Classic Ray Tracing

							Ray Tracing

Today
          Basic algorithms
          Overview of pbrt
          Ray-surface intersection for single surface
Next lecture
          Acceleration techniques for ray tracing large
           numbers of geometric primitives




CS348B Lecture 2                          Pat Hanrahan, Spring 2005
Classic Ray Tracing

Greeks: Do light rays proceed from the eye to the light,
   or from the light to the eye?

Gauss: Rays through lenses

Three ideas about light
     1. Light rays travel in straight lines
     2. Light rays do not interfere with each other if they
     cross
     3. Light rays travel from the light sources to the
     eye (but the physics is invariant under path reversal
     - reciprocity).

CS348B Lecture 2                          Pat Hanrahan, Spring 2005
Ray Tracing in Computer Graphics

Appel 1968 - Ray casting
1. Generate an image by sending one ray per pixel
2. Check for shadows by sending a ray to the light




CS348B Lecture 2                     Pat Hanrahan, Spring 2005
Ray Tracing in Computer Graphics




Whitted 1979
     Recursive ray tracing (reflection and refraction)
CS348B Lecture 2                        Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (g/m 10)




CS348B Lecture 2           Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (mirror 0)




CS348B Lecture 2            Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (mirror 1)




CS348B Lecture 2            Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (mirror 2)




CS348B Lecture 2            Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (mirror 5)




CS348B Lecture 2            Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (mirror 10)




CS348B Lecture 2            Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (glass 0)




CS348B Lecture 2            Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (glass 1)




CS348B Lecture 2            Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (glass 2)




CS348B Lecture 2            Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (glass 5)




CS348B Lecture 2            Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (glass 10)




CS348B Lecture 2           Pat Hanrahan, Spring 2005
Spheres-over-plane.pbrt (glass 10)




CS348B Lecture 2           Pat Hanrahan, Spring 2005
History

Polygons                Appel ‘68
Quadrics, CSG Goldstein & Nagel ‘71
Tori                    Roth ‘82
Bicubic patches         Whitted ‘80, Kajiya ‘82
Superquadrics Edwards & Barr ‘83
Algebraic surfaces      Hanrahan ‘82
Swept surfaces          Kajiya ‘83, van Wijk ‘84
Fractals                Kajiya ‘83
Height fields           Coquillart & Gangnet ’84, Musgrave ‘88
Deformations            Barr ’86
Subdivision surfs.      Kobbelt, Daubert, Siedel, ’98

P. Hanrahan, A survey of ray-surface intersection algorithms

CS348B Lecture 2                                  Pat Hanrahan, Spring 2005
Ray-Surface Intersection
Ray-Plane Intersection

Ray: P  O  t D                        t
                                                D
           0t                                                        N
                                 O                             P

Plane: (P  P)  N  0
                                                                      P
             ax  by  cz  d  0

Solve for intersection
     Substitute ray equation into plane equation
                   (P  P  )  N  (O  t D  P  )  N  0
                       (O  P  )  N
                   t
                          DN
CS348B Lecture 2                                     Pat Hanrahan, Spring 2005
Ray-Polyhedra


 Ray-Slab                    Note: Procedural geometry


                   t1   t2

 Ray-Box                     Ray-Convex Polyhedra




CS348B Lecture 2                       Pat Hanrahan, Spring 2005
Ray-Triangle Intersection 1

                         P1        Barycentric coordinates

                                     P  s1P1  s2P2  s3P3
              s2        s3
                                   Inside triangle criteria
                   P
 P3                s1                 0  s1  1
                              P2      0  s2  1
                                      0  s3  1
    s1  area( PP2 P3 )
                                      s1  s2  s3  1
    s2  area( PP3P1 )

    s3  area( PP1P2 )

CS348B Lecture 2                                         Pat Hanrahan, Spring 2005
Ray-Triangle Intersection 2


