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Translucency Perception: a Review Journal of Vision (2021) 21(8):4, 1–41 1 Translucency perception: A review Norwegian University of Science and Technology, Davit Gigilashvili Department of Computer Science, Gjøvik, Norway Norwegian University of Science and Technology, Jean-Baptiste Thomas Department of Computer Science, Gjøvik, Norway Norwegian University of Science and Technology, Jon Yngve Hardeberg Department of Computer Science, Gjøvik, Norway Norwegian University of Science and Technology, Marius Pedersen Department of Computer Science, Gjøvik, Norway Translucency is an optical and a perceptual phenomenon Internationale de l’Eclairage) total appearance “points that characterizes subsurface light transport through out the visual aspects of objects and scenes” (Pointer, objects and materials. Translucency as an optical 2006). Translucency is among the most essential visual property of a material relates to the radiative transfer attributes of appearance, along with color, gloss, and inside and through this medium, and translucency as a texture (Pointer, 2006; Eugène, 2008), remaining the perceptual phenomenon describes the visual sensation least studied one among those (Anderson, 2011). experienced by humans when observing a given material Although the color information incident on the human under given conditions. The knowledge about the visual retina encodes important information about the objects mechanisms of the translucency perception remains and materials, overall sensation also depends “on the limited. Accurate prediction of the appearance of the appearance of that colour due to the relationship translucent objects can have a significant commercial between the light transmitted, the light reflected, and impact in the fields such as three-dimensional printing. the light scattered by the body of the object” (Pointer, However, little is known how the optical properties of a material relate to a perception evoked in humans. This 2003). Translucency is seen as a phenomenon “between article overviews the knowledge status about the visual the extremes of complete transparency and complete perception of translucency and highlights the opacity” (Eugène, 2008). According to the ASTM applications of the translucency perception research. (2017) translucency is “the property of a specimen by Furthermore, this review summarizes current which it transmits light diffusely without permitting a knowledge gaps, fundamental challenges and existing clear view of objects beyond the specimen and not in ambiguities with a goal to facilitate translucency contact with it.” perception research in the future. The etymology of the term is related to the Latin words “trans” (through) and “lux” (light) — implying light penetration inside the body of the material (Kaltenbach, 2012). A translucent appearance is Introduction usually the result of a visual stimulus incident onto a retina from the objects permitting some degree of the How different objects and materials appear to human subsurface light transport. Translucency is impacted by observers is important not only in commerce, where multiple intrinsic and extrinsic factors. The intrinsic customer choice and satisfaction are often influenced factors are the physical parameters found in the by the visual look of the product, but also in trivial radiative transfer equation (Chandrasekhar, 1960), such daily tasks performed by humans. For instance, we use as the index of refraction, and the absorption and the visual appearance information to judge whether scattering coefficients, as well as the scattering phase materials are fragile or elastic, whether food is spoiled function. They define how the light propagates through or edible. By their appearance, we can effortlessly the media. A photon can get absorbed or scattered, that identify materials within seconds (Sharan et al., 2009; is, redirected toward a different direction when there is a Wiebel et al., 2013). According to the International change in the index of refraction, either at the external Commission on Illumination (the CIE - Commission surface of the object, or inside its volume (Tavel, Citation: Gigilashvili, D., Thomas, J.-B., Hardeberg, J. Y., & Pedersen, M. (2021). Translucency perception: A review. Journal of Vision, 21(8):4, 1–41, https://doi.org/10.1167/jov.21.8.4. https://doi.org/10.1167/jov.21.8.4 Received March 22, 2021; published August 3, 2021 ISSN 1534-7362 Copyright 2021 The Authors Downloaded from jov.arvojournals.org on 09/26/2021This work is licensed under a Creative Commons Attribution 4.0 International License. Journal of Vision (2021) 21(8):4, 1–41 Gigilashvili, Thomas, Hardeberg, & Pedersen 2 1999). How this passage of light through a material knowledge status has developed over time. Second, relates to a visual sensation of translucency remains we overview the role of transparency in translucency unclear to date. The extrinsic factors include, but are perception. Third, we discuss which factors impact not limited to, the illumination direction (Fleming & perceived translucency. Fourth, the potential cues Bülthoff, 2005; Xiao et al., 2014), object shape (Fleming for translucency perception are analyzed. Afterward, & Bülthoff, 2005; Gigilashvili et al., 2018b), and the we discuss the current challenges in the translucency color of the surface a translucent object is placed on perception research and outline the most important (Gigilashvili et al., 2020a). The human visual system questions remaining open, which is followed by a (HVS) is remarkably good at detecting subsurface light concluding section. transport — we can easily tell the difference between a translucent glass and an opaque metal, translucent wax and opaque stone. We can distinguish translucent Background and motivation human skin from an opaque plastic dummy, translucent milk from opaque chalk. One of the fundamental Translucency plays a significant role in a multitude problems is to understand how the HVS interprets the of fields and applications. Thus, it is a research interest surface-reflected and subsurface-scattered light from in different disciplines. In this section, we provide an the stimuli incident on the human retina. The exact overview of the applications and the interdisciplinarity visual and cognitive mechanisms of this ability are of the problem. Afterward, we discuss the gap between far from being fully understood. Because no model the optical and the perceptual properties of a material has yet been able to predict perceived translucency of — motivating the research from the human vision point a given material in an accurate and robust manner, of view. translucency perception remains a topic of active research in academia and industry alike. We would like to highlight that the primary focus Applications of this article is translucency, not transparency —a better understood concept and visual attribute. While To highlight the importance of understanding the two concepts are sometimes used interchangeably underlying visual mechanisms of translucency (e.g., Merriam-Webster Dictionary, n.d.), it is usually perception, we summarize the major applications where accepted that transparent substances, unlike translucent the translucency perception research can make impact. ones, transmit light without diffusing it (Gerbino et al., A broad range of customer products look 1990). According to the CIE, “if it is possible to see an translucent, either customers expecting a translucent object through a material, then that material is said to look from the products, or the degree of translucency be transparent. If it is possible to see only a “blurred” itself can be an indicator of product’s quality. This raises image through the material (due to some diffusion the need for studying translucency in the respective effect), then it has a certain degree of transparency and industries. For example, the foods, such as beer, we can speak about translucency” (Eugène, 2008). This meat and dairy products, are translucent. Therefore, implies that a given material might possess some degree translucent appearance plays an important role in the of transparency and some degree of translucency at the food industry, not only impacting customer satisfaction same time. (Hutchings, 1977, 2011), but also contributing to The contribution of this article is three-fold: the safety assurance (Chousalkar et al., 2010; Ray & Roberts, 2013). Decorative paint manufacturing is (1) Summarize the state-of-the-art about the perception another example, because the hiding power of the of translucency and the review of the recent colorants impacts the appearance and the overall developments in the field. quality of the paints (Krewinghaus, 1969; Midtfjord (2) Discuss the different applications that could benefit et al., 2018; Zhao & Berns, 2009). from the translucency perception research and Translucency has an implication for aesthetic overview of the importance of the topic in and purposes as well. Generation, reproduction, and across different disciplines. perception of translucent appearance has long been (3) Outline the major knowledge gaps and research a topic of interest in visual arts and cultural heritage. challenges in order to facilitate future work. Translucent building materials play an important role in the modern-day architecture and are used The article is organized as follows. We briefly to generate various visual effects of the exterior as summarize the motivation for translucency perception well as interior design (Kaltenbach, 2012; Murray, research in the next section. In four subsequent sections,
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