# Determining line of sight vs cover



## Nikosandros (Oct 15, 2008)

I am trying to figure out if I'm reading the rules correctly.

On page 273 of the PHB we read that when determining LOS, a line can't touch an obstacle. Next page, we see in the drawing that LOS is obstructed by touching the corner of a wall. On the other hand when determining cover: _A line isn’t blocked if it runs along the edge of an obstacle’s or an enemy’s square._ (p. 280). This in confirmed in page 43 of the DMG: _A line that runs parallel right along a wall isn’t blocked._

When a line runs parallel to a wall it in fact, touches the obstacle. So, are the rules for LOS and cover different?

Are those two examples correct?

1) A character is shooting around the corner at someone in a corridor. The target doesn't have any cover, because we have line that runs along the wall.

2) A character is shooting around the corner at someone behind another corner (a U-shaped corridor)

1+++++++++++++2
..........................

for the purpose of cover there are two unobstructed lines from square 1 to square 2, however there is no line of sight.


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## JGulick (Oct 15, 2008)

Nikosandros said:


> I am trying to figure out if I'm reading the rules correctly.




I think the difference here is that Cover is determined from EVERY corner (1 or 2 blocked = cover, 3 or 4 blocked = superior cover) whereas LoS is determined from only the Best corner (even just 1 unblocked = have LoS).  So, in your example, you could have LoS determined along corners not touching the wall (say, from the bottom front corner of 1 to the top front corner of 2).  LoS and (because of the "following the wall" exception in the Cover rules) no Cover.


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## Kordeth (Oct 15, 2008)

JGulick said:


> I think the difference here is that Cover is determined from EVERY corner (1 or 2 blocked = cover, 3 or 4 blocked = superior cover) whereas LoS is determined from only the Best corner (even just 1 unblocked = have LoS).  So, in your example, you could have LoS determined along corners not touching the wall (say, from the bottom front corner of 1 to the top front corner of 2).  LoS and (because of the "following the wall" exception in the Cover rules) no Cover.




Not quite. Cover is determined by one corner of your square and every corner of the target square, LOS is determined by one corner of your square to _any point_ in the target square, not just a corner.


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## Hypersmurf (Oct 15, 2008)

Nikosandros said:


> 2) A character is shooting around the corner at someone behind another corner (a U-shaped corridor)
> 
> 1+++++++++++++2
> ..........................
> ...




There is also no line of effect (PHB p273: "If every imaginary line you trace to a target passes through or touches a solid obstacle, you don't have line of effect"), so someone in square 1 cannot make a ranged attack on square 2 ("You need line of effect to any target you attack").

-Hyp.


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## Ravingdork (Oct 15, 2008)

Thought this might help clear a few things up.


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## CubeKnight (Oct 15, 2008)

raven_dark64 said:


> Thought this might help clear a few things up.



That was extremely helpful. Huge thanks!


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## Syrsuro (Oct 16, 2008)

Note:  The cover rules in the PHB are the 'simplified' version of the rules.

The DMG (page 43) gives an optional approach which restricts the PHB version of cover to ranged, close and area attacks, but gives a different approach to be used for melee attacks.

Thus, for example, under the DMG system a creature that is right around the corner from the combatant (see the bottom left of the earlier posted diagram - "PC#3 has no cover from M#3") has no cover from a ranged attack by M#3, but _does_ have cover from a melee attack by M#3.

Carl


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## Nikosandros (Oct 16, 2008)

What about a defender behind an arrow slit?

He has superior cover in melee, but it would seem that he can't be targeted by a ranged attack. LOE can pass through the slit, but cover must be determined using the corners of the target's square.


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## cjais (Oct 16, 2008)

It would seem that area bursts can easily ignore cover.

In the case of area bursts, cover is determined from the origin square. So firing a scorching burst through an arrow slit would provide no cover to the enemy hiding behind it, as it would instantly place the origin square on the enemy, and as there's no cover between the origin square and the creature (they occupy the same square), he would have no cover, even if the wizard had to aim the burst through a pixie-sized keyhole. 

Is this a correct interpretation of the rules?


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## Nikosandros (Oct 16, 2008)

raven_dark64 said:


> Thought this might help clear a few things up.



Thanks for those diagrams. Quite useful.


