Motivation• Layered materials are ubiquitous - Simple, versatile and effective - Suitable for digital prototyping application• A trial to generalize the previous work: observation & experiment
Background• Digital modeling of material appearance• Rendering in digital image synthesis
Previous Works• Kubelka and Munk 1931, Hanrahan and Krueger 1993• Neumann and Neumann 1989• Kelemen and Szimay-Kalos 2001• Schlick 1993, Lafortune et al 1997• Weidlich and Wilkie 2007, 2009
Evaluation Criteria• Closed mathematical form vs. simulation• Handling of physical behaviors: (ex) scattering, absorption, internal reﬂection, micro-facets, Fresnel• Integration with rendering algorithms: (ex) unbiased MC rendering requires sampling PDFs and a way of quality control
The ﬁnal classiﬁcation can be seen in ﬁgure 4.4, although it should be noted that this is probably notan exhaustive list of what can be achieved by this technique. Note that there are 8 instead of 6 types. ExamplesOne represents the further split into tinted and non-tinted varnish that can be performed for all types,and one stands for the group of materials that consists of more than two layers, i.e. multi-layeredmaterials. a) b) c) d) e) f) g) h) Glossy Paint Tinted Glazing Frosted Paint Metal Foil Metallic Paint Frosted Metal Patina Multi-Layer Interfaces: Diffuse Materials: Metal Tinted Varnish Torrance-Sparrow Smooth Coloured Solid Colourless Solid Clear VarnishFigure 4.4: and Wilkie of various surface types that can be generated by using our layered model in different conﬁgurations. [ Weidlich Examples 2007 ]In order to properly distinguish the various cases the icons do not exhibit the simplifying assumption shown in ﬁgure 3.1which we use for all our actual BRDF computations. The micro-facets are much smaller than the layer thickness in thisdrawing.
Comments on W2L Model• With a few control parameters,• it can represent a various types of materials• with a closed form BRDF model, and• rendering results are physically plausible.
Physical Plausibility• Application of physically correct micro-facet model: Torrance-Sparrow• Application of Beer-Lambert Law to compute absorption assuming that coating is a non-scattering medium, which seems not correct.• Consideration of total internal reﬂection (TIR). Actually, ineffective.
Closed Form Model• Geometric simpliﬁcation of scattering and reﬂection• No additional ray-object intersection test guarantees performance = only a single ray path is considered at each intersection• We argue it as a over-simpliﬁcation that limits handling more general cases
Well...• Basically, W2L is a nice approach to handle layered materials in CG
But,• We need to verify its building blocks as well as basic assumptions.
• Beer-Lambert law is commonly used in spectroscopy to derive the absorption coefﬁcient of non-scattering media based on linear attenuation.• Linearity breaks by the scattering of light due to particulates of the sample or in a dense media• A modiﬁed Beer-Lamber law for a scattering media such as a biological tissue is available [Delpy et al 1988, Sassroli and Fantini 2004]• But, too costly to evaluate the modiﬁed Beer-Lamber law
glossy paint frosted paint tinted glazing metal foil metallic paint frosted metal patina multi-layer coating tinted tinted colorcoating rough rough roughbase material solid metal base color tinted white base diffuse rough rough rough