Alex Cheng of Firaxis explains how Firaxis made each alien race a unique challenge

GDC 13: Inside X-Com’s hidden movement

Alex Cheng of Firaxis spoke at the opening session of the Game AI Summit GDC 2013 about the challenges of making distinct AI for each of the alien races in X-Com: Enemy Unknown.

Based on the classic Gollop brothers’ 1994 title of the same name, X-Com places the player in the shoes of the commander of earth’s resistance against an alien menace.

For lovers of the classic strategy title, it is non-negotiable that different aliens behave distinctly.

To achieve this the Firaxis team implemented a two-action system which, through its simplicity, put special emphasis on the skills unique to each alien race.

For movement, the AI determines what the optimal location for each enemy is by enumerating and scoring game tiles.

Points are awared each tile for player soldiers flanked, proximity to targets, number of available targets, and other variables.

If the alien is already in the best spot, movement ends and a similar check is performed for abilities, enumerating and scoring each, then randomly selecting an action to perform with results weighted towards the highest scoring actions.

These variables – cover, visibility, flanking, and the variety of abilities available to each alien – gave firaxis the reference points it needed to make each alien type behave differently.

Melee units ignore cover, instead prioritizing proximity to X-com soldiers.

Elite units like sectopods similarly prioritize attack over cover, positioning themselves to do the most damage they can.

Ethereals are an alien type that remain in the background, utilizing a bodyguard of Mutons to avoid damage, lashing out with psionic attacks.

The end result of this is a satisfying tactical experience that provides a great deal of variety to the player, remaining true to the founding title of the X-Com franchise.<br /><br />—–<br /><br /><em>GDC 2013 – Live! Breaking news and up-to-the-minute developments: <a href="" target="_blank">HERE</a><br /><br />iPhone owners should <a href="" target="_blank">download our free app</a> (iTunes link). It puts Develop&rsquo;s daily must-read online news content &ndash; including all the stories from GDC &ndash; in the palm of your hand. Sponsored by Microsoft.</em>

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