https://doi.org/10.2352/ISSN.2470-1173.2019.6.MAAP-203 © 2019, Society for Imaging Science and Technology A translucency classification for computer graphics Morgane Gerardin; Institut d’Optique Graduate School; Saint-Etienne, France; Lionel Simonot; Université de Poitiers, Institut Prime UPR CNRS 3346; Futuroscope Chasseneuil, France; Jean-Philippe Farrugia, Jean-Claude Iehl; université de Lyon, CNRS, université Lyon-1, LIRIS, UMR5205, F-69622 Lyon, Villeurbanne, France; Thierry Fournel, Mathieu Hébert; Univ Lyon, UJM-Saint-Etienne, CNRS, Institut d’Optique Graduate School, Laboratoire Hubert Curien UMR 5516, F-42023, SAINT-ETIENNE, France. Abstract oppositions transparency/translucency and translucency/opacity do Translucency is a visual property attributed to objects that not rely on similar perceptual criteria nor similar physical light may cross without transmitting a clear image of the scene phenomena. which is behind. In absence of a more precise definition, this We may agree on the fact that a object is considered as perceptual attribute is often considered as an intermediate translucent when light can go through it while being scattered. As a between transparency, which is the property of objects that light result, the scene that is transmitted through the object appears may cross by transmitting a clear image of the scene behind, and blurred. Two diffusion phenomena may occur: light may be opacity, which is the property of blocking the transmission of light scattered either at the object's surface, which is called surface and therefore masking completely the scene behind. If it is rather scattering, or within the object itself, which is known as subsurface clear that translucency is closely related to light scattering, it is scattering, or volume scattering. difficult to classify the translucent appearance according to one Decomposing translucency among absorption, surface and scale only, due to the different types of scattering, which can occur volume scattering would lead to a more complete, physically- as well as the role of absorbance and thickness of the material. based, intuitive and artist-friendly representation, which may be Through synthetic images rendered by optical models, we show useful for computer graphics or manufacturing. that surface scattering, volume (or subsurface) scattering, possibly In this paper, we define a three-dimensional representation mixed with selective absorption, produce different types of system adapted to a wide range of translucency effects in Section translucency effects and different intermediates between 1. The implementation details are given in Section 2, and some transparency and opacity. We thus propose to represent specific points in this representation space are illustrated through translucency according to three axes related to these three optical generated synthetic images in Section 3. These results are phenomena: surface scattering, volume scattering, and absorption. discussed in Section 4, and Section 5 draws the conclusions. Introduction 1. Three-dimensional representation space While it is used to denote the appearance of a wide range of In 1225, an English theologian Robert Grosseteste wrote a materials such as milk, jade, ceramic, or skin, translucency is a short text in latin entitled “De Colore” which has been interpreted perceptual and physical concept, which is still not well defined. by Ref. [4] as the first intent of a color classification system Many models have been developed in computer graphics to render composed of three axes. The authors have defined these axes based various light effects but translucency has not been fully explored on three oppositions stated in Grosseteste’s text: obscura/clara yet. (dark/light), pauca/multa (little/much), impurum/purum Translucency is often viewed as the intermediate between (pure/impure); and proposed a representation of Grosseteste’s transparency and opacity. Among the rare studies dedicated to this system as a cube, similar to the RGB cube for representing colors concept, the one by Fleming et al. [1] addresses the opposition in a digital image. This idea of assessing a main visual attribute between translucency and opacity through image synthesis. For with independent criteria inspired our approach for assessing example, the authors show that translucency of smooth objects is translucency. more noticeable when the object is illuminated from behind than We have identified three main optical phenomena giving to when illuminated on the front side. Moreover, when illuminated by the objects their transparency, translucency or opacity. To build a directional source, an opaque object appears more contrasted our representation system, each of these phenomena has been than a translucent one. The last point the authors explored is the described independently from one another by using a physical law addition of a blur in the images but this operation is not sufficient or a model. Some assumptions have been made so that only one to explain the perception of translucency because other phenomena parameter is enough to describe the appearance variation over one such as depth of field or penumbra effects can be at the origin of axis: similar blurring effect. Another study from Gkioulekas et al. [2] – absorption, parameterized by the density of absorbers in the shows the importance of single scattering within the material to material, the absorbers being characterized by an absorption explain differences in translucency. Finally, Ref. [3] stipulates that coefficient. opacity, translucency and transparency can be described on the – surface scattering (i.e., the scattering of light at the air-material same scale to carry out a perceptual experiment. interface), parameterized by a surface roughness parameter, However, the elements provided in the literature mentioned – volume scattering, i.e., the scattering of light within the material above are not precise enough to render the different kinds of itself, parameterized by a scatterer density. translucency we can find with objects according to the material in which they are made and their thickness. It is noticeable that the IS&T International Symposium on Electronic Imaging 2019 Material Appearance 2019 203-1 The three-dimensional representation is featured in Figure 1, correct amount of energy leaving a rough surface can be derived and the optical models used to render the three optical phenomena by using the approach that Heitz [8] has developed in order to take are presented below. into account these inter-reflections as the common micro-facet models does not. Along this "surface scattering" axis are represented the clear materials with rough surface, which are translucent but cannot be opaque (Figure 4). Sub-surface scattering For the scattering of light within the material itself, we compute it according to the dipole model proposed by Jensen [9], by assuming inclusions in the medium, which scatter the light in an isotropic way. This is the density of this scatterer, which allows us to control the appearance of the material along this axis. At the origin of the "volume scattering" axis, the density of the diffusing inclusions is zero: the material appears as achromatic and transparent. As the inclusion density increases, the lateral diffusion within the material becomes more important. Beyond a certain Figure 1. Three-dimensional space representing translucent materials. Each limit, the density is so high that the free mean path of the light axis corresponds to one of the physical phenomena among absorption, within the material is very short and the material looks totally surface scattering and volume scattering. The absorption axis is described by the absorber density, the surface scattering axis is controlled by the opaque. This particular case corresponds to the interfaced roughness parameter, and the sub-surface scattering axis is described by the Lambertian model proposed in Ref. [10]. Along this axis are scatterer density. represented the scattering and non-absorbing materials, whose appearance can vary from transparent to white opaque through various degrees of translucency (Figure 5). Absorption The absorbance of an object is described by Beer-Lambert- 2. Implementation Bouguer's law [5]. Let’s denote A the absorbance of the material In computer graphics, the way materials scatter light is and T its transmittance. The Beer-Lambert-Bouguer’s law is characterized by their BSDF (Bidirectional Scattering Distribution defined as in Equation (1), where σ is the attenuation cross Function). These models are valid if the incident light is only scattered locally at the surface of the object (surface scattering). It section, n the absorber density and d the thickness that is crossed is mostly the case for transparent, opaque or thin translucent by the light through the object. objects. This function can be decomposed into two light T =10− A=e− σ nd(1) contributions: the light that is reflected by the surface of the object, The transmittance decreases with the thickness of the object. described by the BRDF (Bidirectional Reflection Distribution Along this axis, we consider that the absorptivity of the material is Function) and the light that is transmitted through the object fixed, and so we defined the absorber density n as the variable described by the BTDF (Bidirectional Transmission Distribution Function). In the case of a completely opaque material, no light is parameter, which controls the absorption of the material. transmitted through the object and only the