Diegetic Interface

Game interface elements that are a part of the game universe. "Diegetic" is a term meaning "within the narrative", usually used in reference to music. Diegetic music is heard by the characters because it's actually being played in the scene (as opposed to non-diegetic background music). Diegetic menus are the same — they actually exist in the game world, rather than simply appearing for the benefit of the player.

This is generally handled in one of two ways. Sometimes the normal player HUD is explained as being part of the character's equipment -- common if he's robotic, a Cyborg, or wearing Powered Armor, and Justified if the game is in first-person perspective. Other times, the game simply uses in-game indications of things that a HUD would normally tell you; a wounded character will limp instead of having a Life Meter, for example.

Vehicle simulators call this a virtual cockpit, and it tends to be the most detailed and realistic interface mode short of a hardware sim with actual panels. In these cases, a 2-D control panel laid out for easier reading without scrolling is usually included as an easier-to-program option.

Generally, this is done in order to increase immersion; it's much easier to believe that your character is a real person in a real situation when the screen isn't cluttered with inexplicable icons representing health and ammo. In some cases, this means that actions done via menus are actually happening in real time -- browsing your inventory may leave the player open to attack, so you can't pause the action midbattle to grab a handy medkit.

A diegetic interface often averts Menu Time Lockout. Robo Cam is when one is applied to a character's view outside of video games. Justified Save Point is related. May justify Interface Screw as well. See also Painting the Fourth Wall, where this is temporarily invoked for the sake of Post Modernism, and usually Played for Laughs. Examples:

Diegetic HUDs

 * Halo, a First-Person Shooter in which you play as a Space Marine with Powered Armor. Your HUD is projected on this inside of your helmet's visor, and some weapons have readouts as well: small LCDs for human weapons, and holograms for the aliens. During Noble Six's Last Stand in Halo: Reach, the visor, and thus the interface, Shows Damage.
 * Half-Life and its sequels and add-ons, also with Powered Armor. Strangely, Gordon is never depicted wearing a helmet.
 * He does wear glasses, though.
 * Crysis, which overlaps with Interface Screw when certain enemies and weapons (like EMPs) cause your HUD to go fuzzy or fail entirely.
 * Crysis 2 also adds a gorgeous new bobbing effect for the HUD when you move, and makes it look more realistic (like a fighter plane HUD). All of this is a wee bit strange when you consider that the Crysis: Legion book calls it a "BUD" (Brain-Up Display), like a neural interface, but there is some acknowledged discontinuity between the two.
 * Metroid Prime, which also has some Interface Screw elements similar to Crysis, though predating it by several years.
 * For example, when Samus gets past certain enemies, they actually cause "interference". Rezbits in Corruption can actually make Samus' Power Suit crash, and the player has to reboot it by pressing a key sequence, much like pressing Ctrl+Alt+Del.
 * The four lights at the top of Samus' field of view, which did nothing in the first two games, represent the power level of the batteries in the Wiimote in the third.
 * Azraels Tear, again with Powered Armor.
 * System Shock, as part of the cyber-interface implanted in the beginning of the game. The player can even improve the interface by finding hardware, such as targeting aids, health monitors, infrared, a widened field of vision, and a multimedia data reader (a CD drive?).
 * Strange Journey has all the interface elements as part of the PC's Demonica Suit.
 * Likewise, the contents of the upper screen of the DS is identical to the display of the characters' Demon Summoning Program in Devil Survivor 2: this is where they find out the names of the Cosmic Horrors they're being attacked by and they directly refer to the skills shown there later in the game.
 * In Deus Ex Human Revolution your HUD doesn't even exist until Adam Jensen gets cybernetic implants (including his eyes) after a brutal beatdown.
 * In Bulletstorm, the player character has no HUD until he puts on the Leash, which then injects him with nanomachines.
 * The Journeyman Project.
 * Star Wars Republic Commando, once again with Powered Armor. As with Crysis, there are certain areas and weapons that cause your HUD to go crazy.
 * Battlefield 2142 has the HUD projected onto everyone's Net Bat Helmet visor from within. Like the above example, the HUD disappears and the entire screen appears washed-out with flickering static if an EMP weapon goes off nearby. Interestingly, this distruption also disables the networked battlefield system that displays the positions of hostiles spotted by one soldier to everyone on the team.
 * F.E.A.R. 2's HUD is projected on the character's glasses and goes missing when they are briefly removed.
 * The Project Eden interface appears to be projected on some kind of contact lens as it is seen booting up when the character does something with their eye.
 * In Borderlands, the cute Claptrap robot gives you the device displaying your HUD before you can even move.
 * Starsiege: Tribes and its sequels outfit everyone with Powered Armor as a rule, but the HUD doesn't look particularly diegetic until Tribes Ascend.
 * I Miss the Sunrise is a Turn-Based Strategy example. Yes, really. The main character is a commander of a fleet who has a unique protein in their body that, when combined with a chemical, greatly augments their mental capabilities, allowing them to take as long as they want to formulate an order without taking any time from an outside perspective. The images displayed on the screen are what the character literally sees from their cockpit (probably not the menus, though).
 * In Perfect Dark, Joanna is equipped with a headset that deploys a small screen over her field of vision which acts as the game's menu, similar to James Bond's wristwatch computer from Goldeneye.

