The X-ray engine uses GSC Gameworld's proprietary ALife artificial intelligence engine. ALife supports more than one thousand characters inhabiting the Zone. These characters are non-scripted, meaning that AI life can be developed even when not in contact with the player.

The NPCs have a full life cycle (task accomplishment, combat, rest, feeding and sleep) and the same applies to the many monsters living in the Zone (hunting, attacking Stalkers and other monsters, resting, eating, sleeping). These monsters will migrate in large groups. The non-scripted nature of the characters means that there are an unlimited number of random quests. For instance, rescuing Stalkers from danger, destroying Stalker renegades, protecting or attacking Stalker camps or searching for treasure. The AI characters travel around the entire zone as they see fit.

Numerous tactics can be employed to complete the game, such as rushing or using stealth and sniping. The NPCs will react in a different way to each of them. S.T.A.L.K.E.R.'s NPCs plan ahead by "Goal-Oriented Action Planning" in order to achieve this.

Bullet physics

S.T.A.L.K.E.R. uses "realistic" bullet physics, similar in nature to tactical shooters such as Ghost Recon Advanced Warfighter or Operation Flashpoint. Bullets are affected by gravity, bounced against solid surfaces at oblique angles, and firearms are highly inaccurate when fired without aiming. To score consistent hits at medium or long range, players must aim using the iron sights
on their guns. Additionally, hit damage is pseudo-realistic, and the player can die after only being shot a few times (although later in the game various armor suits and artifacts can be acquired that increase the player's resistance to damage). Late-game depends heavily on scoped weaponry due to the well-armed and powerful enemies that keep their distance from the player.


S.T.A.L.K.E.R. uses a heavily modified version of the ODE physics engine, supporting hundreds of physics objects on different levels. Ragdoll physics, destructible objects, realistic bullet ballistics and skeletal animation can all be found in the game.


A weather system is integrated into various parts of the landscape and allows a variety of weather effects, such as sunshine, storms and showers. The weapons available, behavior of the AI, game tactics and ranking systems will depend on the weather.

For more direct specifications, check the engine's homepage.

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About S.T.A.L.K.E.R.: Clear Sky with 3 comments by Sclera on Mar 29th, 2011

Allow me to direct your attention to three mods I have recently completed:

1. Clear Sky Music Overhaul

After a few weeks of work and minimal trouble, it's all done. Most of the game's original music has been discarded, since a lot of it was borrowed from Shadow of Chernobyl. Limansk, The Hospital and CNPP remain untouched, but everywhere else gets a bunch of new music. About 59 minutes of music were composed, and (perhaps hypocritically) I borrowed some music from my Shadow of Chernobyl mod as well.

Video preview:

2. Anomaly Sound Replacement for Shadow of Chernobyl

Borne not out of an apparent need for new sounds, but purely out of creative boredom, this mod replaces the most commonly heard anomaly sounds in the game. It gives the game a fresh feel without ruining the intended atmosphere. Also, if you are a mod developer and wish to include these in your mod, please do so, and let me know of course.

Video preview:

3. Interface Mod for Shadow of Chernobyl

I never intended to release this, initially. It started out as a lot of different things I was changing for myself, and ended up being a culmination of these things in a neat little package. The idea of the visual aspect of this mod was to eliminate most of the clutter on-screen, and remove superfluous or immersion-breaking information. On the gameplay screen you can see your health, stamina, current ammo and fire-mode; no, there's no map. That's how I like it (try not to feel tempted to check your Contacts for nearby enemies... I swear I don't do it).

You're given several options here; 2 health bars, 3 cursors, 4 PDAs, and 3 sets of PDA sounds. Don't be overwhelmed though, defaults are installed.

Thanks for reading. Please enjoy my work! Your thoughts are appreciated.

- Rohan

S.T.A.L.K.E.R. Shadow of Chernobyl

S.T.A.L.K.E.R. Shadow of Chernobyl S.T.A.L.K.E.R. Shadow of Chernobyl

Updated 1 month ago Released Mar 19, 2007 Single & Multiplayer First Person Shooter

In 1986, the world's worst nuclear disaster occurred at the Chernobyl power station. Soviet authorities established a 30km 'Exclusion Zone' around this...

