Shading RevisitedLight TransportRadiometryLambert’s wallMore complex wallSlide 6Reflectance ModelingSimple BRDFsLight SourcesGlobal Illumination EquationPhotorealistic LightingClassifying Rendering AlgorithmsClassifying Light PathsSimple Light Path ExamplesMore Complex Light PathsSlide 16The OpenGL ModelRaytracingSlide 19Recursive Ray TracingPowerPoint PresentationSlide 22Slide 23Slide 24Next week…Shading Revisited•Some applications are intended to produce pictures that look photorealistic, or close to it–The image should look like a photograph–A better metric is perceptual: the image should generate a target set of perceptions–Applications include: Film special effects, Training simulations, Computer games, Architectural visualizations, Psychology experiments, …•To achieve the goal of photorealism, we must think carefully about light and how it interacts with surfaces•What you should take away: The various aspects of light interaction and how algorithms capture or ignore themLight Transport•Light transport problems are concerned with how much light arrives at any surface, and from what direction•The physical quantity of interest is radiance: How much light (power) is traveling along a line in space per unit foreshortened area per unit solid angle–We will not go into the theory - it takes 3 hours just to give the definitions and equations•Similar problems arise in radiated heat transport (ie satellites), where some of the technology was originally developedRadiometry•Radiometry: The study of light distribution:–how “bright” will surfaces be? –what is “brightness”?•measuring light•interactions between light and surfaces•Core idea - think about light arriving at a surface•Around any point is a hemisphere of directions•Simplest problems can be dealt with by reasoning about this hemisphereLambert’s wallHow bright are various locations on the plane?More complex wallLight TransportWhich surface gets more light? Why?How much light reaches point “a”?•If the walls are black?•If the walls are mirrors?aabReflectance Modeling•Reflectance modeling is concerned with the way in which light reflects off surfaces–Clearly important to deciding what surfaces look like–Also important in solving the light transport problem•Physical quantity is BRDF: Bidirectional Reflectance Distribution Function–A function of a point on the surface, an incoming light direction, and an outgoing light direction–Tells you how much of the light that comes in from one direction goes out in another direction–General BRDFs are difficult to work with, so simplifications are madeSimple BRDFs•Diffuse surfaces:–Uniformly reflect all the light they receive•Sum up all the light that is arriving: Irradiance•Send it back out in all directions–A reasonable approximation for matte paints, soot, carpet•Perfectly specular surfaces:–Reflect incoming light only in the mirror direction•Rough specular surfaces:–Reflect incoming light around the mirror direction•Diffuse + Specular:–A diffuse component and a specular componentLight Sources•Sources emit light: exitance•Different light sources are defined by how they emit light:–How much they emit in each direction from each point on their surface–For some algorithms, “point” lights cannot exist–For other algorithms, only “point” light can existGlobal Illumination Equation•The total light leaving a point is given by the sum of two major terms:–Exitance from the point–Incoming light from other sources reflected at the pointdLLLioobdooeoocos),,(),,,,(),,(),,( xxxxLight leavingExitance Sum BRDF IncominglightIncoming light reflected at the pointPhotorealistic Lighting•Photorealistic lighting requires solving the equation!–Not possible in the general case with today’s technology•Light transport is concerned with the “incoming light” part of the equation–Notice the chicken and egg problem•To know how much light leaves a point, you need to know how much light reaches it•To know how much light reaches a point, you need to know light leaves every other point•Reflectance modeling is concerned with the BRDF–Hard because BRDFs are high dimensional functions that tend to change as surfaces change over timeClassifying Rendering Algorithms•One way to classify rendering algorithms is according to the type of light interactions they capture•For example: The OpenGL lighting model captures:–Direct light to surface to eye light transport–Diffuse and rough specular surface reflectance–It actually doesn’t do light to surface transport correctly, because it doesn’t do shadows•We would like a way of classifying interactions: light pathsClassifying Light Paths•Classify light paths according to where they come from, where they go to, and what they do along the way•Assume only two types of surface interactions:–Pure diffuse, D–Pure specular, S•Assume all paths of interest:–Start at a light source, L–End at the eye, E•Use regular expressions on the letters D, S, L and E to describe light paths–Valid paths are L(D|S)*ESimple Light Path Examples•LE–The light goes straight from the source to the viewer•LDE–The light goes from the light to a diffuse surface that the viewer can see•LSE–The light is reflected off a mirror into the viewer’s eyes•L(S|D)E–The light is reflected off either a diffuse surface or a specular surface toward the viewer•Which do OpenGL (approximately) support?Radiosity Cornell box, due to Henrik wann Jensen,http://www.gk.dtu.dk/~hwj, rendered with ray tracerMore Complex Light Paths•Find the following:–LE–LDE–LSE–LDDE–LDSE–LSDERadiosity Cornell box, due to Henrik wann Jensen,http://www.gk.dtu.dk/~hwj, rendered with ray tracerMore Complex Light PathsLELDDELDELSDELSELDSEThe OpenGL Model•The “standard” graphics lighting model captures only L(D|S)E•It is missing:–Light taking more than one diffuse bounce: LD*E•Should produce an effect called color bleeding, among other things•Approximated, grossly, by ambient light–Light refracted through curved glass•Consider the refraction as a “mirror” bounce: LDSE–Light bouncing off a mirror to illuminate a diffuse surface: LS+D+E–Many othersRaytracing•Cast rays out from the eye, through each pixel, and determine what they hit first•Cast additional rays from the hit point to
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