1Lighting affects appearance2LightSource emits photonsPhotons travel in a straight lineWhen they hit an object they:• bounce off in a new direction• or are absorbed• (exceptions later).And then some reach the eye/camera.3ReflectanceModel how objects reflect light.Model light sourcesAlgorithms for computingShading: computing intensities within polygonsDetermine what light strikes what surfaces.Basic fact: Light is linearDouble intensity of sources, double photons reaching eye.Turn on two lights, and photons reaching eye are same as sum of number when each light is on separately.This means we can render lights separately4Light Model: Point Source•Light emanates from a point•Equal intensity in all directions• Intensity drops off with distance.• With square of distance: • To simulate effects of non-point sources: 22101dadaa21d5Light model: distant point source•All light in scene comes from same direction.•With same intensitySurfaces reflect light: Lambertian•Amount of light striking surface proportional to cos• Angle between light direction and surface.•Equal brightness in all directions •Albedo is fraction of light reflected. •Diffuse objects (chalk, cloth, matte paint).• Brightness doesn’t depend on viewpoint.Surface normalLight6Lambertian + Point SourceSurface normalLightlˆnormal surface isˆ isradiance is)ˆ(,0max(light ofintensity islight ofdirection isˆˆnalbedoinlilllll  Lambertian ExamplesScene(Oren and Nayar)Lambertian sphere as the light moves.(Steve Seitz)7AmbientAssume Lambertian surface normal receives equal light from all directions.Diffuse lighting, no cast shadows.Ambient (and point) light can be coloredaiAmbient + Point SourceNeeded to avoid artifactsMake sure shadows aren’t black.Reasonable approximation to general Sun + sky. Lamp + light reflected by wallsIn fact, it’s a 1storder approximation.But doesn’t handle many effectsSources of other shapes. Shadows of ambient light in concave objects.8Shadow exampleEnvironment MapEnvironment map: l(   - Light from all directions- Diffuse or point sources- Still distant- Still no cast shadows.- Example: outdoors (sky and sun)Sky9`Specular surfaces• Another important class of surfaces is specular, or mirror-like.– radiation arriving along a direction leaves along the specular direction– reflect about normal– some fraction is absorbed, some reflected– color depends on color of incoming light, not of surface.(http://graphics.cs.ucdavis.edu/GraphicsNotes/Shading/Shading.html)10Specular Directionrnllnnlrnnllr )(2)(2Specular surfaces(http://graphics.cs.ucdavis.edu/GraphicsNotes/Shading/Shading.html)•Brightness depends on viewing direction.•Specularity is spread out.•Mirror, smooth light all bounces same way.•Slightly rougher, direction of bounce varies.•Diffuse, many bouncesnnl)(2nnl)(211Phong’s model• Empirical model (eg., hack)• Phong’s model– reflected energy falls off as:• n very big, this is like mirror.• As n gets smaller, specularity more spread out.• Good model for plastic• Specularity color of source.cosn (Forsyth & Ponce)Lambertian + specular• Two parameters: how shiny, what kind of shiny.• Many objects combine shiny and diffuse material• Wood with veneer; glossy paint, plastic, greasy skin.12Lambertian+Specular+Ambient(http://graphics.cs.ucdavis.edu/GraphicsNotes/Shading/Shading.html)More complex reflectancesPhysically realistic modelsTorrance Sparrow models roughness of surfaces and shadowing of microfacets.Models built from observation.Measurement for every lighting direction and viewing direction.13BRDF Not Always Appropriatehttp://graphics.stanford.edu/papers/bssrdf/(Jensen, Marschner, Levoy, Hanrahan)BRDF BSSRDFLuminescenceSurface shifts color of light.Can reflect more light of a color than is present in source.This is why objects can glow in