Elizabeth’s behaviour in combat situations was a particular challenge for the team at Irrational.
In his talk at GDC 2014, Irrational Games’ John Abercrombie outlined the challenge of bringing Elizabeth to life during BioShock Infinite’s development, urging his contemporaries to retain a focus on player experience at all costs.
Speaking as part of the summit’s AI track, Abercrombie described how the Irrational team saw Elizabeth, the player’s companion throughout most of the game, as a unique opportunity to fill Bioshock Infinite’s world with life, and to avoid ending up with a game world that was simply a mechanical “museum” through which the player passed.
“It’s about perceiving the work from the point of view of the gamer,” he said, admitting that it was a “very difficult proposition” to make entertaining and believable AI. Key to Irrational’s success, he explained, was offering players the ability to understand the character through their own actions, not just observe Elizabeth operating independently as if she was an animatronic.
The Irrational team was inspired by theatre, relying on the techniques that enable an actor to project their character to a large audience such as exaggerated gestures, stage position and improvisation. A fully scripted companion was considered too intensive, and as such unworkable.
Elizabeth’s role in Infinite was to bring its world to life, acting naturalistically alongside the lead, Booker.
One of the most important elements of what Abercrombie called “player-facing design” was that Elizabeth must always be visible to the player, but early iterations saw her following behind the player.
“This was disastrous,” he said. “Any interaction involved the player having to turn around to look at her. Our next idea was to try and predict a location in the player’s field of vision based on the player’s location. This was extremely difficult; she’d travel unnaturally, so just reacting to the player’s movement wasn’t going to work either.”
A solution was eventually found through Irrational’s “goal side” system, which placed fixed positions in Bioshock Infinite’s world that Elizabeth would navigate to based on the player’s location and movement. This still had its issues however, as players were naturally unpredictable. “Players behave erratically,” Abercrombie continued. “Whether the player cares or not, leaping over a railing to make a shortcut is a difficult problem for an AI partner.”
While this was the main foundation of Elizabeth’s non-combat behaviour, she wasn’t only designed as a partner and guide, but to appear to be exploring the world “in a believable fashion and at appropriate times,” helping to emphasise the narrative and performance of a “naive girl transforming into an empowered woman.”
The AI team invested a great deal of effort into the back-end systems which provided markers for Elizabeth to respond to, either generated on the fly or placed by level designers to draw her gaze. Abercrombie revealed that by this point Elizabeth’s behaviour had become so complex her character class was called “WTFLIZ” in the code.
The team also created a “smart terrain” system which, combined with the ‘goal-side’ technology and the direction of her gaze, would ensure she could find objects to interact with either in the player’s view or to draw their attention by giving an exaggerated call-out – a sigh when sitting on a couch, for example, or an announcement that she has found an item.
Irrational admitted that it had to employ a little ‘AI trickery’ to stop Elizabeth acting strangely.
Combat was a trickier proposition. “[Elizabeth] wasn’t making the best choices when trying to stay out of the line of fire,” said Abercrombie. “It was extremely frustrating.” But the team quickly realised that it wasn’t necessary to keep Elizabeth in view during battle. “The player doesn’t even notice that Elizabeth isn’t on screen,” he continued. “We can put her out of the way – even if the player keeps her on the screen, we [can] bring her to the player while staying out of the line of fire and have her find cover.”
The tweak worked. “On the occasions she would run through fire, we would have her throw up her arms and cover her head, so she would perform as you would in the middle of a fire fight,” said Abercrombie, still visibly pleased with the team’s workaround.
Based on these difficult experiences, Abercrombie concluded that AI programmers should strive to balance the integrity of the algorithms governing NPC behaviour with the more immediate need to be entertaining. In his view, it’s no compromise to rely on a workaround if it makes the player experience better. “As AI programmers we want to find the most robust and complete solution,” he argued, “but as game developers we should find the most elegant and player-focused solutions.”
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