Virtual Cockpits

 * Many Racing Games replace the HUD with the car's dashboard when using a first-person viewpoint.
 * Gran Turismo V has realistic simulated interiors for all its cars, a first for the series.
 * The HUD in Ace Combat, especially in Cockpit View. The same applies for Tom Clancy's H.A.W.X..
 * Descent and its sequels provide an optional cockpit view to increase the sense the player was Material Defender in the Pyro-GX and successor ships.
 * The Mechwarrior series. The HUD is presumably a function of the neurohelmet every MechWarrior wears for the 'Mech to keep balanced given its superimposed nature, though.
 * Steel Battalion, in spades. Every VT generation has its own cockpit, and Line of Contact adds even more cockpits with support/indirect-fire and Jaralaccs VTs now having their own. The VT Operations Manual goes into extensive detail on what all of the cockpit lights and gauges mean, and not a single one is there for mere decoration.
 * Steel Battalion: Heavy Armor takes this even further by its use of Kinect, potentially allowing you to reach out and interact with secondary cockpit controls, or interacting with your crewmates.
 * Orbiter has one for the most popular built-in spacecraft, and the space shuttle. Unfortunately (or maybe not) most of the switches on the shuttle panel are dummies, and most of the special functions aren't usable from the mouse interface. A lot of the better realized add-ons have them, but most skip the extra work and only include a 2D panel.
 * Microsoft Flight Simulator has had active virtual cockpits since the 2004 edition, and a basic implementation back in '95.
 * Too many combat flight simulators to count. Fully realistic settings in most of them will even enforce cockpit view, with no non-diegetic gauges to rely on. In extreme cases such as Falcon 4.0, DCS: Ka-50 Black Shark and DCS: A-10C Warthog, the cockpits are fully clickable to the point where the player can even go through a cold start procedure using the virtual cockpit!

