UI, HUD and in-world screens

Everything you read and click on is drawn by the adapter: the heads-up display, the weapon wheel, the menus, and computer screens hanging on the walls of the level. None of it is known to the simulation. Whatever you do through it, pick a gun, press a button on a screen, use a medkit from a menu, reaches the world the same way a key press does: as input for the next tick.

The rules every screen keeps

• Nothing reaches into the simulation directly. A choice leaves as queued input and is acted on at the next tick, which is what keeps a game replayable.
• One thing owns the mouse at a time. The wheel, a screen or a menu must claim the mouse before it opens; if something else holds it, it does not open. While open, it holds back the buttons it uses, and the click that closes it is spent there, so leaving a menu never fires the gun behind it.
• Never offer what a press cannot deliver. A button with nothing behind it is not drawn. The Escape menu shows Restart only when a game has said what restarting means.
• The engine writes no words. Every caption, colour and size is the game's to set; the engine ships plain defaults. UI runs on the real clock, so menus stay responsive while the world is paused or slowed.

The heads-up display

The HUD shows the player you are viewing, read fresh from the simulation every frame, so it can never drift from the truth. A reading appears only while it means something: a sword shows no ammunition count rather than a zero, and the breath bar appears only when you are short of breath.

Every one of these can be restyled, moved, resized, hidden or replaced outright by a game, without changing the engine, and the raw numbers behind each stay readable so a game can draw a HUD of its own.

The weapon wheel

Hold the wheel key and a ring of your weapons opens around the pointer while the world slows down. Point at one and let go, and it is raised over the next ticks. Letting go over a drawer of items pins the wheel open instead, for clicking. The slowdown is a multiplier, never a stop: a fully stopped world would have no tick in which to deliver your choice. A game can set it to no slowdown at all, which a multiplayer game needs.

A game picks how the ring is laid out: a slice for every weapon, or a slice per number key that opens out into the weapons behind it. Carried items, such as medkits, sit in drawers on the same wheel. The wheel refuses to open when there is nothing to choose: a wheel of nothing that still slowed time would be a way to get killed by a key press.

Menus

Two menus are built in. The inventory menu lists what you carry, to use or drop; it pauses the world but lets its sound play on. The Escape menu pauses and silences everything. It has four pages, one inside the other, and Escape backs out one page at a time.

1. QuitResume, quit, and Restart or Unstuck when a game provides them
2. ControlsEvery action and the keys it is on, read live
3. OptionsSliders for volume and gore limits, switches for gore, and how detailed and how far gibs are drawn
4. RebindPress a new key for an action

Which option rows exist is up to each game: the engine registers none and draws whatever it is given.

Screens in the world

A computer screen on a wall is a working panel. Walk up to it and look at it from close enough, and your view locks, your weapon lowers, and the mouse moves a cursor across the screen. Clicking a button there is a press of the Use key on that button, sent at the next tick like any other input. If the cursor is visible, the click works: the screen takes the mouse only where the simulation would accept the press, and lets go the moment it would not.

The ways out are the ones a body would use. Walk away, turn away, push the cursor off the edge of the glass, or press right click or Escape. Your movement keys are never taken, since walking off is the main way out. A screen can also be a plain display that never takes the mouse, such as a camera feed. Screens shrug off blood and soot: nothing marks the glass.

On top of a screen a game can run apps: small programs that fill it, opened and closed like a stack of windows. The game writes the first app, which acts as the machine's own system and launches the others. The last app can never be closed, because a machine showing nothing looks broken.

Input and settings

Each action, moving, firing, opening the wheel, has a main key and a spare. When you rebind an action in the menu, the new key is taken from whatever action had it, and that action falls back to its spare. Escape is never left without a key. A rebind takes effect on the next frame, and it never changes a replay, because the simulation records what you did, not which key you did it with.

Crouch, sprint and walk can each be held or toggled; a game chooses which. A tap is caught when it happens and spent at the next tick, so in toggle mode a quick tap is never lost and never counted twice, however the frames and ticks line up.

Your bindings, volumes and gore limits are saved to a small text file, written a moment after you change something and read back when the game starts.

• Weapons and projectilesThe arsenal the HUD and the wheel read from
• AudioVolume dials, menu clicks and the sound of a paused world
• Level machineryThe doors and lifts a wall screen can drive