Creative Airlocking: streaming in action games
This article will discuss the loading and unloading of areas in linear single-player action titles, and look at contemporary examples of how the best games mask these so they appear seamless.
When designing levels, Level Designers and Environment Artists must consider that their assets all have to fit within memory at once. While older action games like Wolfenstein 3-D and Doom would load the entire level with a Loading Screen at the start of each map, games like Half-Life started a trend of loading smaller sections gradually so they could squeeze in more detail and also provide a more seamless experience for players, making the game feel like one long adventure.
At the time, going from one space to the next in Half-Life resulted in a seconds-long hitch with the word “Loading” on screen. There was no warning that it was going to happen, although Valve’s Level Designers oftenplaced these level transitions in smart places; usually down-time between combat and in a natural chokepoint. In later years, with faster computers, these load times decreased and are now almost seamless.
Half-Life displays a small loading message when transitioning between levels.
Currently, blockbuster series like Gears of War and Uncharted provide truly seamless transitions. After one long initial load for a new chapter with a completely new location (with new art) – sometimes masked behind a pre-rendered movie – “buffer” Streaming Sections are used, in which the previous area is unloaded, and the next loaded, on the fly. Since a lot of the globally-used entities are already loaded, and the environment is usually the same, assets can be shared, which can reduce these transition load times to much less than the initial level load.
Essentially, these games take the smaller loading bar/screen of a more continuously-laid-out game like Half-Life, Portal 2 or Fallout 4 and make the player spend that time in the game world. If done creatively, players won’t even notice it. They might even enjoy the down-time if it’s well-paced, like The Last of Us where it can be spent on a thought-provoking puzzle or with the characters discussing something interesting.
Most action games budget out large areas, and then connect those with these smaller Streaming Sections.
Section (A) is a huge space with lots of combat, Section (C) is another. Players in Streaming Section (B) can’t see into both (A) and (C) at once. Section (B) is where Section (A) is dropped from memory and (C) starts to load in. Section (A) being dropped shouldn’t happen in view of the player, and unless the game supports backtracking it is wise to place a back-gate to stop players returning, for maximum efficiency. As soon as Section (A) has been dropped, Section (C) can start loading in. It must have been loaded by the time the player exits Section (B), so it is also wise to front-gate players in case they rush through.
The best way to think of a Streaming Section is as an airlock; the “door” behind the player is locked, the next area is loaded, and the “door” ahead opens. Ideally, these sections aren’t literal airlocks but instead nicely-disguised puzzles or narrative spaces between the action.
Back-gating, and Unloading
Back-gating, as the term suggests, is when the player is prevented from returning to a previous area. The ‘gate’ behind them is closed, in a lot of cases locked. This doesn’t have to be a literal gate or door, though. A ceiling can collapse causing debris to block the path behind the player, the player can fall through the floor and not be able to climb back up, they can pass through a one-way portal and not get back.
Back-gating after entering the Streaming Section is usually done around a corner where the player can’t see Section (A) being unloaded.
One-way animations are the main manifestation of these in modern action titles. Think of how many doorways your player character has held open, only to have it collapse behind them. The level section behind that door is now being unloaded, to make space in memory for the next large section. In co-op games, these animated interactions are a great way to bring players back together so that Player 2 isn’t left behind, only to fall through the world, in the section that is just about to be unloaded!
The Last of Us has a huge variety of bespoke, painstakingly-animated back-gates.
A cut-scene can also serve as a good back-gate, as long as it makes sense in the context and/or story so as to not feel tacked on, and is within development budget!
One-way drop-downs are also a great and less flow-breaking back-gate. If the L-shaped area just before the drop-down can be kept in memory, as soon as the player drops down a ledge they can never climb back up, the previous area can be unloaded. The only down-sides to this softer back-gate are that they can feel contrived unless the game’s art and world can support it (terrain and collapsed structures are great for this), and that co-op players may have to be teleported to the dropping player so that they don’t fall through the world when Section (A) is unloaded.