Other Examples

 * Jurassic Park Trespasser was probably the first first-person game to have absolutely no HUD. Instead, the player character would verbally call out the amount of ammo left in a gun ("five shots left," "feels half full," etc.) and a tattoo on her chest (which could be viewed by looking down) indicated the amount of health the character had.
 * In Dead Space, everything is diegetic. Health levels and power-up charge are given via displays on Isaac's suit, menus are Holographic Terminals projected by either his suit or the machinery he's interacting with, and even the "go here next" hints are glowing lines on the floor generated by a projector in Isaac's glove, presumably in conjunction with the ship's computer (which would, naturally, know how to get you where you want to go).
 * Metro 2033 handles almost everything diegetically. Damage causes your vision to narrow and red out, while low air causes blurry vision and labored breathing. One button lets you look at your watch (which shows time until air runs out and ambient light level, for sneaking) while another brings up his notes (listing your next objective, with a compass pointing the way). The only non-diegetic part of the interface is your weapon selection and ammo count -- though for some weapons, even the ammo count is visible on the weapon. Even better, the hardest difficulties turn off the non-diegetic parts. Better count those bullets!
 * The Director in Crackdown refers to columns of light and other things visible in the game as being part of a graphical interface attached to the player character's eyes.
 * Fallout 3 and Fallout: New Vegas have diegetic menus in the form of a wrist-mounted computer called the Pip-boy 3000. It somehow lets you do things like use items, manage your equipment and other inventory, and physically examine yourself for wounds. It also runs on an equivalent of MS-DOS. Note, however, that the HUD itself is mostly non-diegetic, as the ammo counter, HP and AP meters, and compass pips aren't justified in-game.
 * The Star Craft 2 research and armory screens are set up something like this. The entire ship may also count as well.
 * In the Sly Cooper game series, the sparkles that mark areas that can Sly and the others can use (Climbing, Crawling Under, etc.) are noted as being visible in-universe, to Sly at least, and represent "thieving opportunities". In the 2nd and 3rd games, the starting locations of missions and the locations of objectives are made visible with holographic markers that are also explicitly said to be visible to the characters.
 * Pretty much everything in the Assassin's Creed series. The HUD, highlighted targets, and even things like the pause menu are explained as being part of the Game Within A Game that is the Animus. Indeed, during the segments outside the Animus, the game goes out of its way to avoid having any sort of HUD at all, except when essential.
 * Escape from Monkey Island displays your items in a circle orbiting around you when you open your inventory. Other characters can apparently see this and make comments like "you better clear up that clutter when you're done".
 * The Getaway doesn't have any kind of HUD to try and make the game more cinematic and immersible. Rather than a health bar, your character develops bloodstains and a limp the more they get hurt. Rather than floating health kits, leaning against a wall recovers you health (and removes bloodstains). And rather than a minimap or GPS arrow pointing you to your destination, your cars indicators will blink when you should turn, and both will flash when you reach your destination. The game did come with an actual map of London to help you find your way around though.
 * James Bond in Golden Eye 1997 could switch weapons using the readout on his laser watch. The bad guys would kindly stop shooting and wait for him to finish what he was up to before resuming the firefight.
 * You could do the same in The World Is Not Enough with a Motorola cell phone.
 * Splinter Cell mixes it with Product Placement in the form of a Palm OPSAT or a Sony Ericsson phone as pause menus.
 * In Hammerfight, the tutorial mentions that flying machines are controlled with a mouse.
 * In Nie R, Grimoire Weiss functions as your menu/journal/inventory. When he's not in your party (such as before you meet him), your menu/journal/inventory is extremely limited.
 * Digital: A Love Story is a Visual Novel that is presented entirely in a GUI reminiscent of Amiga Workbench.
 * Quite a few DS games use this with whatever's on the bottom screen, such as:
 * Pokémon Diamond and Pearl/Platinum's Pokétch, followed by Black and White's C-Gear.
 * And Pokémon Ranger's menu is the very Capture Styler your character uses for, well, everything.
 * Grand Theft Auto: Chinatown Wars shows you Huang's PDA, though that doesn't quite explain why you change weapons and throw grenades with it, or how it knows when he is about to die.
 * Mega Man Star Force uses the Transer.
 * Similarly, Mega Man Battle Network 5: Double Team DS shows the PET on the bottom screen at all times. It even includes a 3D model of the Blue Bomber himself as a bonus.
 * In Minecraft, you have to craft your maps, and they only update if you're holding them.
 * The interface menus in Silent Hill: Shattered Memories are in the form of a shoehorned mobile phone. While it does result in funny things like a messaging menu that can only receive messages, it does add to the immersion—particularly when you are chased by monsters and you need to look at the map.
 * The video-game adaptation of Peter Jackson's King Kong remake deserves some mention because its Diegetic Interface is a total lack of HUD. Despite taking the exact opposite tactic, it still increases immersion by forcing you to pay attention to how many shots you've fired, your character's movespeed and labored breathing, the ambient noise of the game environment, and so on.
 * Silent Hill: Downpour is a minor example; there's no HUD, but there are context commands and an inventory screen. The easiest way to tell how much more damage Murphey's taken is Murphey's appearance--whether he can still run or just barely drag himself around, and the extent of bruising visible on his body are the tell tale signs of damage.