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UW-Madison CS 559 - Lecture 26

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Raytracing ImplementationConstructing RaysRay-Object IntersectionsRay-Sphere IntersectionRay-Plane IntersectionsPoint-in-Polygon TestingMore complex testsCSG Intersection TestingRay-Patch IntersectionDetailsTransforming Normal VectorsNumerical IssuesMapping TechniquesTexture MappingBump MappingBump Mapping ExamplesDisplacement MappingPowerPoint PresentationProcedural TexturingRaytracing Implementation•Raytracing breaks down into two tasks:–Constructing the rays to cast–Intersecting rays with geometry•The former problem is simple vector arithmetic•The intersection problem arises in many areas of computer graphics–Collision detection–Other rendering algorithms•Intersection is essentially root finding (as we will see)–Any root finding technique can be appliedConstructing Rays•Define rays by an initial point and a direction: x(t)=x0+td•Eye rays: Rays from the eye through a pixel–Construct using the eye location and the pixel’s location on the image plane. x0=ey e•Shadow rays: Rays from a point on a surface to the light.–x0=point on surface•Reflection rays: Rays from a point on a surface in the reflection direction–Construct using laws of reflection. x0=surface point•Transmitted rays: Rays from a point on a transparent surface through the surface–Construct using laws of refraction. x0=surface pointRay-Object Intersections•Aim: Find the parameter value, ti, at which the ray first meets object i•Transform the ray into the object’s local coordinate system–Makes ray-object intersections generic: ray-sphere, ray-plane, …•Write the surface of the object implicitly: f(x)=0–Unit sphere at the origin is x2-1=0–Plane with normal n passing through origin is: n•x=0•Put the ray equation in for x–Result is an equation of the form f(t)=0 where we want t–Now it’s just root findingRay-Sphere Intersection•Quadratic in t–2 solutions: Ray passes through sphere - take minimum value that is > 0–1 solution: Ray is tangent - use it if >0–0 solutions: Ray does not hit sphere        012:01 :Substitute01 :Sphere)( :Ray0002000-----xxdxdddxdxxxdxxttttttRay-Plane Intersections•To do polygons, intersect with plane then do point-in-polygon test…  dnxnxndndxnxndxx------0000:0:0 :Substitute0 :Plane)( :RaytttttPoint-in-Polygon Testing•Project point and polygon onto a 2D plane–Find biggest component of normal vector, and just use other two coordinates–For example, if n=(0.2, 0.4, 0.9), just use x,y coordinates, which is like projecting down onto the x-y plane•Cast a ray from the point to infinity and count the number of edges it crosses–Odd number means point is inside–Edge crossing tests are very fast - think clippingMore complex tests•Ray-Polygon test reveals a common strategy–Intersect with something easy - a superset of the actual shape–Do a bounds check to make sure you have actually hit the shape•Also works for cylinders, disks, cones•CSG is well suited to raytracing–Find intersection along ray of all the CSG primitives–Break the ray into intervals according to which primitives it is in–Do set operations to find the first interval that is actually inside the CSG objectCSG Intersection TestingSubtractIntersection intervalsSubtract intervalsRay-Patch Intersection•Equation in 3 parameters, two for surface and one for ray•Solve using Newton’s method for root finding–Have derivatives from basis functions–Starting point from control polygon, or random guess, or try a whole set of different starting values0)()( :Substitute0)()(),( :Patch)( :Ray30i30030i300  jjiijjjiijvBuBtvBuBvuttPdxPxdxxDetails•Must find first intersection of ray from the eye–Find all candidate intersections, sort them and take soonest–Techniques for avoiding testing all objects•Bounding boxes that are cheap to test•Octrees for organizing objects in space–Take care to eliminate intersections behind the eye–Same rules apply for reflection and transmission rays•Shadow ray just have to find any intersection shadowing the light source–Speedup: Keep a cache of shadowing objects - test those firstTransforming Normal Vectors•Normal vectors are not transformed the same way points are–Ray directions behave like normal vectors•Plane equation should still be true with transformed points!•Transform normal vectors with the inverse transpose of the transformation matrix–For rotations, matrix is its own inverse transpose0 : EqnPlane xnT  1TTTTTMKIKMIMK0MxKn0MxKnTNumerical Issues•Shadow, reflection and transmission rays have to be sure they don’t intersect the surface they are leaving–Can’t just ignore the objects - some objects self-shadow–Solution: Use a tolerance - offset the starting point off the surface a little in the normal direction•Finding all the intersections with a spline surface patch is difficult•CSG can have problems when doing set operations–Make sure pieces being subtracted protrude above surfacesBad GoodMapping Techniques•Raytracing provides a wealth of information about the visible surface point:–Position, normal, texture coordinates, illuminants, color…•Raytracing also has great flexibility–Every point is computed independently, so effects can easily be applied on a per-pixel basis–Reflection and transmission and shadow rays can be manipulated for various effects–Even the intersection point can be modifiedTexture Mapping•Define texture parameters over the surface•Determine texture parameters at each hit point•Use them to look up a texture map, just as with OpenGL•But…–Can also use them to look up other things…Bump Mapping•Vary the surface normal vector according to some map–Influences shading and reflection and refraction•Variation may be random - for effects like stucco - or structured - for effects like tilesBump Mapping ExamplesDisplacement Mapping•Bump mapping changes only the normal, not the intersection point–Silhouettes will not show bumps, even though shading does•Displacement mapping actually shifts the intersection point according to a map–Gives bump map effects and also correct silhouettes and self shadowing, if implemented fullyFrom RmanNoteshttp://www.cgrg.ohio-state.edu/~smay/RManNotes/index.htmlProcedural Texturing•Define a shading language that can be


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UW-Madison CS 559 - Lecture 26

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