Slowing the player down, and Loading
As Streaming Sections are usually connectors between two larger areas, they naturally make for slower-paced breaks in the action. Since Section (C) is being loaded in, slowing the player down in (B) – either literally as with Gears of War’s infamous forced walks or cerebrally with light puzzle gameplay – can be more efficient and interesting than just making a large footprint which has to cater for a player, say, sprinting for 30 seconds.
Even when rushed, this plank puzzle in The Last of Us takes time and offers a nice respite.
“Popcorn” encounters with just 1-2 enemies can be a good trick to allow loading to finish and slow players down and prevent them from simply rushing through a short Streaming Section. They also keep players on their toes and vary the flow from, for example, combat to puzzle to combat.
Interactive Objects such as the slow-turning valves in Killzone 2 and the Gears of War games can also buy some loading time, as can environmental obstacles such as jumps or mantles or animations where the player’s buddy looks around for, and then finds, a ladder to kick down for the player to climb (also a good front-gate).
Interactions like the valve in Gears of War slows players down and can also act as a front-gate.
These approaches can also be combined in ways that fit the feel of the game, such as a Grub locking the player in a room and flooding it with frightening enemies in the first Gears of War game.
Batman Arkham Asylum does a great job with additional ‘softer’ methods of slowing players down by playing a captivating well-acted taunt on a monitor from The Joker, or by encouraging exploration with The Riddler’s location-specific riddles or any number of collectibles.
Front-gating, and Loaded
As with Back-gates, front-gates are quite self-explanatory – the exit to the area the player is currently in is locked until certain conditions, such as all the enemies in the room being dead or the next area having loaded in, are met. Again, this doesn’t have to be a literal gate or door, just an obstacle in the world that can change its state from closed and locked to open.
A lot of games from the Call of Duty series to Killzone 2 to The Last of Us extensively use friendly characters to unblock a front-gate; chain-link fences are cut through, doors are kicked open, wooden beams are lifted. New waves of enemies can also open a front-gate for the player and offer the bonus in that noisy, gun-firing AI attract players, like carrots on a stick, to the newly-opened exit. Many action games have excellent examples of enemies blow-torching open a door to get in or a huge monster bursting in through a wall; not only are these cool enemy entrances, but oftentimes their new unorthodox entrance-ways become cool exits, sign-posted by their un-gating event.
Previously-locked doors in Halo often flash and make noise when opened by new enemies.
Not all games front-gate the exits of their Streaming Sections because the time needed to load a Section (C) can usually be accurately gauged, and the acceptable fallback is a slight hitch. However, front-gates do provide that extra failsafe to ensure the next area is loaded before leaving a Streaming Section – in this case, a player with a scratched disk or corrupted file could see out of the world, at best, or get stuck or fall out of the world, at worst (though it could be argued someone with a scratch or corrupted files might see worse issues regardless).
The biggest issue here is that front-gates need to fit the game or the level art – neat doorways or bottlenecks aren’t always possible. The other big issue is repetition; if a specific door interaction animation is always used, the game needs to provide a lot of variety in that animation!
One trick that can be used to alleviate repetition, however, is if the front-gate is out of sight near the end of the Streaming Section (A). A check can be done to see if Section (C) has loaded, and if it has, the door can potentially be pre-opened saving the player another potentially-repetitive interaction but also holding as a true front-gate if a player does rush through.
Batman Arkham Asylum had an interesting front-gate in the penitentiary sections; a security camera scanned Batman once before opening the door. Given the backtracking-heavy structure of the game, when racing through at full pelt, if the next area had not finished loading, the camera would loop the camera’s scanning animation. This is a great compromise because the camera scan completely fits the fiction of the world, and an extra scan animation would probably go unnoticed by many players.
Batman Arkham Asylum’s Penitentiary’s doors only open when loading is complete.
In most linear action games, keeping the player immersed in the world is preferable to seeing a loading screen. If developers can create interesting activities, take advantage of slower pacing through narrative, or just make smart use of assets and an interesting space to traverse, Streaming Sections can be part of the world and not feel like generic winding corridors that stand out even to uninitiated players as padding.
Copyright © Martin 'Alf-Life' Badowsky 2016