S.T.A.L.K.E.R.: Shadow of Chernobyl Developer Diary #6

Designer Alexei Sytyanov discusses the artificial intelligence behind the creatures and characters of GSC Game World's ambitious game.


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While some first-person shooters give you a big clip of ammunition and a huge gun and drop you into a level full of monsters to blast to smithereens, GSC Game World's S.T.A.L.K.E.R.: Shadow of Chernobyl will be more than a little different. The developer refers to it as a "survival action" game in which you play as a scavenger hunting irradiated objects at ground zero--specifically, the Chernobyl reactor after a fictitious second-reactor meltdown in the near future. You must explore this dangerous area (known as the "exclusion zone," or "the Zone" for short), but you won't be alone--you'll be hunted by mutant monsters and rival scavengers who want to get to the loot first. In this edition of our designer diaries, designer Alexei Sytyanov discusses the artificial intelligence that will be used for the game's characters and enemies.

Think or Die

By Alexei Sytyanov
Designer, GSC Game World

Throughout this series, we have spoken of the exclusion zone, describing it as a place of uncontrolled horrors that have emerged from the appalling tragedy of the Chernobyl nuclear explosion. The full story, however, is only just unfolding. What little control humans had in creating the Zone has been surpassed by the rules we set in motion. Our advanced artificial intelligence has made a region where nothing is predictable and everything is in constant flux. We may have populated the Zone, but thanks to the game's advanced AI, what happens next is anyone's guess. To look at how this kind of AI works in S.T.A.L.K.E.R. we must first look at the Zone itself.

This bloodsucker doesn't take kindly to strangers.
This bloodsucker doesn't take kindly to strangers.

Monsters, mutated and malevolent, stalk the land. Whether they are indigenous or the result of some other phenomenon is not known, but all of them hide in this shell of civilization. The varied terrain gives shelter to an equally varied fauna whose mutations and abilities have adapted at a phenomenal rate due to the irradiated landscape. Amid the wastelands and frozen tundra is an enemy of unified purpose. Survival is their game and whether it is a pack of rabid dogs alert to the approach of potential prey, or a darker, more self-aware intelligence with designs on the same secrets you hunt for, danger is paramount. Among the undergrowth or beneath the hillsides cracked with cramped tunnels, these monsters know their environments. Every crack, crook, nook, and brook in the Zone shelters those who respect its laws and surprises those who fumble in the dark.

All of this is made possible by our goal of simulating realism in every move and every decision those monsters should make. Each of them is capable of reacting to a situation in much the same way as you or I would. They are capable of an individual choice through emotional or strategic parameters. They evaluate the situation in ways that elevate them beyond the level of the usual run of non-player characters into the realm of true competitors. In other words, it would be a simple matter to create invulnerable enemies that could not be beaten by a human player, but where's the challenge or the enjoyment in that? The creation of a humanlike intellect, on the other hand, with all of its emotions--panic, fear, elation, and the ability to think--is a goal worth pursuing. In S.T.A.L.K.E.R., we hope that the enemies will not disappoint anyone.

For example: An opponent with instincts beyond its species loses sight of its prey. Our AI gives the creature the ability to guess the location of the opponent rather than unfairly having to know it for certain. The creature, with senses honed by the environment, metaphorically places itself in the mind of the enemy and tries to figure out where it would go. The aim, and this bears repeating, is not to create faultless NPCs, but rather to create worthy opponents. These NPCs possess virtual senses that are as affected by the environment as their real-world counterparts. Obstacles will affect line of sight, brittle undergrowth will affect the audible approach of predators (or prey), and NPCs will have the same desire to survive a hostile encounter as we do. This includes formulating strategies that take into account the landscape or the lighting, and taking their approaches from the safest side. At the same time, these creatures will also feel the same feelings of fear players do in ominous moments during the game.

In fact, most creatures in the Zone don't take kindly to strangers.
In fact, most creatures in the Zone don't take kindly to strangers.

Computer-controlled combatants will constantly evaluate the situation and retreat whenever necessary. This could happen when the NPC is injured or the odds are overwhelming. Conversely, they will be able to gauge their opponent's strengths and weaknesses and develop a suitable strategy. NPCs are as capable as a human in outflanking the enemy or laying ambushes. It becomes fascinating to witness how effective they are at maintaining distances, judging the physical state of the opponent, assessing morale levels, and then acting upon all the information gathered. Even a creature that appears weak or tactically inferior will be able to seize any opportunity that could arise--such as the stalker running out of ammunition or a weapons malfunction. In situations such as these you had better hope you have a good understanding of the terrain and an even better set of legs.

Remember too that vehicle physics are brought into the equation in the bid for cohesive realism. Just as the range of parameters for the creatures makes for varied and interesting gameplay, so too will the vehicles become something more than just ways of getting around the land. With cargo capacity, fuel requirements, and a whole destructible framework, the vehicles represent a powerful ally--but they can be just as much of a hindrance. Of course, all these elements are balanced with the underlying desire to make S.T.A.L.K.E.R. fun, and in future diaries we'll tell you exactly how that will be achieved.

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