/*  *  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 tbInit() in before any other tb call *  o  call tbReshape() from the reshape callback *  o  call tbMatrix() to get the trackball matrix rotation *  o  call tbStartMotion() to begin trackball movememt *  o  call tbStopMotion() to stop trackball movememt *  o  call tbMotion() from the motion callback *  o  call tbAnimate(GL_TRUE) if you want the trackball to continue  *     spinning after the mouse button has been released *  o  call tbAnimate(GL_FALSE) if you want the trackball to stop  *     spinning after the mouse button has been released * *  Typical setup: * *    void    init(void)    {      tbInit(GLUT_MIDDLE_BUTTON);      tbAnimate(GL_TRUE);      . . .    }    void    reshape(int width, int height)    {      tbReshape(width, height);      . . .    }    void    display(void)    {      glPushMatrix();      tbMatrix();      . . . draw the scene . . .      glPopMatrix();    }    void    mouse(int button, int state, int x, int y)    {      tbMouse(button, state, x, y);      . . .    }    void    motion(int x, int y)    {      tbMotion(x, y);      . . .    }    int    main(int argc, char** argv)    {      . . .      init();      glutReshapeFunc(reshape);      glutDisplayFunc(display);      glutMouseFunc(mouse);      glutMotionFunc(motion);      . . .    } * * *//* functions */extern void tbInit(GLuint button);extern void tbMatrix(void);extern void tbReshape(int width, int height);extern void tbMouse(int button, int state, int x, int y);extern void tbMotion(int x, int y);extern void tbAnimate(GLboolean animate);extern void tbAnimateFunc(void (*func)(int animate));extern void tbStepAnimation(void);