The cover system and LOS: the lost rule book

[shorn]

I have noticed a lot of questions regarding the cover system and line of sight system in the game. In another post I plotted out a room and recorded the exact line of sight/cover for 3 aliens and one squadie. As others pointed out this was a bit intimidating and although interesting needed to be simplified so people could make sense out of it.

Because of this I have gone over several missions, recorded information, and tried my best to relay that info to you in the following three page PDF and in the JPG file. (the information is the same in every file...just trying to give you guys options for your viewing pleasure)

Web PDF with black background
https://dl.dropbox.com/u/1589284/D%2...d%20you%20.pdf

More printer friendly version with white background
https://dl.dropbox.com/u/1589284/D%2...20white%29.pdf

Hi Res JPG
https://dl.dropbox.com/u/1589284/D%2...ng-and-you.jpg

(Files updated 1/28/2013)

The first page is a description of the various systems the game uses for cover, flanking, and line of sight. The second page contains recreated examples from actual game play.

Some things to note:

1: The line of sight in the game seems to follow similar rules to miniature board game play on a grid. Different grid games deal with LOS in various ways but it essentially boils down to drawing a line from one character (either from the center or a corner) to the enemy and if that line does not cross anything you have LOS. Xcom seems to work in the same manner and from the looks of it they are using the center method.

2: To get my data I did not do multiple reloads off a save. I merely played until i found a situation that seemed appropriate and then used a separate program to allow me to move a squadie an unlimited distance around the battle field during my turn. If anyone cares to experiment around with this do a Google search for XCOM: Enemy Unknown (AOB). If your interested in testing aspects of the game this it is a handy way to do it and a heck of a lot faster than loading saves every step.

3: I have some data for height above the ground by have been having a problem modeling this well as I am unsure how tall a vertical movement box is. Once I figure this out I can diagram it and add it to my rule book thing. Also if anyone has already done this pm me the info and ill add it with credits.

4: Some people may be interested in the raw data I gathered concerning the three aliens. If you are then look in the following link. I have fixed any discrepancies that came up through the several posts and double checked a few things so everything should be accurate now. http://forums.2kgames.com/showthread...36#post2350236

Also i do welcome input on improving the file. Before any improvements are made I will test the theory in game however to be sure it is as accurate as possible.

UPDATE (11/13/2012): I have added a 3rd page with 3 more examples of how the cover/los system works. In the first and second example I look at how two different types of cover effect LOS and bonuses. In the third example I look at how cover LOS works through a door.

UPDATE (1/25/2013): i found some people were unable to see the PDF and so have included a JPG file. Also corrected a typo on page 3 example 6..squadie has full cover from 6 and 1 not 5 and 1.

UPDATE (1/28/2013): Corrected a sentience to be more correct. "By enacting this ability (hunker down) your squadie is immune to critical hits" not flanking

Update (1/31/2013) another sentience correction

[Kadjunga]

Really good effort.

One more thing is that if you are standing behind cover then your soldier is effectively standing in the 2 tiles next to it as well, for shooting purposes (peaking out, etc). If you do not want him to be in LOS, move him more than 1 tile away from the edge of the hard cover and he will no longer be able to see the enemies (or them him).

[Darkendragon]

really good explanation, good job, just one error, in your final example, you said squaddie 1 is flanking sectoid 2, I think you meant to say squaddie 3 is flanking sectoid 2

also I donno why or how, or if my game glitched but I still believe that hunker down can also reduce line of sight while behind low cover. i've given this example out a bit but in 1 of my games, I was in a ufo wreck, I approached the door and overwatched, and 3 mutons showed up and of course I shot and it missed horribly and things went wrong. so I restart and knowing that they were coming out and that my overwatch shots were gonna miss, I decided to use hunker down. when I did they walked right infront of me without noticing me (and of course I didnt see them either cuz I was hunker down and my vision was shortened) and they did not react until I had my first shot, so they were still clumped up out in the open.

because of this I felt that while hunker down, your not exposed, and since your not looking around corners, its considered full cover and your unseen by the enemy.

