toward a Perceptually-relevant Theory of Appearance

Two approaches are commonly employed in Computer Graphics to design and adjust the appearance of objects in a scene. A full 3D environment may be created, through geometrical, material and lighting modeling, then rendered using a simulation of light transport; appearance is then controlled in ways similar to photography. A radically different approach consists in providing 2D digital drawing tools to an artist, whom with enough talent and time will be able to create images of objects having the desired appearance; this is obviously strongly similar to what traditional artists do, with the computer being a mere modern drawing tool.In this document, I present research projects that have investigated a third approach, whereby pictorial elements of appearance are explicitly manipulated by an artist. On the one side, such an alternative approach offers a direct control over appearance, with novel applications in vector drawing, scientific illustration, special effects and video games. On the other side, it provides an modern method for putting our current knowledge of the perception of appearance to the test, as well as to suggest new models for human vision along the way

The perception of hazy gloss

Most previous work on gloss perception has examined the strength and sharpness of specular reflections in simple bidirectional reflectance distribution functions (BRDFs) having a single specular component. However, BRDFs can be substantially more complex and it is interesting to ask how many additional perceptual dimensions there could be in the visual representation of surface reflectance qualities. To address this, we tested materials with two specular components that elicit an impression of hazy gloss. Stimuli were renderings of irregularly shaped objects under environment illumination, with either a single Ward specular BRDF component (Ward, 1992), or two such components, with the same total specular reflectance but different sharpness parameters, yielding both sharp and blurry highlights simultaneously. Differently shaped objects were presented side by side in matching, discrimination, and rating tasks. Our results show that observers mainly attend to the sharpest reflections in matching tasks, but they can indeed discriminate between single-component and two-component specular materials in discrimination and rating tasks. The results reveal an additional perceptual dimension of gloss-beyond strength and sharpness-akin to ''haze gloss'' (Hunter & Harold, 1987). However, neither the physical measurements of Hunter and Harold nor the kurtosis of the specular term predict perception in our tasks. We suggest the visual system may use a decomposition of specular reflections in the perception of hazy gloss, and we compare two possible candidates: a physical representation made of two gloss components, and an alternative representation made of a central gloss component and a surrounding halo component.Perceptual Representation of Illumination, Shape and Materia

Dynamic Stylized Shading Primitives

Honorable Mention in RenderingInternational audienceShading appearance in illustrations, comics and graphic novels is designed to convey illumination, material and surface shape characteristics at once. Moreover, shading may vary depending on different configurations of surface distance, lighting, character expressions, timing of the action, to articulate storytelling or draw attention to a part of an object. In this paper, we present a method that imitates such expressive stylized shading techniques in dynamic 3D scenes, and which offers a simple and flexible means for artists to design and tweak the shading appearance and its dynamic behavior. The key contribution of our approach is to seamlessly vary appearance by using a combination of shading primitives that take into account lighting direction, material characteristics and surface features. We demonstrate their flexibility in a number of scenarios: minimal shading, comics or cartoon rendering, glossy and anisotropic material effects; including a variety of dynamic variations based on orientation, timing or depth. Our prototype implementation combines shading primitives with a layered approach and runs in real-time on the GPU

Designing Gratin, A GPU-Tailored Node-Based System

International audienceNodal architectures have received an ever-increasing endorsement in computer graphics in recent years. However, creating a node-based system specifically tailored to GPU-centered applications with real-time performance is not straightforward. In this paper, we discuss the design choices we took in the making of Gratin, our open-source node-based system. This information is useful to graphics experts interested in crafting their own node-based system working on the GPU, either starting from scratch or taking inspiration from our source code. We first detail the architecture of Gratin at the graph level, with data structures permitting real- time updates even for large pipelines. We then present the design choices we made at the node level, which provide for three levels of programmability and, hence, a gentle learning curve. Finally, we show the benefits of our approach by presenting use cases in research prototyping and teaching

Improving Shape Depiction under Arbitrary Rendering

International audienceBased on the observation that shading conveys shape information through intensity gradients, we present a new technique called Radiance Scaling that modifies the classical shading equations to offer versatile shape depiction functionalities. It works by scaling reflected light intensities depending on both surface curvature and material characteristics. As a result, diffuse shading or highlight variations become correlated to surface feature variations, enhancing concavities and convexities. The first advantage of such an approach is that it produces satisfying results with any kind of material for direct and global illumination: we demonstrate results obtained with Phong and Ashikmin-Shirley BRDFs, Cartoon shading, sub-Lambertian materials, perfectly reflective or refractive objects. Another advantage is that there is no restriction to the choice of lighting environment: it works with a single light, area lights, and inter-reflections. Third, it may be adapted to enhance surface shape through the use of precomputed radiance data such as Ambient Occlusion, Prefiltered Environment Maps or Lit Spheres. Finally, our approach works in real-time on modern graphics hardware making it suitable for any interactive 3D visualization

