Light and Matter For Computer GraphicsOverviewOpticsReflection and TransmissionTypes of ReflectionSlide 6Types of GlossSlide 8Computing The Specular Reflection VectorTypes of TransmissionIndex of RefractionSnell’s Law of RefractionTotal Internal ReflectionComputing The Specular Transmission VectorSurface ModelsBasic Surface ModelsSlide 17What’s Missing?Surface RoughnessSlide 20Reflectance Distribution ModelAnisotropyBRDFProperties of the BRDFDimensionality of BRDFRadiometryRadiometry vs. PhotometryColorSampling WavelengthWhere to Sample?Slide 31Light TransportLocal vs. Global IlluminationFor Next Time…ReferencesMore Detail: ScatteringLight and MatterFor Computer GraphicsComp 770 LectureSpring 2009OverviewA very high-level introduction to some concepts and definitions underlying image synthesis.OpticsMaterials and SurfacesRadiometry and PhotometryColorEnergy TransportOpticsThe study of light has 3 sub-fields.Physical optics: study of the wave nature of light. Geometric optics: study of the particle nature of light. Quantum optics: study of the dual wave-particle nature of light and attempt to construct unified theories to support duality. Wave “packets” called photons.Computer graphics most concerned with geometric optics (but need some of the others, too).Reflection and TransmissionReflection: “process whereby light of a specific wavelength incident on a material is at least partly propagated outward by the material without change in wavelength.”Transmission (or refraction): “process whereby light of a specific wavelength incident on the interface (boundary) between two materials passes (refracts) through the interface without change in wavelength.”(Definitions from Glassner1995).Types of ReflectionSpecular (a.k.a. mirror or regular) reflection causes light to propagate without scattering.Diffuse reflection sends light in all directions with equal energy.Mixed reflection is a weighted combination of specular and diffuse.Types of ReflectionRetro-reflection occurs when incident energy reflects in directions close to the incident direction, for a wide range of incident directions.Gloss is the property of a material surface that involves mixed reflection and is responsible for the mirror like appearance of rough surfaces.Types of GlossGloss factors measured by the ratio of energy () in the reflected and incident directions for certain standard angles (i and r).Specularity measures the brightness of a highlight: r /i (i = r = 60°). Sheen measures the brightness of glancing highlights: r /i (i = r = 85°).Types of GlossContrast is the brightness of a glancing highlight relative to the brightness in the surface normal direction r /n. (i = r = 85°).Distinctness of Image measures the clarity of the highlight or the sharpness of its borders: dr / dr , or the rate of change of reflected energy with reflected direction.Absence of Bloom measures the haziness around the highlight: r2 /r1, where r1 and r2 are only a few degrees different.Computing The Specular Reflection VectorriINRIN’N’RGiven: I, N, R are coplanar. I N = R NN’ = (I N)NFrom the parallelogram shown at right, see:R + I = 2N’OrR = 2N’ – I = 2(I N)N - ITypes of TransmissionSpecular transmission causes light to propagate w/o scattering, as in clearglass.Diffuse transmission sends light in all directions with equal energy, as infrosted glass.Mixed transmission is a weighted combination of specular and diffuse transmission.Index of RefractionThe speed of light is not the same in all media.Reference medium is a perfect vacuum.IOR: i() = c / v. c = speed of light in vacuum, v is speed of light of wavelength  in the medium. Surface where two media touch called the interface.Light appears to bend when passing through the interface, due to change in speed.Amount of bending, or refraction, determined by the IOR of both materials.Snell’s Law of RefractionGoverns the geometry of refraction.i()sini = t()sint i = IOR of incident medium t = IOR of medium into which the light is transmittedIf the light is transmitted intoa denser medium, it is refracted toward the normal of the interface.If the light is transmitted into a rarer medium, it is refracted away from the normal of the interface.sini i t I T N sintTotal Internal ReflectionAt some angle, called the critical angle, light is bent to lie exactly in the plane of the interface.At all angles greater than this, the light is reflected back into the incident medium: total internal reflection (TIR).Snell’s law gives critical angle ci()sinc = t()sin( / 2)sinc = t () / i()Computing The Specular Transmission VectoritITNR-M MII||TT||      occurred. has TIR negative, is cos11 Ifcos11cos:ngSubstituticos11sin1sin1coscoscoscoscossinsin :Law sSnell'Apply coscossinsincossincossin1:so ,sin ,cos ,sincos222222222||||itiitiititiitiititttitititititiittiitttiittiiNITNINNINNINMTTTNIMTTINIISurface ModelsPerfect mirrors and reflections don’t exist.Perfect transmission requires a perfect vacuum.Real surfaces have some degree of roughness.Even most basic simulation must account for specular and diffuse reflection / transmission.More realism requires accounting for more factors.Wavelength dependence: dispersion, diffraction, interferenceAnisotropy: angular-dependence of reflectance.Scattering: absorption and re-emission of photons.Basic Surface ModelsNon-physically based, as used in OpenGL.Materials have ambient, diffuse, and specular colors.Ambient is a very coarse approx. Of light reflected from other surfaces. (Global illumination).Diffuse usually just the “color” of the surface.Specular determines highlight color.Basic Surface Models light. surface; global; orlight vect