[shorn]

really good explanation, good job, just one error, in your final example, you said squaddie 1 is flanking sectoid 2, I think you meant to say squaddie 3 is flanking sectoid 2

also I donno why or how, or if my game glitched but I still believe that hunker down can also reduce line of sight while behind low cover. .......

because of this I felt that while hunker down, your not exposed, and since your not looking around corners, its considered full cover and your unseen by the enemy.

Good point on Hunker Down. I have added that to the file. Hunker down reduces your sight from 17 squares from the unit to 3 in either cover. Also since it doubles the benefit from cover, your low cover gives you +40 defense and the shield changes to a full cover shield (i imagine it is coded to change the shield at +40 defense from low to full).

I was curious and so tested your theory that hunker down reduced your exposure and found that it does not seem to have any effect.

https://dl.dropbox.com/u/1589284/D%2...est-hunker.jpg

If you look at the picture you see player 1 hunkers in two positions and player 2 who changes place can see player 1 every time. Either the game glitched for you, your luck was due to how the aliens move (to speed the game up they teleport from one bit to another bit instead of walking the path), or there is a hidden bonus to hunker. I do like the idea that hunkering down would keep you from looking around corners though...it would be a nice little benefit.

[Darkendragon]

if you want to do more testing then try the scenario I was in, think of a a UFO cargo crash sight for instance, there you have a few low cover in front of the door. the enemies came in from the door towards the low cover, so they were directly in front of me with the cover in between us. they never came within 3 squares of me since I did not see them during their move.

in your example you you had them looking around a corner. I have a feeling the reason why it didnt work in yours was cuz they were looking at from an angle. with my situation, my guys were in the middle of cover, not at the edges.

this just another scenario to test out

[ellgieff]

This looks awesome, thanks.

Is there a possibility to get it with a different colour scheme? I don't want to use that much ink.

[shorn]

This looks awesome, thanks.

Is there a possibility to get it with a different colour scheme? I don't want to use that much ink.

Just uploaded one that should be a bit easier on your printer

[ellgieff]

And thank you. :-D

[Operations]

This is a picture I found which explains the rules well too.
http://i.imgur.com/eqnBg.gif

[shorn]

This is a picture I found which explains the rules well too.
http://i.imgur.com/eqnBg.gif

Big props to MarinusWA and Gaggle of Chryssalids for this. I had not seen this latest post from them but as you said it does a good job of visually explaining the rules.

[tayo]

Man I gotta really pound this into my head, every time I think I know it perfectly there's an example that stumps me.

[shorn]

So I had an email question from a fellow user and thought I would write a quick post explaining whats going on.

I think there's a mistake though.
Example 4 - it says Squaddie 2 would have LOS if he took a step south - I'm not sure why. I would think if he took a step north he'd have LOS because of the "lean".

This is an interesting dynamic of the LOS in the game and is something that is not very intuitive. First off please look at the following picture.

https://dl.dropbox.com/u/1589284/D%2...th-stepout.jpg
I will denote position by calling out the upper left corner of where the unit will be. For instance the sectiod is at f2.

It is important to remember that a unit that is in full cover at a corner calculates their line of sight from the square adjacent to the corner. Our sectoid in example 4 then would calculate their LOS at d2. As you can see from the LOS lines only line 3 is unblocked by the full cover. The only reason the sectoid can hit our squadie at f6 is because they are also leaning out into square d6. Once they move to square f8 they are no longer leaning, as you need cover to do this, and thus they lose LOS to the alien.

[stokad]

How do you explain that you can shoot through walls then ? (i just had that happens two times - once for both side) on the graveyard map for one of the central building wall.

[Busard]

This is a picture I found which explains the rules well too.
http://i.imgur.com/eqnBg.gif

First line of sight example, shouldn't the XCOM guy lean up and see the middle alien?
Also very last example is wrong, both aliens are flanked (even though the lean positions are not flanking positions; I know it shouldn't be that way but it is).

[shorn]

How do you explain that you can shoot through walls then ? (i just had that happens two times - once for both side) on the graveyard map for one of the central building wall.

Stokad. There are two systems working side by side when you take a shot.

System 1: Line of sight.

This system looks to see if you can see the target. Its done by drawing a line from the center of the square your standing on to the center of the square the alien is standing on. I explain this in the post right above yours.
System 2: Graphics.

