static int mouse_x, mouse_y;		/* previous values of mouse location */

// This function records the current mouse location when the button goes
// down, facilitating the motion function.
void twMouseFunction(int button, int state, int x, int y) {
    y=windowHeight-y;
    switch(button) {
    case GLUT_LEFT_BUTTON:
	if(GLUT_DOWN==state) {
	    mouse_x = x;
	    mouse_y = y;
	}
	break;
    case GLUT_MIDDLE_BUTTON:
	if(GLUT_DOWN==state) {
	    twOrientVPN(x,y);
	}
	break;
    case GLUT_RIGHT_BUTTON:
	break;
    default:
	printf("invalid button in mouse callback: %d\n",button);
	break;
    }
    // I hoped this would fix the area around the mouse that isn't
    // updated, but no luck.
    glutPostRedisplay();
}
    
void twMotionFunction(int x, int y) {
    y=windowHeight-y;
    twTrackballOrientation(mouse_x,mouse_y,x,y);
    mouse_x = x;
    mouse_y = y;
}