empties as the player becomes hurt such as Tekken, Street Fighter, and many others. Armour levels are also commonly monitored, either through a separate readout, or as part of the health system. For example, some Halo games use one recharging shield bar, acting as the health level. When this is depleted the player can only take a few more hits before their death. The same goes in Destroy All Humans!, but in the form of the player's shields. Traditionally, games used lives to represent health. Every time the main character was injured they would lose one of their limited lives. Another way to display the life in the HUD is demonstrated in Gears of War, where the characters life is displayed only when they are taking damage, in which case a red cog known as the Crimson Omen appears in the center of the screen. The more visible the Crimson Omen is, the more damage the player has sustained and the closer they are to death. This health system is known as the 'Red Ring' system. There is also a lot of variance with regards to the display of other information. Some games permanently display all the weapons a character is currently carrying, others rely on a pull up weapon selector. Inventory or storage space may also be permanently overlaid over the screen, or accessed via a menu. Alternatively, only a limited number of items stored in the inventory might be displayed at once, with the rest being rotated into view using the [ and ] keys. In order to maintain the suspension of disbelief, some games make the HUD look like a real HUD within the context of the game's world. Many first-person vehicle simulation games use this technique, showing instruments and displays that the driver of the vehicle would be expected to see. The displays in the helmet in the first-person adventure game Metroid Prime or Star Wars: Republic Commando also mimic the player's point of view. A similar method is used in Tom Clancy's Ghost Recon Advanced Warfighter and Crysis. In some of these circumstances where the player and character within the game are meant to see the same "HUD" information, such as Halo, the term "HMD" (Helmet-Mounted Display) would technically be more accurate. Some games, in an attempt to increase player immersion and reduce potential screen clutter, have most or all elements disappear when not needed