This system is responsible for the actual animation of the shot and it is this system that causes the error. See, usually you only see shots through walls when you or the alien take a shot from the corner. When you take a shot from a corner the animation is supposed to show the character stepping out from behind cover and taking a shot. If you are shooting at an alien behind cover it is also supposed to show that alien leaning out and getting hit. The problem is that the shot you take is always at the aliens center of mass (sometimes head) and if the animation for the unit leaning out does not happen you will see shot go through a wall.

Also remember that some cover, cars/trucks/trailers mostly, have spaces in them like windows or gaps between the trailers and cab. Again you have LOS but sometimes the animation shows a shot through a wall. So this is just a graphical glitch..hopefully they will patch it at some point as it does look a bit cheese but its not a game breaker..just remember the LOS rules and you will be ok.

[Schuesseled]

@Shorn, example 5 is incorrect, units/aliens in full cover are treated as stepping out to one or both sides of that cover to determine flanking and line of sight. Because of that everyone but the guy hunkering down can see them and squaddies 3 and 4 will be flanking them as well.

[shorn]

@Shorn, example 5 is incorrect, units/aliens in full cover are treated as stepping out to one or both sides of that cover to determine flanking and line of sight. Because of that everyone but the guy hunkering down can see them and squaddies 3 and 4 will be flanking them as well.

Looking back at the scenario I did make a mistake. Squadie 5 can see both aliens due to the step out as you said. I also made a mistake with the line of sight lines as they were not from the center of the unit or the step out position and will fix both in the main files.

Let me explain though, and show you with pictures, why or why not the rest of the guys do not have LOS.

First off remember that a unit only steps out in the direction of cover. So if we look at Squadie 1 it will step out down the B and D column but can only do so towards the bottom of the map and not towards the top. Because of this when you draw a line of sight from squadie 1 to sectoid 2 you do so from the sectoids location because it cannot step out in that direction.
Squadie 6 is hunkered down and cannot see either. Squadie 7 is hunkered and by doing so its LOS is reduced to 3 squares from the squadie. Sectoid 2 happens to be just outside of this area. It appears the alien has to be at least halfway into the hunker down circle to be seen.

Also remember that a unit cannot take cover on the diagonal. Squadie 4 and 3 are in such a position and thus have no cover bonus at all. The cover of the tree however does block their line of sight to sectoid 1 and sectoid 2 however. Squadie 2 is in a similar position but because of the step out effect of sectoid 2 he has LOS on sectoid 2. Even with the step out effect from sectoid 1 there is no LOS between sectoid 1 and squadie 2.

The following is the picture: I created example 5 from this forest location using the in game grid system. You should be able to better see from the screen shots what is going on perhaps.

https://dl.dropbox.com/u/1589284/D%2...S-exsample.jpg

***Original PDF's are updated with the above changes now.

[ToastyBiggins]

Excellent guide. I picked up most of these rules by "feel" after playing the game for a long time, but it was cool to see them spelled out in detail, and it will be a huge help to new players still grappling with the cover system. Honestly this should probably be stickied.

There do seem to be some bugs in the way it calculates flanking sometimes, though. Yesterday I had a soldier in the exact same position as Squaddie 1 in example 2 (leaning out from the side of the truck to shoot a muton taking cover towards the front of the truck) which clearly should have been flanking, but was not, or at the very least was not indicated as such on the HUD (red shield icon, no extra crit chance, etc.). If you absolutely, positively have to be flanking, I think the best option is to just run right up in their grill.

[shorn]

Excellent guide. I picked up most of these rules by "feel" after playing the game for a long time, but it was cool to see them spelled out in detail, and it will be a huge help to new players still grappling with the cover system. Honestly this should probably be stickied.

There do seem to be some bugs in the way it calculates flanking sometimes, though. Yesterday I had a soldier in the exact same position as Squaddie 1 in example 2 (leaning out from the side of the truck to shoot a muton taking cover towards the front of the truck) which clearly should have been flanking, but was not, or at the very least was not indicated as such on the HUD (red shield icon, no extra crit chance, etc.). If you absolutely, positively have to be flanking, I think the best option is to just run right up in their grill.