                    P  s1P1  s2 P2  s3P3

                                    s1 
                   P
                    1    P2   P3  s2    P 
                                    
                                    s3 
                                    

                         s1   P2  P3 
                        s    P  P  P
                         2  3       1  

                         s3   P1  P2 
                                      
CS348B Lecture 2                             Pat Hanrahan, Spring 2005
2D Homogeneous Coordinates


                                   x1    y1     z1
                    P1  P2  P3  x2    y2    z2
                                   x3    y3    z3


                                    x1    y1     z1
                   area(P1P2 P3 )  x2    y2     z2
                                    x3    y3     z3


CS348B Lecture 2                               Pat Hanrahan, Spring 2005
Ray-Triangle Intersection 3

                        P P2 P3       P2  P3
                   s1            P          P  P2  P3
                        P1 P2 P3         
                        P1 P P3       P3  P1
                   s2            P          P  P3  P1
                        P1 P2 P3         
                        P1 P2 P       P P
                   s3            P  1 2  P  P1  P2
                        P1 P2 P3        


                              s1   P2  P3 
                             s    P  P  P
                              2  3       1  

                              s3   P1  P2 
                                           

CS348B Lecture 2                                     Pat Hanrahan, Spring 2005
Ray-Polygon Intersection

1. Find intersection with plane of support
2. Test whether point is inside 3D polygon
       a.   Project onto xy plane
       b.   Test whether point is inside 2D polygon




CS348B Lecture 2                          Pat Hanrahan, Spring 2005
Point in Polygon
   inside(vert v[], int n, float x, float y)
   {
     int cross=0; float x0, y0, x1, y1;
       x0 = v[n-1].x - x;
       y0 = v[n-1].y - y;
       while( n-- ) {
         x1 = v->x - x;
         y1 = v->y - y;
         if( y0 > 0 ) {
           if( y1 <= 0 )
             if( x1*y0 > y1*x0 ) cross++;
         }
         else {
           if( y1 > 0 )
             if( x0*y1 > y0*x1 ) cross++;
         }
         x0 = x1; y0 = y1; v++;
       }
       return cross & 1;
   }
CS348B Lecture 2                       Pat Hanrahan, Spring 2005
Ray-Sphere Intersection


              D              Ray:      P  O  tD
      P
O
                                               
                                                    2
                             Sphere:       P  C  R2  0
                   C
                                                       
                                                            2
                                           O  t D  C  R2  0

             b  b2  4ac             at 2  bt  c  0
          t
                  2a
                                       a  D2

                                       b  2 (O  C)  D
                                       c  (O  C) 2  R 2
CS348B Lecture 2                               Pat Hanrahan, Spring 2005
Geometric Methods

Methods                                                    N
          Find normal and tangents
                                           P, u, v
          Find surface parameters
                                                                T
                                                     S
e.g. Sphere
     Normal          N  PC


     Parameters      x  sin  cos      tan 1( x, y)
                     y  sin  sin      cos1 z
                     z  cos 

CS348B Lecture 2                         Pat Hanrahan, Spring 2005
Ray-Implicit Surface Intersection


   f ( x, y, z)  0                1. Substitute ray equation
                                   2. Find positive, real roots

                   x  x0  x1 t
                                   Univariate root finding
                   y  y0  y1 t           Newton’s method

                   z  z0  z1 t
                                           Regula-falsi
                                           Interval methods
                                           Heuristics

       f (t )  0
         *




CS348B Lecture 2                                Pat Hanrahan, Spring 2005
Ray-Algebraic Surface Intersection

                                   Degree n
  pn ( x, y, z)  0                    Linear: Plane
                                       Quadric: Spheres, …
                   x  x0  x1 t       Quartic: Tori

                   y  y0  y1 t
                   z  z0  z1 t   Polynomial root finding
                                          Quadratic, cubic, quartic
                                          Bezier/Bernoulli basis
       p (t )  0
         *
         n
                                          Descartes’ rule of signs
                                          Sturm sequences

CS348B Lecture 2                               Pat Hanrahan, Spring 2005