Last Time Animation Key frame hand animation Motion capture Procedural Evaluations 5 6 04 University of Wisconsin CS559 Spring 2004 Today Advanced rendering Raytracing Photon mapping Radiosity 5 6 04 University of Wisconsin CS559 Spring 2004 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 them 5 6 04 University of Wisconsin CS559 Spring 2004 Global 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 2004 Light 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 region 5 6 04 University of Wisconsin CS559 Spring 2004 Reflectance 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 made 5 6 04 University of Wisconsin CS559 Spring 2004 Simple 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 direction 5 6 04 University of Wisconsin CS559 Spring 2004 Light 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 paths 5 6 04 University of Wisconsin CS559 Spring 2004 Classifying 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 S or D Pure diffuse D Pure specular S S or D S or D Assume all paths of interest Start at a light source L End at the eye E Light Eye 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 eye 5 6 04 University of Wisconsin CS559 Spring 2004 Simple 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 2004 More Complex Light Paths Find the following 5 6 04 University of Wisconsin CS559 Spring 2004 LE LDE LSE LDDE LDSE LSDE Radiosity Cornell box due to Henrik wann Jensen http www gk dtu dk hwj rendered with ray tracer More Complex Light Paths LE LDDE LSDE LSE LDSE LDE 5 6 04 University of Wisconsin CS559 Spring 2004 The 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 others 5 6 04 University of Wisconsin CS559 Spring 2004 Raytracing 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 color 5 6 04 University of Wisconsin CS559 Spring 2004 Raytracing Shadow rays Reflection ray Transmitted ray 5 6 04 University of Wisconsin CS559 Spring 2004 Recursive 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 required Send out a new refracted ray if required Generally reduce the weight of each additional ray when computing the contributions to surface color Stop when the contribution from a ray is too small to notice What light paths does recursive ray tracing capture 5 6 04 University of Wisconsin CS559 Spring 2004 PCKTWTCH by Kevin Odhner POV Ray 5 6 04 University of Wisconsin CS559 Spring 2004 Kettle Mike Miller POVRay 5 6 04 University of Wisconsin CS559 Spring 2004 5 6 04 University of Wisconsin CS559 Spring 2004 Which paths are missing Ray traced Cornell box due to Henrik Jensen http www gk dtu dk hwj 5 6 04 University of Wisconsin CS559 Spring 2004 5 6 04 University of Wisconsin CS559 Spring 2004 Missing Paths Raytracing cannot do LS D E Light bouncing off a shiny surface like a mirror and
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