ToastyBiggins, Thanks for the kind words. It took me a bit of playing to figure all this stuff out and my hope is that maybe some one will not have to go through the same suffering . I also agree if you have to flank you might as well get right up on the aliens grill..its the only way to be sure of the bonus..

[shorn]

I was having a conversation in another thread about LOS and the person, marts2753, posted how he was unsure that the LOS was taken from center of unit to unit. Instead he posed that maybe if you could see any part of the units square you had LOS.

After some testing I came up with this.

https://dl.dropbox.com/u/1589284/D%2...r-example2.jpg

This is from the multilayer bar map. One enemy squadie is inside the room while the good squadies are outside. As you can the blue LOS line crosses many squares occupied by squadies but it is only when i move a squadie from G5 to I8 that i get LOS on the bad guy. You might notice that Squadie 1 looks like he should have LOS. To help you see better i highlighted the center point of squadie 1 in red. As you can see it is outside the blue LOS line.

Ill update the PDF to contain this..maybe after another mission of dishonored though :P

[shorn]

I have updated the pdf file. See the end of the first post for what has changed. Also note if you have the PDF book marked you should just have to exit out of the PDF if you have it opened and reload it to have all the new info. Enjoy!

[Zauron]

shorn,

I'm confused about example #7 in your pdf. You say there that the sectoid has full cover from squadie 4. Squadie 4 looks like he should be flanking the sectoid being that he's on the opposite side of the sectoid from his full cover. How does the sectoid get full cover from him instead of being flanked by him?

Also that example seems to cut off in the middle of a sentence at the end of it.

[shorn]

shorn,

I'm confused about example #7 in your pdf. You say there that the sectoid has full cover from squadie 4. Squadie 4 looks like he should be flanking the sectoid being that he's on the opposite side of the sectoid from his full cover. How does the sectoid get full cover from him instead of being flanked by him?

Also that example seems to cut off in the middle of a sentence at the end of it.

Zauron, Good Catch! The text should say that the sectoid has half cover from squadie 4 not full cover and has been changed in the pdf's. Also Im not sure what the next sentence was going to be so i just cut it out

As for your question: Why is squadie 4 not flanking the sectoid. Flanking can only happen in 3 situations. If the squadie is to the side of the sectoid, if the squadie is behind the sectoid, or if the squadie is at a corner of cover assuming the sectoid is using the same cover. So squadie 4 would have to be at position F6, and column G or column H to flank the sectoid.

Hope that helps

[Zauron]

Okay right, I think I understand it all now. So he WOULD be flanking the sectoid if that half cover to the side wasn't there, because he's be behind him, but since it is, the sectoid has half cover.

The other image I've seen talks about flanking being related to "facing" - i.e. have to be directly to the side or behind whatever direction the opponent is facing to flank them. With cover, is it thus assumed that the character is "facing" in all directions they have at least some cover in? Thus the Sectiod here is technically facing north and west, not just north, so squaddie 4 isn't "behind him".

But I wondered, does facing make a difference when you are completely uncovered? You can't change your facing really so I assumed it didn't make any actual difference, but it made me wonder, when out of cover does the character's facing make it so if they are directly facing an opponent, are they uncovered but still not "flanked?" Or are flanked and uncovered the same in terms of crit bonuses and the like and talking about "facing" is a bit misleading since its more about "which directions you have cover in"?

[shorn]

Okay right, I think I understand it all now. So he WOULD be flanking the sectoid if that half cover to the side wasn't there, because he's be behind him, but since it is, the sectoid has half cover.

The other image I've seen talks about flanking being related to "facing" - i.e. have to be directly to the side or behind whatever direction the opponent is facing to flank them. With cover, is it thus assumed that the character is "facing" in all directions they have at least some cover in? Thus the Sectiod here is technically facing north and west, not just north, so squaddie 4 isn't "behind him".

But I wondered, does facing make a difference when you are completely uncovered? You can't change your facing really so I assumed it didn't make any actual difference, but it made me wonder, when out of cover does the character's facing make it so if they are directly facing an opponent, are they uncovered but still not "flanked?" Or are flanked and uncovered the same in terms of crit bonuses and the like and talking about "facing" is a bit misleading since its more about "which directions you have cover in"?

