Modeling and rendering of layered materials (다층 재질의 모델링 및 렌더링)
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The document discusses modeling and rendering of layered materials. It analyzes the previous W2L model for representing layered materials and proposes a generalized version called GW2L. The W2L model is found to oversimplify scattering and reflection behavior. GW2L decouples microfacet normals and uses implicit ray bundles to better approximate scattering over multiple paths within coating layers. Experiments show GW2L produces more physically plausible results than W2L. Future work is needed to verify GW2L, derive an analytic form, and accelerate performance through baking or GPU methods.
![W2L Model
• [Weidlich and Wilkie 2007] = W2L Model
• "Arbitrarily layered micro-facet surfaces", GRAPHITE 2007 +
SIGGRAPH ASIA 2009 Course](https://image.slidesharecdn.com/gw2l-scce2010-gist-130219094124-phpapp02/85/Modeling-and-rendering-of-layered-materials-8-320.jpg)
![The final classification can be seen in figure 4.4, although it should be noted that this is probably not
an exhaustive list of what can be achieved by this technique. Note that there are 8 instead of 6 types.
Examples
One 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-layered
materials.
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 Varnish
Figure 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 3.1
which we use for all our actual BRDF computations. The micro-facets are much smaller than the layer thickness in this
drawing.](https://image.slidesharecdn.com/gw2l-scce2010-gist-130219094124-phpapp02/85/Modeling-and-rendering-of-layered-materials-9-320.jpg)
![• 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](https://image.slidesharecdn.com/gw2l-scce2010-gist-130219094124-phpapp02/85/Modeling-and-rendering-of-layered-materials-20-320.jpg)
![• TIR = (1-G) + T*G = (1-G) + (1-F)*G = 1 - F*G
• The range of G is [0.94,1] when IOR < 1.41.
• Otherwise, G=1.
• Hence, the effect of G in TIR is not noticeable.](https://image.slidesharecdn.com/gw2l-scce2010-gist-130219094124-phpapp02/85/Modeling-and-rendering-of-layered-materials-23-320.jpg)
Modeling and rendering of layered materials (다층 재질의 모델링 및 렌더링)
- 1. 재질감 표현을 위한 렌더링 기술 동향 Modeling and Rendering of Layered Materials 이주행 ETRI 한국CAD/CAM학회 이론 및 응용 연구회 2010년 3월 26일 (금)
- 2. Agenda • Motivation / Background / Previous Works • Analysis of Previous Method: W2L Model • Proposed Method / Results • Summary / Q & A
- 3. Motivation • Layered materials are ubiquitous - Simple, versatile and effective - Suitable for digital prototyping application • A trial to generalize the previous work: observation & experiment
- 4. Background • Digital modeling of material appearance • Rendering in digital image synthesis
- 5. 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
- 6. 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
- 8. W2L Model • [Weidlich and Wilkie 2007] = W2L Model • "Arbitrarily layered micro-facet surfaces", GRAPHITE 2007 + SIGGRAPH ASIA 2009 Course
- 9. The final classification can be seen in figure 4.4, although it should be noted that this is probably not an exhaustive list of what can be achieved by this technique. Note that there are 8 instead of 6 types. Examples One 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-layered materials. 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 Varnish Figure 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 3.1 which we use for all our actual BRDF computations. The micro-facets are much smaller than the layer thickness in this drawing.
- 10. 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.
- 11. 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.
- 12. 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
- 13. Well... • Basically, W2L is a nice approach to handle layered materials in CG
- 14. But, • We need to verify its building blocks as well as basic assumptions.
- 17. roughness 1 0.1 0.01 0.001 0.0001 3.5 1.5 ior Direct reflection from coating layer
- 18. Absorption inside a Coating
- 19. Beer-Lambert Law • Foundation of spectroscopy
- 20. • 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
- 23. • TIR = (1-G) + T*G = (1-G) + (1-F)*G = 1 - F*G • The range of G is [0.94,1] when IOR < 1.41. • Otherwise, G=1. • Hence, the effect of G in TIR is not noticeable.
- 24. G
- 26. Basic Idea of W2L
- 27. V View direction wo is given.
- 28. V Get wor from Snell's law
- 29. V Get wir from pdf sampling from coating
- 30. V Get wi from Snell's law
- 31. L V Get incoming radiance L from wi
- 32. L V Get BRDF of coating layer, absorption, and attenuation
- 33. L V In W2L, everything happens between a pair of "big" micro-facets!
- 34. Generalization
- 35. V View direction wo is given.
- 36. V Get wor from Snell's law
- 37. V Get wir from pdf sampling from coating
- 38. V Note (1): Micro-facets are much smaller than the coating depth.
- 39. V Note (1): Micro-facets are much smaller than the coating depth.
- 40. V Get wi from Snell's law
- 41. L V Get incoming radiance L from wi
- 42. L V Get BRDF of coating layer, absorption, and attenuation
- 43. V Note (2): A ray path can be further subdivided to approximate scattering
- 44. V Note (2): A ray path can be further subdivided to approximate scattering
- 45. V Note (2): A ray path can be further subdivided to approximate scattering
- 46. V Note (2): A ray path can be further subdivided to approximate scattering
- 47. L V For implicit ray bundles, get BRDF, absorption, and attenuation in a MC way
- 48. Generalization of W2L = GW2L • More general configuration of micro-facets to decouple normals • Implicit ray bundles to approximat scattering in multiple paths
- 49. The most promising trial so far. Experiments with GW2L
- 50. Rendering Setups • PBRT-v2 for rendering • Metropolis / IGI • Extension through custom plug-ins in C++ • Coating layer has no diffuse/ambient terms to be more physically acurate
- 51. cfg_1 = W2L cfg_3 = GW2L
- 53. cfg_1 cfg_3 base + coating coating base
- 54. b+c ior=1.5 ior=3.5 cfg_1 b d=1 d=3 d=1 d=3 b+c ior=1.5 ior=3.5 cfg_3 b d=1 d=3 d=1 d=3
- 55. Effect of Implicit Ray Bundles
- 56. nb 1 10 100 b+c b time 1:46 2:40 11:50
- 58. Does it have an analytic form?
- 59. Can it be baked?
- 60. Results
- 61. glossy paint frosted paint tinted glazing metal foil metallic paint frosted metal patina multi-layer coating tinted tinted color coating rough rough rough base material solid metal base color tinted white base diffuse rough rough rough
- 62. glossy paint frosted paint tinted grazing metallic foil metallic paint frosted metal patina
- 63. Summary • Analysis of W2L model • Generalization of W2L model as GW2L • Presentation of experimental results
- 64. Future Works • Verification of GW2L algorithm • Derivation of analytic form for GW2L • Baking or GPU-based acceleration for re-shading/re-lighting • Comparison with “real” coated materials