# Ray tracing. Computer Graphics COMP 770 (236) Spring Instructor: Brandon Lloyd 3/19/07 1

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1 Ray tracing Computer Graphics COMP 770 (236) Spring 2007 Instructor: Brandon Lloyd 3/19/07 1

2 From last time Hidden surface removal Painter s algorithm Clipping algorithms Area subdivision BSP trees Z-Buffer Ray casting 3/19/07 2

3 Today s topics Recursive ray tracing Intersection tests Plane Polygon Sphere 3/19/07 3

4 Recursive ray casting Ray casting generally dismissed early on: Takes no advantage of screen space coherence Requires costly visibility computation Only works for solids Forces per pixel illumination evaluations Not suited for immediate mode rendering Gained popularity in when Turner Whitted (1980) recognized that recursive ray casting could be used for global illumination effects 3/19/07 4

5 Generalized ray casting Ray casting used to compute visibility at the eye Visibility for arbitrary points needed for shading Reflections Refraction and transparency Shadows We need to compute the first surface hit along a ray represent ray with origin and direction compute intersections of objects with ray return closest object p(t) = o + td o d p 3/19/07 5

6 Generating primary rays ntan( θ width 2) height ntan( θ 2) x = r y = u width height ( width ) ( height ) c = x y + n l 2 2 d= c + ix + jy i [0,width),j [0,height) u c y o l θ d 3/19/07 6 n

7 Generating secondary rays The origin is the intersection point p 0 Direction depends on the type of ray shadow rays use direction to the light source reflection rays use incoming direction and normal to compute reflection direction transparency continue with same direction refraction use snell s law 3/19/07 7

8 Intersection tests Go through all of the objects in the scene to determine the one closest to the origin of the ray (the eye). Strategy: Solve of the intersection of the Ray with a mathematical description of the object. 3/19/07 8

9 Simple strategy Parametric ray equation Gives all points along the ray as a function of the parameter p(t) = o + td Implicit surface equation Describes all points on the surface as the zero set of a function f (p) = Substitute ray equation into surface function and solve for t 0 f (o + td) = 0 3/19/07 9

10 Ray plane intersection Implicit equation of a plane: Substitute ray equation: Solve for t: n p d= 0 n(o + td) d= 0 t(n d) = d n o d n o t = nd 3/19/07 10

11 Generalizing to Triangles Find of the point of intersection on the plane containing the triangle Determine if the point is inside the triangle barycentric coordinate method half-plane method v 1 o v 3 p v 2 3/19/07 11

12 Barycentric coordinates Points in a triangle have positive barycentric coordinates: p = α v + α v + α v α + α + α = Expressed as a matrix equation: v 1 V -1 can be precomputed to accelerate this calculation px v1x v2x v3x α1 p = v v v α y 1y 2y 3y 2 pz v1z v2z v3z α 3 p = Vα 1 α = V p v 3 p v 2 3/19/07 12

13 Barycentric coordinates Since we only care about the signs of the barycentric coordinates, we don t have to compute a complete inverse V (v v ) T Adjoint( V) T = Adjoint( V) = (v3 v 1) Det( ) V T (v1 v 2) 3/19/07 13

14 Issues Numerical robustness. There are lots of places that errors could creep in. What if the point p is slightly off the plane? What if the weights don t sum exactly to 1? Could be caused by p being off the plane as above, or by slight errors in computing Adjoint(V). Manifest itself when triangle is viewed near a grazing angle. No advantage in computing the Adjoint(V) rather than the full inverse if inverse is precomputed Ray-tracing is, in general, dominated by ray-intersection test It pays to precompute as much as possible Barycentric coordinates can be used for interpolating vertex parameters normals, colors, texture coordinates, etc. v 3 v 1 p v 2 3/19/07 14

15 Half-Plane Method Test intersection point against planes connecting each triangle edge with the eye s position Closely related to the Adjoint(V) of barycentric coordinates but more general v 1 v3 p v 2 o 3/19/07 15

16 Implementation Steps Compute plane equations: n p d = 0 n = (v a o) (v b Find d value: n o = d o) Pick a plane orientation: Several choices here. The easiest is to evaluate p in the for the first plane, and make sure that all subsequent planes have the same sign. Assuming that you traverse the edges in a consistent order. 3/19/07 16

17 Tradeoffs of Half-Plane Method Can still preprocess all triangles, but the cross products depend on the viewing position Numerically more robust including the eye as in our computation avoids problems when the intersection point lies slightly off the plane. Relation to the barycentric coordinate method cross products of half plane method are the same as in Adjoint(V) if the eye position is the origin 3/19/07 17

18 Spheres Intersecting a sphere with a ray originating from the outside: t v A sphere is defined by its center, s, and its radius r. The intersection of a ray with a sphere can be computed as follows: A vector from the eye to the sphere s center. b c 2 = = Apply the Pythagorean law to find b s eye c c v 2 t And once more to find t = v v = c dx r 2 b 2 How many dx -sized steps to the rays point of closest approach from the sphere s center. 3/19/07 18

19 Speeding up Intersection Test Early Rejection Ray tracing dominated by intersection tests Try to reduce unnecessary work Test for sphere within range of ray When testing all the objects, we want the closest. At any point we usually have a t best that is the closest intersection we have found so far. 3/19/07 19

20 More Trivial Rejections Test the radicand r 2 -b 2 < 0 Have to test anyway to avoid generating an exception Means that the ray misses the sphere (no real roots) t = v r 2 b 2 b e dx c r s 3/19/07 20

21 Next time Robustness issues Optimizations Acceleration structures Distribution ray tracing anti-aliasing depth of field soft shadows motion blur Volume effects Code structure 3/19/07 21

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