You have pretty much the idea of it all. Facing really is just what direction you have cover in. If you do not have cover then your facing does not matter and you cannot be flanked. At this point you are in the open and have a +50% chance to take crit damage. If we think about what direction you have cover in you see that squadie 4 is not to the side or behind the sectoid as the sectiod is behind the low cover in regards to that squadie.

The sectiod can only take cover to the north or west though so it is not facing north and west..I think i see where you are going with that idea but i dont want you thinking you can have cover on the diagonal as you only have cover via rows (high cover row a3-h3) or columns (low cover column f4-f8)..

If your interested I modeled the positions you have cover and no cover against 3 sectiods and a squadie from an in game book store map. You can see the position results from each alien here as well as the hit percentage at various distances.

http://forums.2kgames.com/showthread...ge-LOS-and-you

[Hamsterminator]

Bit of a necro here, but this thread is fantastic- surprised it's not stickied

[shorn]

Bit of a necro here, but this thread is fantastic- surprised it's not stickied

Thanks! It would be nice if it was stickied as I think it could help some of the people that come in asking questions about LOS/flanking. I figure at some point an admin will get around to it :P

As an aside I found some people were unable to see the PDF and so have included a JPG file. Also corrected a typo on page 3 example 6..squadie has full cover from 6 and 1 not 5 and 1.

[natas]

Hi, new user here Came upon this thread while I'm looking for LOS info in Xcom. Great information here but regarding the "Hunker Down" information by Shorn, it say that squadie is immune to flanking. Isn't it immune to critical instead?

Ok, the main question I wanted to post is about the stepping out and LOS behind cover. All the information I read is that stepping out only happen when behind full cover and at the edge. Unit behind half cover can see from cover and stepping out don't happen. However, I came across this clip on youtube:



At at around 11:35 min into the video, a muton shot a soldier behind a corner that it don't seem to have a LOS.



At first I thought the side that the muton facing have gap/window that it can get it's LOS from but it seem not the case.



I was puzzled about this and replay it a few time and finally notice that the soldier that got hit, panic and step out to return fire.



My question is that stepping out for that soldier behind half cover should not happen or is that stepping out can still happen behind half cover under special condition?

[shorn]

Natas,
Hey there and welcome to the forum. You are right that it is more correct to say that a hunker downed squadie is immune to critical vs they are immune to flanking. My reasoning was that since flanking only causes criticals…anyway..i have changed that in the above files

As for your second bit.

The lean out effect does happen in low cover it is usually just not as pronounced as it is with high cover. That is you will generally just see the unit shoot over the cover instead of leaning out to take a shot. As you can seen in the next two pictures though the leaning effect works almost the same in low cover as it does in high cover. The only difference is with low cover there are areas where the cover blocks your LOS on a target like you can get with high cover. If you look at bookstore hit for alien 1 you will see the alien is next to low cover and as our squadie moves around the cover they gain a flanking bonus or are flanked in the same way that one flanks or is flanked with high cover.

https://dl.dropbox.com/u/1589284/D%2...e%20corner.jpg
https://dl.dropbox.com/u/1589284/D%2...e-corner-2.jpg

What is really interesting about the picture and video you posted is that the squadie leans out across the cover. This is honestly the first time I have seen this. Perhaps there is a special leaning effect in regards to low cover or perhaps this is simply an interesting glitch. I’ll do some testing and see what I can find.

[Nogen]

What is really interesting about the picture and video you posted is that the squadie leans out across the cover. This is honestly the first time I have seen this. Perhaps there is a special leaning effect in regards to low cover or perhaps this is simply an interesting glitch. I’ll do some testing and see what I can find.

I am more tempt to believe that explanation.

[natas]

Hi Shorn,

Thanks for the reply. After some thinking I think I understand why did you say hunker down is immune to flanking.

