Casting shadows

Figure: Shadows cast by a distant point light source can be obtained by using a view from a different angle.

The only difference between visibility and illumination, in the scheme of parallel projection used by GEOM, lies in the direction of view. Therefore maintaining a double calculation, one for each direction, yields criteria for suppressing (or changing the color of) the picture elements representing the spherical surface.

/* shadsf - present a family of spheres in color */
/* s[n][7] - family of spheres                   */
/* u[3][3] - direction of shadow                 */
/* t[n] -    color codes                         */


void shadsf(s,t,u,n) double s[][7], u[3][3]; int *t, n; {
int    i, j, k, m, mi;
double b1, b2, g, th, dh, ri;
double emi, p[3], q[3], a[3], b[3], o[3][3];
g=(double)sspch;
for (i=0; i<n; i++) {
  ri=s[i][3];
  emi=((double)sslen)*ri;
  mi=(int)emi;
  dh=3.14159/emi;
  th=0.0;
  spheu(o,&s[i][4]);
  for (k=0; k<mi; k++) {
    th+=dh;
    sphrv(a,ri,th,g*th);
    sphmv(p,o,a);
    if (p[1]>0.0) continue;
    sphmv(q,u,p);
    if (q[1]>0.0) continue;
    sphvs(p,p,&s[i][0]);
    sphmv(b,u,p);
    m=1;
    for (j=0; j<n; j++) {
      if (i==j) continue;
      sphdj(&b1,&b2,p,&s[j][0],s[j][3]);
      if (b1<0.0)        {m=0; break;}
      if (p[1]<=s[j][1]) continue;
      if (b2<=0.0)       {m=0; break;}
      sphmv(q,u,&s[j][0]);
      sphdj(&b1,&b2,b,q,s[j][3]);
      if (b1<0.0)        {m=0; break;}
      if (b[1]<=q[1])    continue;
      if (b2<=0.0)       {m=0; break;}
      }  /* end for j */
//    geomms(p[0],p[2],m,t[i]);
    pltms(p[0],p[2],m);
    }  /* end for k */
//  if (kbdst()) {kbdin(); break;}
  }    /* end for i */
}