Alien sight mechanics and breaking LOS

[Grendel13G]

Back in the day of LW1 and I/I EW, I remember how crucial it was for me to learn how to break line of sight from the aliens. Occasionally I'll try the same thing in LW2, like hiding behind the non-edge tiles of a large van, only for some Muton to reward my cleverness by lobbing a plasma grenade at my conveniently clustered soldiers that I didn't think the aliens could even see.

How do the mechanics of sight work for the aliens in XCOM2/LW2, re: knowing your soldiers' locations? Do they have perfect knowledge of where your soldiers are? But this doesn't seem to be right, either, because I've seen aliens wander about aimlessly when my non-stealthed soldiers are outside of their range of vision. Do they have perfect knowledge, but only within their vision range? Perfect knowledge, but only soldiers they could see at the beginning of their turn? Or do large cover objects like vans no longer hide units at all? Or is it something else?

The follow-up question, of course, is whether there are any useful tactics regarding alien sight for (non-stealthed) soldiers. What have you found that does or doesn't work?

[Daergar]

There are mods that help you out, Gotcha! and others. LoS is logical and finicky at the same time.

Aliens to not know where you are unless they see you. Pods vary in state as well, from wandering to searching for you, drawn by sound and so on.

[JulianSkies]

Back in the day of LW1 and I/I EW, I remember how crucial it was for me to learn how to break line of sight from the aliens. Occasionally I'll try the same thing in LW2, like hiding behind the non-edge tiles of a large van, only for some Muton to reward my cleverness by lobbing a plasma grenade at my conveniently clustered soldiers that I didn't think the aliens could even see.

How do the mechanics of sight work for the aliens in XCOM2/LW2, re: knowing your soldiers' locations? Do they have perfect knowledge of where your soldiers are? But this doesn't seem to be right, either, because I've seen aliens wander about aimlessly when my non-stealthed soldiers are outside of their range of vision. Do they have perfect knowledge, but only within their vision range? Perfect knowledge, but only soldiers they could see at the beginning of their turn? Or do large cover objects like vans no longer hide units at all? Or is it something else?

The follow-up question, of course, is whether there are any useful tactics regarding alien sight for (non-stealthed) soldiers. What have you found that does or doesn't work?

For what's worth, with grenades and other Area of Effect abilities as long as they saw one soldier they could target it so that it'd hit anyone else they didn't see as long as it hit the target they saw too. This weird behaviour will be changed in 1.3.
That aside, they only know the location of the soldiers they can see, but there are some weird LoS behaviours in this game that don't really match EW at all, there's a lot of complaints about that but it comes from the base game and little that PI can do to fix it.

[Manifest]

That aside, they only know the location of the soldiers they can see.

Are you sure? I remember it was either EW or X-Com 2 where if they could see one of your soldiers they could see them all, but if all your soldiers were hidden you were fine.

[Tuhalu]

There are multiple issues.

There is a vanilla behaviour where AOE abilities can hit your guys out of the fog of war as long as there is LOS. This allows the aliens to shoot rockets or poison spit at the underside of roof tiles or a wall right next to your guys without any enemy having LOS. This is being fixed in 1.3, so as to require the enemy to have LOS with at least 1 unit before that rocket or spit can be used.

- Fix vanilla "rule" allowing cheaty visibility for AI AOE attacks. Added two new config vars to XComGame.Helpers_LW:
1) RequireVisibilityForAoETarget - if true, the AI will only consider targets that are visible when evaluating AoE abilities. When false (e.g. vanilla behavior) the AI may target anyone on the XCOM team regardless of visibility. Set to true for LW2.
2) AllowSquadVisibilityForAoETarget: Only has an effect when RequireVisibilityForAoETarget is true. If false, a unit may only target units it can directly see for AoE effects. If true, a unit may target anyone visible to anyone on the alien team. Set to true for LW2.

Walls, roofs and vehicles often have buggy design.

