An HMD is provided with two piezoelectric vibrating gyros which detect vertical and horizontal angular velocities, and a horizontal sensor which detects a vertical angle. While the user's head rotates, shooting direction of a three-dimensional camera is varied in accordance with the angles...http://www.google.com.hk/patents/US5841409?utm_source=gb-gplus-share專利 US5841409 - Image display apparatus
