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UW-Madison CS 559 - CS 559 Lecture Notes

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Last TimeTodayShading RevisitedGlobal IlluminationLight SourcesReflectance ModelingSimple BRDFsLight TransportClassifying Light PathsSimple Light Path ExamplesMore Complex Light PathsSlide 12The OpenGL ModelRaytracingSlide 15Recursive Ray TracingSlide 17Slide 18Slide 19Slide 20Slide 21Missing PathsLight CachingCausticsRefraction causticRefraction causticsStill Missing…Radiosity ExampleRadiosity AssumptionsRadiosity Meshing5/6/04 © University of Wisconsin, CS559 Spring 2004Last Time•Animation–Key-frame (hand animation)–Motion capture–Procedural•Evaluations5/6/04 © University of Wisconsin, CS559 Spring 2004Today•Advanced rendering–Raytracing–Photon-mapping–Radiosity5/6/04 © University of Wisconsin, CS559 Spring 2004Shading 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 them5/6/04 © University of Wisconsin, CS559 Spring 2004Global Illumination•Light sources emit light•Surface reflect or absorb light•We want to know how much light reaches the image plane, and what color it is–Depends on the geometric arrangement and the surface properties•First, we need to describe lights and surfaces?5/6/04 © University of Wisconsin, CS559 Spring 2004Light Sources•Sources emit light: exitance•Different light sources are defined by how they emit light:–The power they emit in each direction from each point on their surface–For some algorithms, “point” lights cannot exist–For other algorithms, only “point” lights can exist•For example:–A frosted incandescent light globe emits roughly equal amounts in all directions from all points on the surface–A flashlight emits low power in a small set of directions from a small round region5/6/04 © University of Wisconsin, CS559 Spring 2004Reflectance Modeling•Reflectance modeling is concerned with the way in which light reflects off surfaces•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 made5/6/04 © University of Wisconsin, CS559 Spring 2004Simple 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 component•Fresnel surfaces:–Reflect some and transmit some depending on incoming direction5/6/04 © University of Wisconsin, CS559 Spring 2004Light Transport•Light transport is the problem of figuring out where the light’s power goes•Rendering algorithms solve the transport problem•One way to classify rendering algorithms is according to the type of light interactions they capture•We would like a way of classifying interactions: light paths5/6/04 © University of Wisconsin, CS559 Spring 2004Classifying 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)*E–Light, followed by either diffuse or specular, zero or more times, ending at the eyeS or DS or DS or DLightEye5/6/04 © University of Wisconsin, CS559 Spring 2004Simple 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?5/6/04 © University of Wisconsin, CS559 Spring 2004Radiosity 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–LSDE5/6/04 © University of Wisconsin, CS559 Spring 2004More Complex Light PathsLELDDELDELSDELSELDSE5/6/04 © University of Wisconsin, CS559 Spring 2004The 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 others5/6/04 © University of Wisconsin, CS559 Spring 2004Raytracing•Cast rays out from the eye, through each pixel, and determine what they hit first–Builds the image pixel by pixel, one at a time•Cast additional rays from the hit point to determine the pixel color–Shadow rays toward each light. If they hit something, then the object is shadowed from that light, otherwise use “standard” model for the light–Reflection rays for mirror surfaces, to see what should be reflected in the mirror–Transmission rays to see what can be seen through transparent objects–Sum all the contributions to get the pixel color5/6/04 © University of Wisconsin, CS559 Spring 2004RaytracingShadow raysReflection rayTransmitted ray5/6/04 © University of Wisconsin, CS559 Spring 2004Recursive Ray Tracing•When a reflected or refracted ray hits a surface, repeat the whole process from that point–Send out more shadow rays–Send out new reflected ray (if


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UW-Madison CS 559 - CS 559 Lecture Notes

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