VR thus originally referred to fully immersive experiences based on HMD technology. Another parent technology of current generation VR was the simulators commonly used for military training in circumstances where real-world training was difficult, expensive, or dangerous. With the advent of the HMD, the pre-VR simulators that made use of flat photography and mechanical gadgetry gave way to video technology, and finally to the flexibility and interactivity made possible by computer graphics and animation.