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/* 
  *  Simple trackball-like motion adapted (ripped off) from projtex.c
  *  (written by David Yu and David Blythe).  See the SIGGRAPH '96
  *  Advanced OpenGL course notes.
  *
  *
  *  Usage:
  *  
  *  o  call gltbInit() in before any other gltb call
  *  o  call gltbReshape() from the reshape callback
  *  o  call gltbMatrix() to get the trackball matrix rotation
  *  o  call gltbStartMotion() to begin trackball movememt
  *  o  call gltbStopMotion() to stop trackball movememt
  *  o  call gltbMotion() from the motion callback
  *  o  call gltbAnimate(GL_TRUE) if you want the trackball to continue 
  *     spinning after the mouse button has been released
  *  o  call gltbAnimate(GL_FALSE) if you want the trackball to stop 
  *     spinning after the mouse button has been released
  *
  *  Typical setup:
  *
  *
     void
     init(void)
     {
       gltbInit(GLUT_MIDDLE_BUTTON);
       gltbAnimate(GL_TRUE);
       . . .
     }

     void
     reshape(int width, int height)
     {
       gltbReshape(width, height);
       . . .
     }

     void
     display(void)
     {
       glPushMatrix();

       gltbMatrix();
       . . . draw the scene . . .

       glPopMatrix();
     }

     void
     mouse(int button, int state, int x, int y)
     {
       gltbMouse(button, state, x, y);
       . . .
     }

     void
     motion(int x, int y)
     {
       gltbMotion(x, y);
       . . .
     }

     int
     main(int argc, char** argv)
     {
       . . .
       init();
       glutReshapeFunc(reshape);
       glutDisplayFunc(display);
       glutMouseFunc(mouse);
       glutMotionFunc(motion);
       . . .
     }
  ***/


 /* functions */
 void tbInit(GLuint button);

 void tbMatrix(void);

 void tbReshape(int width, int height);

 void tbMouse(int button, int state, int x, int y);

 void tbMotion(int x, int y);

 void tbAnimate(GLboolean animate);