/* This is a source code for 'project' program   by Jan Kybic, 1996. It projects a set of 3D points into a plane.      It is to be invoked by 'project x y z', where (x,y,z) is    a vector of real numbers perpendicular to the project plane.   It then reads data from standard input, each line representing one point    in 3D space containing three real numbers - coordinates x,y,z.   For each input line it produces an output line consisting of a pair   of real numbers - coordinates u,v of the projected point in the plane.      The normal vector does not define the projecting function entirely,   however, while any such a function is satisfactory for the intended   application, I prefer to leave the detailed choice to the program.*/#include <stdio.h>#include <stdlib.h>#define EPS 1e-50    /* smaller than this is zero */typedef double vector[3] ;double product(vector a,vector b) /* dot product of two vectors */{   return a[0]*b[0]+a[1]*b[1]+a[2]*b[2] ;}void times(vector c,vector a,vector b) /* vector product c of two vectors a,b */{  c[0]= a[1]*b[2]-a[2]*b[1] ;  c[1]=-a[0]*b[2]+a[2]*b[0] ;  c[2]= a[0]*b[1]-a[1]*b[0] ;}double mag(vector a) /* square of a magnitude of a vector */{ return product(a,a) ;}vector n,k,l ;double n2,k2,l2 ;int init()  /* given vector n, compute k,l */   {  if ((n2=mag(n))<EPS)    { fputs("!!! project: Vector is too small.\n",stderr) ;      return -1 ;    }    /* Create vector k, perpendicular to n */  if (abs(n[0])>abs(n[1]))    if (abs(n[0])>abs(n[2]))         { k[0]=-n[1]/n[0] ; k[1]=1 ; k[2]=0 ; }     else          { k[0]=0 ; k[1]=1 ; k[2]=-n[1]/n[2] ; }   else          if (abs(n[2])>abs(n[1]))         { k[0]=0 ; k[1]=1 ; k[2]=-n[1]/n[2] ; }      else             { k[0]=0 ; k[1]=-n[2]/n[1] ; k[2]=1 ; }           times(l,k,n) ;  k2=mag(k) ; l2=mag(l) ;             return 0 ;} #define BUFLEN 256int main(int argc, char *argv[]){  vector q ; char buf[BUFLEN] ;  if (argc!=4)    { fputs("!!! project: Usage: 'project x y z'\n",stderr) ;      return 1 ;    }   n[0]=atof(argv[1]) ; n[1]=atof(argv[2]) ; n[2]=atof(argv[3]) ;         if (init()) return 1 ;    while(fgets(buf,BUFLEN,stdin)!=NULL)    { if (sscanf(buf,"%lf %lf %lf",&q[0],&q[1],&q[2])==3)               printf("%g %g\n",product(k,q)/k2,product(l,q)/l2) ;            else { fprintf(stderr,"!!! project: Real number expected: %s\n",buf) ;               return 1 ;             }                 }        return 0 ;    }