A large VAN may appear to be 4-5 tiles of heavy cover, but only the corner tiles actually provide heavy cover. The rest of them are actually no cover and this can get your guys killed (they'll show the full cover icon, but provide LOS through them and provide no cover). There are certain walls that exhibit the same behaviour.

Certain roofs allow the aliens to see through them from positions where they really shouldn't be able to as well. This is mostly due to large roofs actually being several roof sections stitched together. The aliens wind up seeing through the edges somehow (even though you can't!), which often leads to wierd detection mechanics.

[sadh]

Is enemy knowledge of a soldier's position completely "binary" (meaning they know with 100% or 0% certainty) or can they make educated guesses, same as we do?

Example: We have LOS to an enemy. That enemy breaks LOS and runs behind a tall obstacle, but we don't see them emerge on the other side. In such a case, we could assume their likely general location even if we don't know the exact tile they're on, and strategize accordingly.

Do enemies use that same logic or do they completely lose situational awareness the moment a soldier breaks LOS?

[Grendel13G]

Yikes, I had no idea there were so many different overlapping issues.

Daergar: I've been using the Gotcha Again mod, and I love how much it helps with determining line of sight on the aliens, where overwatch triggers, and when your move will activate a new pod (that you can see). Is there a way to use it to determine when the aliens have LOS on you? Or can you just assume LOS is symmetrical most of the time?

A large VAN may appear to be 4-5 tiles of heavy cover, but only the corner tiles actually provide heavy cover. The rest of them are actually no cover and this can get your guys killed (they'll show the full cover icon, but provide LOS through them and provide no cover). There are certain walls that exhibit the same behaviour.

Woah! This is nuts! But it does explain a lot of what has been happening to me, and why I've been so confused about alien sight mechanics. Thanks for the info on the fixes for 1.3; requiring visibility for AOE attacks sounds like a significant improvement.

[LordYanaek]

Is enemy knowledge of a soldier's position completely "binary" (meaning they know with 100% or 0% certainty) or can they make educated guesses, same as we do?

Binary which is probably why firaxis coded the cheating AoE attacks to "simulate" educated guesses that AI cannot do. This causes a lot of other issues unfortunately such as firing at soldiers nobody have ever seen. Glad Pavonis got rid of it.

Daergar: I've been using the Gotcha Again mod, and I love how much it helps with determining line of sight on the aliens, where overwatch triggers, and when your move will activate a new pod (that you can see). Is there a way to use it to determine when the aliens have LOS on you? Or can you just assume LOS is symmetrical most of the time?

It's symmetrical all the time (i think) except for some edge cases where an enemy can shoot at you from the edge of the vision range and you don't see it but those are (fortunately) very rare.

A large VAN may appear to be 4-5 tiles of heavy cover, but only the corner tiles actually provide heavy cover. The rest of them are actually no cover and this can get your guys killed (they'll show the full cover icon, but provide LOS through them and provide no cover). There are certain walls that exhibit the same behaviour.

Woah! This is nuts! But it does explain a lot of what has been happening to me, and why I've been so confused about alien sight mechanics. Thanks for the info on the fixes for 1.3; requiring visibility for AOE attacks sounds like a significant improvement.

Totally nuts but blame Firaxis for this. It's extremely annoying and the main reason i don't play Ironman. This bug (yes, it's a bug that was never fixed by Firaxis) is extremely frequent.

[chrisb]

One thing that should be changing in 1.3 is that the aliens will require having LOS before using AoE abilities. So this should prevent aliens you can't see from launching perfect grenades/rockets out of fog.

Bug Report

Thanks. I've added a fix for this. It's a vanilla bug/feature - the AI is allowed to AoE you even if it can't directly see you, and the AI always knows exactly where every non-concealed XCOM soldier is. With the fix they should only consider units as targets if *someone* on the AI team can see them, so as not to break archers bombarding you from far away.