In a recent commentary on Valve's Half-Life 2 Episodes, Marsh Davies criticizes much of Episode One for its "failure to make your navigation comprehensible, either spatially or narratively." He goes on to praise Episode Two for remembering to provide the player with an overview of its regions, so that the spaces allow the player to see the places he has been, or is going to. As I was reminded in my own recent replay of the original Half-Life and its companion games, this is not a recent improvement by Valve, but a return to form.
Half-Life extensively uses a gameplay cycle I think of as the "Preacher Loop," after a well-known formula for public speaking. In the South, particularly, the most common public speeches are sermons, and over the centuries preachers have developed a simple, three-point plan for writing compelling orations week in and week out:
- Tell 'em what you're gonna tell 'em.
- Tell 'em.
- Tell 'em what ya told 'em.
The preacher begins by telling the audience the ultimate point of the sermon, then (often) launches off on what seems like a tangent to this idea, centered around a personal anecdote or current event. Through the application of a scriptural reading, the preacher reaches the promised conclusion. He then emphasizes how that conclusion was really evident all along, if we only paid attention to the words and deeds of the Lord. Rhetorically, this structure reassures the audience that the surprising conclusion is sound. Moreover, the repetition of the intended lesson ensures that it will be remembered.
Building a sense of spatial continuity in linear levels relies on a gameplay loop that's analogous to this sermon plan. The Preacher Loop has three elements that serve purposes analogous to the three points of the classic sermon structure.
Overview: Quickly reveal the destination and one or two waypoints.
Traverse: Let the player cross the space, while maintaining its coherence.
Review: Allow the player to see his starting point from the destination.
The overview gives the player a sense of purpose and an initial idea of how to work towards that goal. In the traverse, the player confronts and overcomes the obstacles of the route. The review emphasizes that the player has succeeded, and by re-examining a familiar space from a new perspective, fixes the traverse's start and end points in a definite relationship in the player's mind. This helps the player see the game world as a continuous, sensible, and therefore immersive space.
A simple example of the preacher loop from early on in Half-Life shows most of the principles. As a player enters a shaft, a falling creature destroys part of the walkway. Immediately, the player desires to go to the walkway's original destination. From the starting position, a vent is clearly visible: earlier segments have already trained the player to recognize this as a potential traversal route. Reaching the vent by jumping on pipes, the player can quickly reach his destination and look back on what he just did to get there.
Just around the corner, a more complex Preacher Loop awaits. Here, the player sees a room where the bridge is already destroyed, and expects that he must get to the other side (he has been trained to reach this conclusion by the immediately preceding room). This time, the route is more complicated, but illustrates some additional points.
First, note that this Preacher Loop contains another one, where the player takes a hidden route at one end of the sewer to cross from one side of the room to the other. Second, once the player exits the main room, it seems like the spatial continuity breaks down, as the player moves through twisting passages to a platforming challenge and then through more hallways. In this case, the space is actually continuous, but even were it not, individual pieces of the route (jumping down boxes, then climbing up a ladder) maintain its coherence in the mind of the player. Just like a preacher's sermon, the traverse does not have to be reasonable, it must merely seem so to the player at the time. So long as the player believes the path taken could have connected starting point and destination, he will probably buy into the continuity of the space.
Subtle defects may, however, produce an unsettling impression in those who notice them, as Rob Ager argues happened in The Shining. By contrast, the rigorous use of the Preacher Loop produces a sense of spatial continuity in Dear Esther that helps anchor it in the physicality of the island that is central to its dreamlike narrative web.
Also, the aerial serves to anchor the player's spatial perceptions of the island.
Preacher Loops, and various riffs on the construct, such as a series of external areas where an attacking helicopter "boss" performs the overview and review roles, continue to appear throughout most of Half-Life. The office complex features a striking number of loops that take the player through a circuitous route from one side of a door to another. These loops help establish progression, particularly for players who manage to assemble a large team of guards and scientists and lead them through the area. With relatively few elevators to break it up, this long pathway creates a sense of spatial continuity that mirrors the gameplay continuity created by the game's cutscene-free storytelling.
The use of these loops drops off significantly in the latter half of the game, however, and by the time the player reaches Xen this construction has vanished almost completely. Traversing Xen frequently requires the player to cross an area in search of a point that is invisible from the starting position, and may remain so until the player is almost on top of it. The numerous teleportation devices also make Xen discontinuous and incoherent, although one could argue that this aspect helps establish it as alien and incomprehensible.
It might seem impossible to employ a Preacher Loop when building linear, unidirectional segments, but this isn't actually the case. As Davies points out in his analysis of the antlion lair in Episode Two, areas that don't physically meet at the start and end points can still fulfill the loop so long as the two ends are visible from one another. This works especially well in vertically-oriented spaces such as shafts.
What Davies values in Episode Two and misses in Episode One is the careful use of the Preacher Loop. The underground segments of Episode One, with a few limited exceptions, do not reveal the player's goal immediately, and do not look back to establish the linkage of initial and final spaces. Even though the spaces the game moves through are continuous, they do not seem so to the player, who cannot assemble them into any coherent world due to lack of a shared context between the starting and ending points. In such disjoint spaces, continuity and purpose must be established through means external to the level design, as players of "cinematic" immersive shooters are likely well aware.
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