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→Overview of the BxDF functions: Move BDF after its component function descriptions (now it's like the BSS ordering). |
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|doi=10.1117/12.959611
|year=1980|series=Radiation Scattering in Optical Systems
|s2cid=128406154
}}
</ref> Most often it is used to name the general mathematical function which describes the way in which the light is scattered by a surface. However, in practice, this phenomenon is usually split into the reflected and transmitted components, which are then treated separately as '''BRDF''' ('''[[bidirectional reflectance distribution function]]''') and '''BTDF''' ('''bidirectional transmittance distribution function''').
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| title = Proceedings of the 27th annual conference on Computer graphics and interactive techniques - SIGGRAPH '00
| isbn = 978-1581132083
| s2cid = 2860203
}}
</ref> was one of the last key breakthroughs on the way to fully [[virtual cinematography]] with its ultra-photorealistic [[digital look-alike]]s. The team was the first in the world to isolate the [[subsurface scattering]] component (a specialized case of BTDF) using the simplest [[light stage]], consisting on moveable light source, moveable high-res [[digital camera]], 2 [[polarizers]] in a few positions and really simple [[algorithm]]s on a modest [[computer]].<ref name="Deb2000"/> The team utilized the existing scientific knowledge that [[light]] that is reflected and scattered from the air-to-oil layer retains its [[Polarization (waves)|polarization]] while light that travels within the [[skin]] loses its polarization.<ref name="Deb2000"/> The subsurface scattering component can be [[computer simulation|simulated]] as a steady high-scatter glow of light from within the [[3D model|model]]s, without which the skin does not look realistic. [[ESC Entertainment]], a company set up by [[Warner Brothers Pictures]] specially to do the visual effects / virtual cinematography system for [[The Matrix Reloaded]] and [[The Matrix Revolutions]] isolated the parameters for an approximate analytical BRDF which consisted of [[Lambertian diffusion]] component and a modified [[specular]] [[phong reflection model|Phong]] component with a [[Fresnel equations|Fresnel]] type of [[Visual effects|effect]].<ref name="HaD2004">{{cite book
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| title = Proceedings of the conference on SIGGRAPH 2004 course notes - GRAPH '04
| isbn = 978-0111456781
| s2cid = 33684283
}}</ref>
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* '''BTDF''' ('''Bidirectional transmittance distribution function''')<ref name="bartell1980"/> is similar to BRDF but for the opposite side of the surface. (''see the top image'').
* '''BDF''' ('''Bidirectional distribution function''') is collectively defined by BRDF and BTDF.
* '''BSSRDF''' ('''Bidirectional scattering-surface reflectance distribution function''' or '''Bidirectional surface scattering RDF''')<ref name="nicodemus1977" /><ref name="jensen2001">{{cite book|last1=Jensen|first1=H. W.|url=https://archive.org/details/siggraph2001conf00fium/page/511|title=Proceedings of the 28th annual conference on Computer graphics and interactive techniques - SIGGRAPH '01|last2=Marschner|first2=S. R.|last3=Levoy|first3=M.|last4=Hanrahan|first4=P.|website=graphics.ucsd.edu/~henrik/papers/bssrdf/|publisher=Proceedings of ACM SIGGRAPH 2001|year=2001|isbn=978-1581133745|pages=[https://archive.org/details/siggraph2001conf00fium/page/511 511–518]|chapter=A Practical Model for Subsurface Light Transport|citeseerx=10.1.1.503.7787|doi=10.1145/383259.383319|s2cid=11408331 |chapter-url=http://jbit.net/~sparky/bssrdf.pdf|accessdate=14 July 2014}}</ref> describes the relation between outgoing radiance and the incident flux, including the phenomena like [[subsurface scattering]] (SSS). The BSSRDF describes how light is transported between any two rays that hit a surface.
* '''BSSTDF''' ('''Bidirectional scattering-surface transmittance distribution function''') is like BTDF but with subsurface scattering.
* '''BSSDF''' ('''Bidirectional scattering-surface distribution function''') is collectively defined by BSSTDF and BSSRDF. Also known as '''BSDF''' ('''Bidirectional scattering distribution function''').
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