S.T.A.L.K.E.R.: Call of Pripyat

S.T.A.L.K.E.R.: Call of Pripyat S.T.A.L.K.E.R.: Call of Pripyat

Updated 1 month ago Released Feb 4, 2010 Single Player First Person Shooter

Call of Pripyat takes you once again into the vicinity of the Chernobyl nuclear reactor that exploded in 1986. This so-called "Zone" is a highly contaminated...

S.T.A.L.K.E.R. Build 1935

S.T.A.L.K.E.R. Build 1935 S.T.A.L.K.E.R. Build 1935

Updated 5 months ago Released Oct 17, 2004 Single Player First Person Shooter

This is build 1935 of S.T.A.L.K.E.R. Shadow of Chernobyl, formally called S.T.A.L.K.E.R. Oblivion Lost. This was made by GSC Game World before they remade...

S.T.A.L.K.E.R.: Clear Sky

S.T.A.L.K.E.R.: Clear Sky S.T.A.L.K.E.R.: Clear Sky

Updated 6 months ago Released Aug 21, 2008 Single Player First Person Shooter

A group of stalkers, for the first time, reaches the very heart of the Zone-the Chernobyl Nuclear Power Plant-and triggers a cataclysm on the brink of...

S.T.A.L.K.E.R: Oblivion Lost

S.T.A.L.K.E.R: Oblivion Lost S.T.A.L.K.E.R: Oblivion Lost

Updated 4 years ago Released Aug 27, 2002 Single Player First Person Shooter

Ones of the first concepts of S.T.A.L.K.E.R Shadow of Chernobyl.

Post comment Comments  (10 - 20 of 65)
Stealth-Killer Dec 12 2011, 2:47pm says:

Yeah brolee!?... get a better machine, ***!

+2 votes     reply to comment
Opinionator Aug 24 2013, 4:56pm replied:

How old is SOC/CS/COP how many gamers have had and still have major issues with these games no matter the high or low their hardware specs. X-ray its a bag of broken unoptimized **** and always has been and always will be, get real and eat some H pie.

-4 votes     reply to comment
Opinionator Nov 4 2011, 9:32am buried:


Xray? Crapyray more like. Stalkers are really old games and they still run awful on modern PCs even after so many patches. In game frame rate stuttering and freezing while its loads mid game just kills off the emersion and your aim, awh crap dead again :(

-17 votes     reply to comment
SurvivorSTK Apr 8 2013, 4:22am replied:

your word just mean your PC is ****

-2 votes     reply to comment
Opinionator Aug 24 2013, 4:58pm replied:

OMG GSC and Steam forums take a look, facepalm and fail to you too dude.

-4 votes     reply to comment
hukondejo Sep 25 2012, 10:01am replied:

I bought the game "Shadow of Chernobyl" and "Call of pripyat" bundle on steam, and gues what.
No more lag, stuttering, freezing.

+5 votes     reply to comment
Blakluma Jan 28 2012, 1:10pm replied:

x-ray is a great engine and probably the most capable engine I have ever seen.
Sadly though it is hard on the proccesor and RAM.

+5 votes     reply to comment
Opinionator Aug 24 2013, 4:59pm replied:

Just say unoptimized will ya sheesh.

+1 vote     reply to comment
Cossack-HD Dec 10 2011, 8:37am replied:

If you're not Stalker modder then you can't understand all awesomeness of X-RAY...

+9 votes     reply to comment
Opinionator Aug 22 2013, 9:32am replied:

Yeah the Xray engine its so awesome all the games need community patches and bug fixes so its a little less broke and a little less ******. Don't get me wrong almost everything is great about Stalker series except the Xray engine, they should of called the CTD engine lol. Bury me fanboys in your own denial.

I don't need to be a mechanic to know if I am driving a **** car or not, facepalm to your come back.

+1 vote     reply to comment
Protos.Angelus Online
Protos.Angelus Aug 9 2011, 9:02am says:

First of all S.T.A.L.K.E.R. 2 is coming out on a NEW ENGINE--- much better etc. so let's all focus our thoughts on that game not these past games. OK? Pray that the upcoming stalker will be a brilliant, bugless game.

-2 votes     reply to comment
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GSC Game World
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Released Mar 19, 2007
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Mar 23 2011, 1:30pm by .sverek

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