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
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
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.
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
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
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
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