Before we begin Paper summaries for today? Material Properties 2 - PDF document

Before we begin  Paper summaries for today? Material Properties 2 Announcement Announcement  SIGGRAPH animation screenings  Career Fair  Every Monday  The straight dope  12:30pm -- 2pm  Fair: Tuesday, September 26th  10am -- 4pm  07-1315  Interviews: Wednesday, September 27th  Sponsored by RIT digital studio program (College Imaging Arts and Sciences) and  http://www.rit.edu/co-op/careers Rochester Chapter of SIGGRAPH. Announcement Logistics  Electronic Arts on campus  Checkpoint 2  Due Today  Checkpoint 3  Wednesday, Sept 27th  To be given today  6:00pm - 8pm  Project Proposals  Golisano Auditorium  All should have received e-mail feedback.  Please address issues (if any) 1

Plan for today Computer Graphics as Virtual Photography  Material Properties real camera photo Photographic Photography: scene (captures processing print  Bi-directional reflectance distribution functions light) (BRDFs)  Advanced Illumination Models processing  Beyond BRDFs camera Computer 3D synthetic tone  Checkpoint 3 of the ray tracer model Graphics: models reproduction image (focuses  Ray tracer help simulated lighting) Shading BRDF  Bi-directional Reflectance Function  Computing the light that leaves a point  Shading point - point under investigation BRDF f ( , , , ) = � � � �  Illumination model - function or algorithm r i i r r used to describe the reflective characteristics of a given surface. At a given point, gives relative reflected illumination in any  Shading model – algorithm for using an direction with respect to incoming illumination coming from illumination model to determine the color of a any direction; point on a surface.  For efficiency’s sake, most illumination models Note: The θ ’s are elevation, ϕ ’s are measured about the are approximations. surface normal. The i ’s refer to the incident ray; the r ’s to the reflected ray. BRDF Geometry BRDF  Can return any positive value.  Generally wavelength specific. BRDF = f ( , , , , ) � � � � � r i i r r 2

Anisotropic Models Ansiotropic Models  Anisotropy  anisotropic (adj.) an · i · so · trop · ic 1.  Isotropic - surfaces reflect equally from any Physics. of unequal physical properties direction of view along different axes.  Anisotropic - reflection varies not only with angle of incidence, but also with the angle of the incident light w.r.t some viewing angle.  Surfaces considered to possess an intrinsic grain  http://www.neilblevins.com/cg_educatio  Examples: satin, velvet, hair, brushed aluminum n/aniso_ref/aniso_ref.htm Anisotropic Models Why does ansiotropic reflection occur?  Anisotropic reflection -- example  Occurs on objects with fine grain in a given direction. Blevins Blevins Ward Anisotropic Models Anisotropic Models  Ward Model [Ward92]  Ward Model - Isotropic  Designed for both accuracy and ease of use  Includes model for anisotropic reflection 2 2 − (tan γ ) / α ρ 1 e ρ = d + ρ ( • ) s 2 π 4 πα cos θ cos δ diffuse specular 3

Anisotropic Model Anisotropic Models  Ward Model -- anisotropic  Ward Model  ρ d - Diffuse reflectance coefficient (can vary with wavelength)  ρ s - Specular reflectance coefficient (can vary with wavelength)  α - Standard deviation of surface slope 2 2 2 2 2 − (tan γ (cos φ / α + sin φ / α )) ρ 1 e x y ρ = d + ρ ( • ) s π cos θ cos δ 4 πα α x y diffuse specular Anisotropic Models Ward’s Anisotropic Model  Ward Model w/ ansiotropy  α x - Standard deviation of surface slope in x-direction  α y - Standard deviation of surface slope in y-direction Anisotropic Models Anisotropic Models  Ward Model - example  Other anisotropic models (all based on physics)  [Kajia85]  [Poulin90]  [He91] Photo Isotropic Anisotropic 4

BRDF Subsurface Scattering  Simplifying Assumptions wrt the BRDF  Light enters and leaves from the same point.  Not necessarily true  Subsurface scattering  Skin, marble  Light of a given wavelength will only reflect back light of that same wavelength  Not necessarily true  Light Interference Jensen, et al  Oily patches, peacock feathers 2001 bidirectional surface scattering Subsurface Scattering distribution function (BSSDF)  Example: Skin  Relates outgoing reflectance in a given direction (at a given point) to the incoming luminance arriving at another point. Blevins,2001 bidirectional surface scattering distribution function (BSSDF) BSSDF -- Examples incoming BSSDF Outgoing luminance at x i luminance at x o in the direction in the direction of w i of w o When x o == x i the BSSDF is simply a BRDF Using BSSDF Using BRDF Jensen, et al 2001 5

BSSDF -- Examples BSSDF -- Examples Using BSSDF Using BSSDF Using BRDF Using BRDF Jensen, et al 2001 Jensen, et al 2001 BSSDF Modeling Light transport functions  Won Henrik Wann Jensen an academy  BSSRDF (Bidirectional surface scattering reflectance distribution function) describes award in 2004. the relation between outgoing radiance and the incident flux, including the phenomena  Practical model described in [Jensen, et. like subsurface scattering (SSS). al. 2001]  BRDF (Bidirectional reflectance distribution function) is a simplified BSSRDF, assuming that light enters and leaves at the same point Light Transport Functions Light transport functions  BTDF (Bidirectional transmittance distribution function) is similar to BRDF but for the opposite side of the surface. (see the top image).  BSDF (Bidirectional scattering distribution function) is the most general function. Wikipedia 6

Light transport functions Summary  Advanced models of reflection  Anisotropic Models  BSSDF – subsurface scattering  Complete transport functions.  Adding to ray tracer.  Break. 7