text: /*
   * (c) Copyright 1993, 1994, Silicon Graphics, Inc.
   * ALL RIGHTS RESERVED
   * Permission to use, copy, modify, and distribute this software for
   * any purpose and without fee is hereby granted, provided that the above
   * copyright notice appear in all copies and that both the copyright notice
   * and this permission notice appear in supporting documentation, and that
   * the name of Silicon Graphics, Inc. not be used in advertising
   * or publicity pertaining to distribution of the software without specific,
   * written prior permission.
   *
   * THE MATERIAL EMBODIED ON THIS SOFTWARE IS PROVIDED TO YOU "AS-IS"
   * AND WITHOUT WARRANTY OF ANY KIND, EXPRESS, IMPLIED OR OTHERWISE,
   * INCLUDING WITHOUT LIMITATION, ANY WARRANTY OF MERCHANTABILITY OR
   * FITNESS FOR A PARTICULAR PURPOSE.  IN NO EVENT SHALL SILICON
   * GRAPHICS, INC.  BE LIABLE TO YOU OR ANYONE ELSE FOR ANY DIRECT,
   * SPECIAL, INCIDENTAL, INDIRECT OR CONSEQUENTIAL DAMAGES OF ANY
   * KIND, OR ANY DAMAGES WHATSOEVER, INCLUDING WITHOUT LIMITATION,
   * LOSS OF PROFIT, LOSS OF USE, SAVINGS OR REVENUE, OR THE CLAIMS OF
   * THIRD PARTIES, WHETHER OR NOT SILICON GRAPHICS, INC.  HAS BEEN
   * ADVISED OF THE POSSIBILITY OF SUCH LOSS, HOWEVER CAUSED AND ON
   * ANY THEORY OF LIABILITY, ARISING OUT OF OR IN CONNECTION WITH THE
   * POSSESSION, USE OR PERFORMANCE OF THIS SOFTWARE.
   *
   * US Government Users Restricted Rights
   * Use, duplication, or disclosure by the Government is subject to
   * restrictions set forth in FAR 52.227.19(c)(2) or subparagraph
   * (c)(1)(ii) of the Rights in Technical Data and Computer Software
   * clause at DFARS 252.227-7013 and/or in similar or successor
   * clauses in the FAR or the DOD or NASA FAR Supplement.
   * Unpublished-- rights reserved under the copyright laws of the
   * United States.  Contractor/manufacturer is Silicon Graphics,
   * Inc., 2011 N.  Shoreline Blvd., Mountain View, CA 94039-7311.
   *
   * OpenGL(TM) is a trademark of Silicon Graphics, Inc.
   */
  /*
   * trackball.h
   * A virtual trackball implementation
   * Written by Gavin Bell for Silicon Graphics, November 1988.
   */
  
  /*
   * Pass the x and y coordinates of the last and current positions of
   * the mouse, scaled so they are from (-1.0 ... 1.0).
   *
   * The resulting rotation is returned as a quaternion rotation in the
   * first paramater.
   */
  void
  trackball(float q[4], float p1x, float p1y, float p2x, float p2y);
  
  /*
   * trackball function that results a openGL matrix
   *
   */
  void
  trackball_opengl_matrix(double m[16], float p1x, float p1y, float p2x, float p2y);
  
  /*
   * Given two quaternions, add them together to get a third quaternion.
   * Adding quaternions to get a compound rotation is analagous to adding
   * translations to get a compound translation.  When incrementally
   * adding rotations, the first argument here should be the new
   * rotation, the second and third the total rotation (which will be
   * over-written with the resulting new total rotation).
   */
  void
  add_quats(float *q1, float *q2, float *dest);
  
  /*
   * A useful function, builds a rotation matrix in Matrix based on
   * given quaternion.
   */
  void
  build_rotmatrix(float m[4][4], float q[4]);
  
  /*
   * This function computes a quaternion based on an axis (defined by
   * the given vector) and an angle about which to rotate.  The angle is
   * expressed in radians.  The result is put into the third argument.
   */
  void
  axis_to_quat(float a[3], float phi, float q[4]);
  
  