Comparative study of layered material models

International audienceThe accurate reproduction of layered materials is an important part of physically-based rendering applications.Since no exact analytical model exists for any configuration of layer stacks, available models make approximations.In this paper, we propose to evaluate them with a numerical approach: we simulate BRDFs and BTDFs for layered materials in order to compare existing models against a common reference.We show that: (1) no single model always outperforms the others and (2) significant differences remain between simulated and modeled materials.We analyse the reasons for these discrepancies and introduce immediate corrections

Utilisation du rendu expressif pour l'illustration et l'exploration de données archéologiques

National audienceLe rendu expressif est une branche relativement jeune de la synthèse d'images qui s'intéresse non pas à créer des images qui sont le résultat de simulations de phénomènes physiques réalistes, mais qui tend à communiquer visuellement des informations sur les objets représentés par le biais de styles variés (dessin, aquarelle, etc). Ce type de rendu semble particulièrement adapté au domaine de l'archéologie pour deux raisons : il permet d'illustrer les hypothèses de reconstruction 3D archéologiques sans pour autant biaiser l'interprétation par une représentation trop réaliste et peut aussi apporter une méthode visuelle intuitive d'exploration de données archéologiques. Dans cet exposé, nous allons tout d'abord présenter les travaux que nous avons réalisé par le passé portant sur la création d'illustrations (effets de papier, aquarelle). Puis nous allons introduire les aspects du rendu expressif qui permettraient l'exploration sémantique de données archéologiques

Reparametrization Uniforme de l’Apparence des Matériaux par Redistribution de Densité

Variations of material parameters such as roughness, sheen, etc. do not usually produce visually-uniform changes in rendered images. This might make it detrimental to artistic edition or prefiltering. In this work, we provide a methodology to reparametrize non visuallyuniform parameters, by uniformly distributing the visual changes across the range of parametervalues. For 1D and 2D parameters, our solution boils down to inverting a Cumulative Distribution Function. We provide three concrete applications of this method to reparametrize the complex refractive index, roughness and sheen.Des variations de paramètres de modèles matériaux tels que la ruguosité ne produisent généralement pas de changements qui apparaissent visuellement uniformes en synthèse d’images. Cela a pour conséquence de compliquer le contrôle artistique ainsi que les méthodes de pré-filtrage. Dans ce rapport, nous proposons une méthodologie qui permet de reparamétriser des modèles de matériau en redistribuant de manière uniforme les changements d’apparence en jouant sur les valeurs des paramètres. Pour des paramètres à une ou deux dimensions, notre solution revient à inverser une fonction de distribution cumulative. Nous présentons trois applications concrètes de cette méthode portant sur trois types de paramètres: l’indice de réfraction complexe, la rugosité et les effets de lustre diffus

Bringing an Accurate Fresnel to Real-Time Rendering: a Preintegrable Decomposition

International audienceWe introduce a new approximate Fresnel reflectance model that enables the accurate reproduction of ground-truth reflectance in real-time rendering engines. Our method is based on an empirical decomposition of the space of possible Fresnel curves. It is compatible with the preintegration of image-based lighting and area lights used in real-time engines. Our work permits to use a reflectance parametrization [Gulbrandsen 2014] that was previously restricted to offline rendering

Simplification et abstraction de dessins au trait

National audienceDans cet article, nous proposons une méthode pour simplifier un ensemble de lignes vectorielles tout en conservant la structure du dessin de départ et en incorporant des choix de style de la part de l'utilisateur. Cette approche a pour avantage d'être assez modulaire pour s'adapter à de nombreuses applications : de l'édition interactive de tracés à la génération de niveaux de détail pour le rendu non-photoréaliste, en passant par la gestion de la densité de dessins. Nous présentons ainsi un cadre commun à l'ensemble de ces méthodes et illustrons son potentiel par le biais de deux applications : un outil de tracé progressif et une méthode de mise à l'échelle d'un dessin