Flanking = +Hit% (ignore cover bonus) + 50% critical

When a unit is "flanked" while hunker down, enemy unit will only have +hit% and not +50% critical since it's immune to it. This make the targeted unit same as it is in the open and not behind cover which can't be flanked (but a bit better with no critical chance). Since both conditions for flanking can't be meet while hanker down thus that's what you mean by immune to flanking. I hope I get it right

Hope you can figure out the puzzle behind the leaning across cover issue. Not understanding LOS system is causing me to lose troops like crazy in my C/I

[Bibor]

That muton shooting in the video at 11.30s is definitely a glitch, probably caused by the door and window which would provide the muton with LOS *if* the door were open.

[shorn]

Hi Shorn,
Thanks for the reply. After some thinking I think I understand why did you say hunker down is immune to flanking……
Hope you can figure out the puzzle behind the leaning across cover issue. Not understanding LOS system is causing me to lose troops like crazy in my C/I

Nats,

Yep, that’s pretty much why I phrased it that way. Still it is more correct to say they are immune to critical hits or immune to the bonus gained by flanking. I did the former wording so it was more in line with what hunker down says in the tool tips.
The leaning across cover thing is interesting and a bit hard to test as I have to first find a situation where such a thing might be possible. See usually you just shoot over the cover which is why people say the leaning effect does not take place with low cover, although, I did see a sectoid do it yesterday but it was at a corner not.

I think the leaning across cover is not a glitch but really really rare. First you have a unit behind low cover and then you have to have that units line of sight to the alien blocked unless it steps out across the cover, as in the video you presented. Unfortunately since I can’t place the sectoids where I want I have to hope they just move there on their own accord when im fishing for conformation.

Regardless of this possibly rare event the rest of the guide should help you understand the LOS system just fine. If it helps bring up your grid view and think, can I draw a line between my character and the enemy character without it intersecting high cover or low cover with a change in terrain. Remember that the lean out effect around corners of cover is real so account for that. Also remember even though they lean out some times the game does not show this happening. You can still take a shot in this case and it will often show the shot going through the building to hit the alien. This is just a graphic thing not a system bug.

That muton shooting in the video at 11.30s is definitely a glitch, probably caused by the door and window which would provide the muton with LOS *if* the door were open.

Bibor,

Good call, and at first as I was watching the video I thought it might be a door/window LOS glitch like we find on the office paper map.

http://forums.2kgames.com/showthread...21#post2692221

The problem is that when you look at the lines that muton would not have any LOS if the door was open while in the above map they would…so it should not be that particular glitch.

LOS through the door = https://dl.dropbox.com/u/1589284/D%2...20dynamics.jpg

Watching the video its apparent that the squadie steps across cover to take the shot. This means when the muton fires it is actually firing at the square the squadie is stepping to but as the squadie is not there the shot looks like its going through the wall. Firing through walls is a known graphic glitch but only happens if you have your squadie in a position where they can lean out…like at the corner of a building.

[shorn]

I have not as of yet figured out a good reliable way of calculating LOS with changes in altitude. In looking into this though I did create this and found it a bit interesting.

https://dl.dropboxusercontent.com/u/...r-obstical.jpg

The set up is as follows: Line of 5 sectiods behind a piece of full cover in such a way they cannot lean around to get a shot and a line of 6 squdies in flight armor at max altitude. The top left row corresponds to the sectoids in the top right row and the same is true for the bottom rows.

The lables represent what squadie or alien is seen by a corrosponding unit. So squadie 1 sees alien 1-5 while squadie 2 only sees alien 4 and 5.

Looking at the numbers there are a few anomolies that pop up. For instance: On the upper row Squadie 1 can see aliens 1-5 yet alien 1 cannot see squadie 1. Like wise on the bottom row squadie 5 and 6 can see nothing yet alien 5 can see everyone. Also on the bottom row the squadies who can see aliens only see 4-5 but alien 3 can see squadie 1..

Conclusion: The first thing that is obvious is that a container of that size casts a big shadow. Row 1 shows that if a squadie flying at max alt is 5 spaces from the edge of container they can only see things that are 4 space or more...which means there is a 3 space area next to the container they cannot see.

The second thing i notice is how sometimes a squadie has no los to an alien but an alien will have los to the squadie. There are times where one could almost justify such a thing but honestly it feels like a LOS bug to me.

[Huwmane]

Love the work you've done, thanks!