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ARITH3(9.2)                                                        ARITH3(9.2)



NAME
       add3,  sub3,  div3,  mul3,  eqpt3, closept3, dot3, cross3, len3, dist3,
       unit3, midpt3, lerp3, reflect3, nearseg3, pldist3, vdiv3, vrem3, pn2f3,
       ppp2f3, fff2p3, pdiv4, add4, sub4 - operations on 3-d points and planes

SYNOPSIS
       #include <&lt;libg.h>&gt;

       #include <&lt;geometry.h>&gt;

       Point3 add3(Point3 a, Point3 b)

       Point3 sub3(Point3 a, Point3 b)

       Point3 div3(Point3 a, double b)

       Point3 mul3(Point3 a, double b)

       int eqpt3(Point3 p, Point3 q)

       int closept3(Point3 p, Point3 q, double eps)

       double dot3(Point3 p, Point3 q)

       Point3 cross3(Point3 p, Point3 q)

       double len3(Point3 p)

       double dist3(Point3 p, Point3 q)

       Point3 unit3(Point3 p)

       Point3 midpt3(Point3 p, Point3 q)

       Point3 lerp3(Point3 p, Point3 q, double alpha)

       Point3 reflect3(Point3 p, Point3 p0, Point3 p1)

       Point3 nearseg3(Point3 p0, Point3 p1, Point3 testp)

       double pldist3(Point3 p, Point3 p0, Point3 p1)

       double vdiv3(Point3 a, Point3 b)

       Point3 vrem3(Point3 a, Point3 b)

       Point3 pn2f3(Point3 p, Point3 n)

       Point3 ppp2f3(Point3 p0, Point3 p1, Point3 p2)

       Point3 fff2p3(Point3 f0, Point3 f1, Point3 f2)

       Point3 pdiv4(Point3 a)

       Point3 add4(Point3 a, Point3 b)

       Point3 sub4(Point3 a, Point3 b)

DESCRIPTION
       These  routines do arithmetic on points and planes in affine or projec-
       tive 3-space.  Type Point3 is

              typedef struct Point3 Point3;
              struct Point3{
                    double x, y, z, w;
              };

       Routines whose names end in 3 operate on vectors or ordinary points  in
       affine  3-space,  represented  by  their Euclidean (x,y,z) coordinates.
       (They assume w=1 in their arguments, and set w=1 in their results.)

       Name   Description

       add3   Add the coordinates of two points.

       sub3   Subtract coordinates of two points.

       mul3   Multiply coordinates by a scalar.

       div3   Divide coordinates by a scalar.

       eqpt3  Test two points for exact equality.

       closept3
              Is the distance between two points smaller than eps?

       dot3   Dot product.

       cross3 Cross product.

       len3   Distance to the origin.

       dist3  Distance between two points.

       unit3  A unit vector parallel to p.

       midpt3 The midpoint of line segment pq.

       lerp3  Linear interpolation between p and q.

       reflect3
              The reflection of point p in the segment joining p0 and p1.

       nearseg3
              The closest point to testp on segment p0 p1.

       pldist3
              The distance from p to segment p0 p1.

       vdiv3  Vector divide -- the length of the component of a parallel to b,
              in units of the length of b.

       vrem3  Vector  remainder  --  the  component  of  a perpendicular to b.
              Ignoring roundoff,  we  have  eqpt3(add3(mul3(b,  vdiv3(a,  b)),
              vrem3(a, b)), a).

       The  following  routines  convert  amongst  various  representations of
       points and planes.  Planes are represented identically  to  points,  by
       duality;     a    point    p    is    on    a    plane    q    whenever
       p.x*q.x+p.y*q.y+p.z*q.z+p.w*q.w=0.  Although when dealing  with  affine
       points  we  assume p.w=1, we can't make the same assumption for planes.
       The names of these routines are extra-cryptic.  They contain an f  (for
       `face')  to  indicate a plane, p for a point and n for a normal vector.
       The number 2 abbreviates the word `to.'  The number 3  reminds  us,  as
       before,  that  we're  dealing  with  affine points.  Thus pn2f3 takes a
       point and a normal vector and returns the corresponding plane.

       Name   Description

       pn2f3  Compute the plane passing through p with normal n.

       ppp2f3 Compute the plane passing through three points.

       fff2p3 Compute the intersection point of three planes.

       The names of the following routines end in 4 because  they  operate  on
       points  in projective 4-space, represented by their homogeneous coordi-
       nates.

       pdiv4  Perspective division.  Divide p.w into p's coordinates, convert-
              ing  to  affine  coordinates.  If p.w is zero, the result is the
              same as the argument.

       add4   Add the coordinates of two points.

       sub4   Subtract the coordinates of two points.

SOURCE
       /sys/src/libgeometry

SEE ALSO
       tstack(9.2)

BUGS
       Spotty coverage.



                                                                   ARITH3(9.2)