extend_to_perpendicular()

Make one vector perpendicular to a second vector.

Strewth! I can't for the life of me remember what all this is about or why I wrote it... I can't think what you'd use it for but here it is...

#include 

void extend_to_perpendicular( VECTOR *p_line1, VECTOR *p_line2, long *p_squared_dis )
   {
   // V1.0
   // (c) Jon P 2001
   //
   // MAKE TWO LINES PERPENDICULAR TO EACH OTHER IN THEIR EXISTING PLANE.
   //
   // *p_line1 and *p_line2 are end points of two lines that have an origin of 0,0,0.
   // Each line must have a length of 4096.
   // *** Note: the two lines must already be at least 45 degrees appart and not
   //           more than 90 degrees.
   //
   // *p_line2 is changed so that its line becomes perpendicular to *p_line1 in the
   // same plane.  *p_line1 is not changed.
   //
   // *p_suared_dis is the distance between *p_line1 and *p_line2 squared.
   // If *p_suared_dis is passed as 0 then this routine will calculate it and return
   // the answer in that parameter incase it might prove useful in the calling code.
   // Thus p_suared_dis must be a valid read-writable address if specified as zero.

   // Eplaination:-
   //   Take two lines (and pass them as *p_line1 and *p_line2) , both 4096 in length,
   // with the same origin of 0,0,0 ...  then calculate the squared distance between
   // the two end points and pass it as as *p_squared_dis.
   // This length is effectively a squared chord length of a circle with a 4096 radius
   // and with the two end points being points on its circumference
   // (see NOTES12# for examples).
   //
   // The squared length is then used to calculate two multipliers...
   // the first is applied to *p_line2 to shorten it (relative to 0,0,0) to where it
   // intersects a new chord (which is a chord as if the two lines are perpendicular).
   // The second multiplier is then used to extend the new point in *p_line2 (relative
   // to point *p_line1) into the final perpendicular position
   //
   //
   
   register long temp, temp2;
   


   if ( *p_squared_dis == 0 )
      { // squared chord length not supplied so calculate it.
      temp = p_line1->vx - p_line2->vx;
      temp *= temp;
      temp2 = p_line1->vy - p_line2->vy;
      temp += temp2 * temp2;
      temp2 = p_line1->vz - p_line2->vz;
      temp += temp2 * temp2;
      *p_squared_dis = temp;
      }
   
   if ( *p_squared_dis <= 22943587 )
      {
      temp = *p_squared_dis >> 13;     //  /8192
      temp += temp * temp / 7984;
      temp += ( 33554432 - *p_squared_dis ) / 6369;
      temp -= 2208;

      p_line2->vx *= temp;
      p_line2->vx >>= 12;
      p_line2->vy *= temp;
      p_line2->vy >>= 12;
      p_line2->vz *= temp;
      p_line2->vz >>= 12;

      temp = ( *p_squared_dis / 9077 ) + 1823;
      }
   else
      {
      temp = *p_squared_dis >> 13;
      temp = -( temp - 422 ) * ( 6458 - temp );
      temp /= 4551;
      temp += 3327;
      temp += *p_squared_dis / 12559;

      p_line2->vx *= temp;
      p_line2->vx >>= 12;
      p_line2->vy *= temp;
      p_line2->vy >>= 12;
      p_line2->vz *= temp;
      p_line2->vz >>= 12;

      temp = *p_squared_dis >> 13;
      temp = temp * temp / 6177;
      temp += 3071;
      }

   p_line2->vx = ( p_line2->vx - p_line1->vx ) * 5793 / temp  + p_line1->vx;
   p_line2->vy = ( p_line2->vy - p_line1->vy ) * 5793 / temp  + p_line1->vy;
   p_line2->vz = ( p_line2->vz - p_line1->vz ) * 5793 / temp  + p_line1->vz;
   
   }