Today More raytracing stuff Soft shadows and anti aliasing More rendering methods The text book is good on this I ll be using images from the CDROM in the back of the book Soft Shadows Light sources that extend over an area area light sources should cast soft edged shadows Some points see all the light fully illuminated Some points see none of the light source the umbra Some points see part of the light source the penumbra To raytrace area light sources cast multiple shadow rays Each one to a different point on the light source Weigh illumination by the number that get through Soft Shadows Penumbra Umbra Penumbra Soft Shadows All shadow rays go through No shadow rays go through Some shadow rays go through Anti Aliasing Raytracing can alias badly Each ray is a single point sample Problem is made worse by recursive rays the point sample depends on other point samples Common solutions Super sampling Cast multiple rays per pixel and average their contribution in some way Jittered sampling Frequently used with super sampling randomly jitters each ray within the pixel Adaptive sampling Cast extra rays through the pixel if some initial sample rays indicate that they are needed Anti Aliasing Uniform Jittered Adaptive Distribution Raytracing Distribution raytracing casts more than one ray for each sample Originally called distributed raytracing but the name s confusing Multiple rays for each pixel distributed in time Gives you motion blur a strong visual clue for motion Cast multiple reflection rays at a reflective surface Gives you rough blurry reflections Simulate multiple paths through the camera lens system Gives you depth of field an important visual clue for depth Distribution Raytracing Depth of Field Watt learning Study12 Raytracing Can t Do Recall light paths Which does a raytracer capture Which can it not capture Ray traced Cornell box due to Henrik Jensen http www gk dtu dk hwj Missing Paths Basic recursive raytracing cannot do LS D E Light bouncing off a shiny surface like a mirror and illuminating a diffuse surface LD E Light bouncing off one diffuse surface to illuminate others Basic problem The raytracer doesn t know where to send rays out of the diffuse surface to capture the incoming light Also a problem for rough specular reflection Fuzzy reflections in rough shiny objects Bi directional Raytracing Cast rays from the light sources out into the scene Light pass When a ray hits a diffuse surface accumulate some light there Surfaces record the amount of light that hits them Store the light in texture maps Store the light in quadtrees Store the light in photon maps Cast rays from the eye out into the scene Eye pass When a ray hits a diffuse surface look up the amount of light that hit it in the light ray phase What paths does it capture What sort of visual effects do you see Caustics Standard raytracer Bi directional raytracer Note the LS DS E paths Watt learning Study13 More rays in the light pass Refraction caustic Henrik Jensen http www gk dtu dk hwj Refraction caustics Henrik Jensen http www gk dtu dk hwj Still Missing LD E paths Diffuse diffuse transport Formulated and solved with radiosity methods L S D E paths Solved with Monte Carlo renderers very very inefficient Also solvable with multi pass methods but also very very inefficient and subject to aliasing An unsolved unsolvable problem Real World LD E Paths Watt learning Study15 Museu4 tif Radiosity Assumptions All surfaces are perfectly diffuse Means that is doesn t matter which way light hits or leaves a surface Illumination is constant over a patch Can break the world up into a discrete number of pieces Problems at sharp illumination boundaries shadows Ways around these problems but less efficient and less able to manage scene complexity Assumptions allow us to solve for LD E paths Radiosity Equation N Bi Ei i Fij B j j 1 Derived from the global illumination equation using radiosity assumptions Bi is the radiosity brightness of patch i i is the diffuse reflection coefficient Fij is the form factor which quantifies how much light patch j contributes to patch i The brightness of each patch depends on how much light it gets from all the others and its diffuse reflection Solving the Radiosity Eqn Radiosity algorithms use one of several methods to solve the radiosity equation Basically a very large linear system so techniques can all be mapped onto linear system solvers A large part of the computation is in finding form factors Describe how much light gets from each patch to every other patch Geometric in nature do not depend on the illumination just the layout of the scene Another key factor is finding good meshing strategies ways of laying out the patches Radiosity Example Color bleeding is extreme in this example Textures are applied after solving for illumination Some meshing artifacts are visible note the banding around the pictures on the wall Watt learning Study15 Rdcb2 tif Radiosity Meshing Each patch is colored with its illumination Note the discrete nature of the solution The previous image was obtained by pushing color to vertices and then Gourand shading Watt learning Study15 Rdcb1 tif Next time We switch from rendering to animation last regular class
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