{"id":4871,"date":"2010-07-09T11:49:08","date_gmt":"2010-07-09T11:49:08","guid":{"rendered":"http:\/\/www.labri.fr\/perso\/barla\/blog\/?p=4871"},"modified":"2012-07-09T16:04:19","modified_gmt":"2012-07-09T16:04:19","slug":"digital-modeling-of-material-appearance","status":"publish","type":"post","link":"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=4871","title":{"rendered":"Digital modeling of material appearance"},"content":{"rendered":"<p id=\"top\" \/><em>Julie Dorsey, Holly Rushmeier, Francois Sillion<\/em><\/p>\n<h2>Background<\/h2>\n<ul>\n<li>Despite the absence of universal definitions, directional effects are clearly important. Terms and classifications can be found in a variety of contexts and fields of study. In home decorating, the term surface finish is often used. Paints of the same color are designated as matte or gloss finish, metal is weathered or polished, and ceramics are glazed or unglazed. &#8211; p.18<\/li>\n<li>Descriptions of texture are even more varied than those used for directional variations. Even the word &#8220;texture&#8221; does not have a uniform meaning; for example, it is applied to microscopic phenomena in material science and tactile phenomena in foods. &#8211; p.20<\/li>\n<\/ul>\n<h2>Observation and classification<\/h2>\n<ul>\n<li>While there are simple light interactions at each surface, either a pure specular reflection or specular transmission, the combination of multiple reflections and redirections of each\u00a0 as it passes through the object and reaches the eyes results in a complex appearance. &#8211; p.34 &#8211; <em>3D appearance<\/em> <em>may actually require moving the head around to understand what&#8217;s going on.\u00a0 The simpler (specular) the appearance, the more complex patterns and shape understanding becomes!<br \/>\n<\/em><\/li>\n<li>If objects being reflected are relatively close to the glossy object, the cross section of the cone from which a point is reflecting light is relatively small, and lines like those between the yellow and blue surfaces above are only blurred a bit. As the objects get further away, the cross section of the cone becomes large. Distant objects do not appear with any detail at all when reflected in such a shiny surface. &#8211; p36 <em>&#8211; Is the amount of blur observed with distant objects used by the HVS?<\/em><\/li>\n<\/ul>\n<h2>Mathematical terms<\/h2>\n<ul>\n<li>We could devise completely perceptually-inspired models for individual materials and define parameters to adjust them. However, that would not guarantee that objects looked correct when viewed in combination or as conditions change. &#8211; p.47 &#8211; <em>Find cases where perceptual categorization is challenged (even harder with motion!)<\/em>.<\/li>\n<\/ul>\n<h2>General material models<\/h2>\n<ul>\n<li>Understanding how light interacts with a smooth surface provides some basic bounds on forms of the BRDF. The smooth surface results also provide a complete model for shiny opaque surfaces like stainless steel and clear transmitting surfaces like glass. &#8211; p.62 &#8211; <em>Is there a continuous range of materials between steel and glass? Fresnel effect then depends on slant: it enhances silhouettes, but also forces observers to move their head to perceive small details on flat surfaces.<\/em><\/li>\n<li>For metals, there is not a dramatic change as the angle of incidence nears grazing, since the reflectance is high at all angles. [&#8230;] All of the metals have a high reflectance at all wavelengths at near grazing angles. The implicaton of high reflectance at all wavelengths is that all of the metals tend to look gray or white when viewed at grazing angles. &#8211; p.67 &#8211; <em>Grazing cue used for perception of metals?<\/em><\/li>\n<li>The principle features of the Blinn and Cook-Torrance models as far as directional effects is the prediction of off-specular peaks and the increase in specular reflectance as the angle of incidence increases. An off-specular peak means that the maximum value of the reflectance function is not in the mirror direction, but in a direction closer to a grazing angle. The off-specular peaks are the consequence of shadowing and masking causing asymmetries. &#8211; p.88 &#8211; <em>Are variations due to grazing angle noticeable or disturbing?<\/em><\/li>\n<li>Appropriately assigning the color of the specular component is a mechanism that can be used in any of the reflectance models to obtain metallic versus nonmetallic appearance. &#8211; p.89 &#8211; <em>This sounds like an important cue to material recognition! There is a need for a general model that would allow testing such cues&#8230;<br \/>\n<\/em><\/li>\n<li>The principle feature of the Oren-Nayar model is the prediction of backscattering, which is a consequence of facets oriented toward the light source diffusely reflect some light back to the source. [&#8230;] The result in each case are BRDF functions with lobes that have more complicated structures than those used in empirical models. &#8211; p.90 &#8211; <em>Results do look smoother and softer.<\/em><\/li>\n<li>Unlike the surface problem, in which geometry and reflectance properties are treated entirely separately, the definition of the geometry of a participating medium and its properties are closely coupled. &#8211; p.111<\/li>\n<li>A subsurface scattering object will not have sharp shadows cast on it, since some light always diffuses from the lit area to the unlit area. &#8211; p.120<\/li>\n<\/ul>\n<h2>Specialized material models<\/h2>\n<p><em>In general, how do the special effects of these section impact perceived appearance?<\/em><\/p>\n<ul>\n<li>Natural materials such as rocks (and indeed many manufactured materials such as ceramics) cannot be adequately modeled as a surface that is a set of randomly oriented small facets. Within the faceted surface there may be many deeper pits or pores. [&#8230;] Wet materials often look darker, brighter, and more specular, depending on the type of material and the viewing conditions. &#8211; p.145<\/li>\n<li>The effect of the liquid that enters into a material is to alter the paths of light within cavities and pores. If the pore is filled with a fluid of a higher index of refraction, total internal reflection at the surface when rays are traveling from the higher to lower index fluid results in both more rays being absorbed (causing darkening) and rays undergoing more internal reflections. More internal reflections accentuate the effect of spectral variations and results in changes in hue and saturation. &#8211; p.146<\/li>\n<li>Interactive formulation and application of metallic paints facilitate assessing how paint and form will work together. A particular combination of face, flop, and travel may produce an appealing color variation accentuating the form of a vehicle, or may be perceived as uneven application of paint. &#8211; p.155 &#8211; <em>Color variations may be assigned to form or material in their own way by the HVS.<\/em><\/li>\n<li>An interesting feature of pearl appearance is that the color variations due to interference are only a function of the position of the viewer, not the light source. &#8211; p.159<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Julie Dorsey, Holly Rushmeier, Francois Sillion Background Despite the absence of universal definitions, directional effects are clearly important. Terms and classifications can be found in a variety of contexts and fields of study. In home decorating, the term surface finish is often used. Paints of the same color are designated as matte or gloss finish, &#8230; <a title=\"Digital modeling of material appearance\" class=\"read-more\" href=\"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=4871\" aria-label=\"Read more about Digital modeling of material appearance\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[621],"tags":[],"class_list":["post-4871","post","type-post","status-publish","format-standard","hentry","category-books"],"_links":{"self":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4871","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4871"}],"version-history":[{"count":6,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4871\/revisions"}],"predecessor-version":[{"id":22781,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4871\/revisions\/22781"}],"wp:attachment":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4871"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4871"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4871"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}