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## infocynic (Oct 16, 2008)

In this case you might just need a little DM fiat... it's incredibly hard to shoot back through an arrow slit when you're not adjacent to it. If someone really wanted to, I'd do it at a steep penalty, at least -5 and maybe -10. Or just let the wizard fry them with a scorching burst.


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## Nikosandros (Oct 16, 2008)

infocynic said:


> . If someone really wanted to, I'd do it at a steep penalty, at least -5 and maybe -10.



You mean -5 or -10 on top of the penalty for superior cover?


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## tiornys (Oct 17, 2008)

infocynic said:


> In this case you might just need a little DM fiat... it's incredibly hard to shoot back through an arrow slit when you're not adjacent to it. If someone really wanted to, I'd do it at a steep penalty, at least -5 and maybe -10. Or just let the wizard fry them with a scorching burst.



I wouldn't penalize like this.  An arrow slit is a classic example of superior cover--you have line of effect (which requires that at least one line traced from somewhere within your square to somewhere within the target square be unblocked), and all four lines from one of the attacker's corners to the four defender's corners are blocked.  Superior cover, -5 penalty.

Also:  







			
				PHB said:
			
		

> *Superior Cover (-5 penalty to Attack Rolls):*  The target is protected by a significant terrain advantage, such as when fighting from behind a window, a portcullis, a grate, or an arrow slit.



(underlined for emphasis)

t~


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## Nikosandros (Oct 17, 2008)

In principle, I agree with you completely. However the rules state that: _if four lines are blocked from every corner, you can’t target the defender._ (DMG, p. 43).


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## Larrin (Oct 17, 2008)

Nikosandros said:


> In principle, I agree with you completely. However the rules state that: _if four lines are blocked from every corner, you can’t target the defender._ (DMG, p. 43).




by pretty much this same logic you  can't attack from behind an arrow slit, even if you are sitting right behind it.....at the very least they'd have superior cover against you.  This would make arrow slits pointless.  

The best solution is to simply assume that the arrow slit is located at one corner of the square, thus the attacker inside can use that as his corner for getting a clean shot at the enemy, and the enemy can use that as the one corner he can see (so its still superior cover)


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## tiornys (Oct 17, 2008)

Nikosandros said:


> In principle, I agree with you completely. However the rules state that: _if four lines are blocked from every corner, you can’t target the defender._ (DMG, p. 43).



Interesting, and a clear contradiction of the PHB.



			
				PHB said:
			
		

> To determine whether you can see a target, pick a corner of your space and trace an imaginary line from that corner to any part of the target's space.  You see the target if at least one line doesn't pass through or touch an object or an effect....
> 
> *Line of Effect:*  You can target a creature or a square if there's an unblocked path between it and you--that is, if you have line of effect to it.  If every imaginary line you trace to a target passes through or touches a solid obstacle, you don't have line of effect to the target.




So, the PHB clearly states that for LoS you can trace to any part of the target's square, not just the corners.  It's less explicit for LoE, but it implies at least as liberal an interpretation as for LoS.  Arguably, the rules for LoE are even more liberal than for LoS, since LoS explicitly references a path starting at a corner of your space, where as LoE just wants an unblocked line from you to the target.

I'll note that for obstacles that fill squares, the PHB and DMG rules are identical.  As Larrin points out, if the arrow slit is at the corner of a square, we have no problems.  Discrepancy arises when you have obstacles that fill the corners of a square but leave openings along the border of those squares.  According to the DMG, in the following situation, I can't be targeted by anyone:

```
||         ||         
         ||         ||         
         ||**     **||         
.........||.........||.........
         ||**     **||         
         ||         ||         
         ||    ME   ||         
         ||**     **||         
.........||.........||.........
         ||**     **||         
         ||         ||         
         ||         ||         
         ||         ||         


** = a 6 inch protrusion that blocks the corner of my square
```

This is clearly absurd.  Therefore, I'd rule that the PHB trumps the DMG in cases where partial-square obstacles are concerned.

t~


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## Ravingdork (Oct 17, 2008)

tiornys said:


> This is clearly absurd.  Therefore, I'd rule that the PHB trumps the DMG in cases where partial-square obstacles are concerned.
> 
> t~




And here I thought that everybody knew that the DMG rules for cover were messed up, in need of errata, and shouldn't be used.

Guess that's just true on the WotC forums.

Or perhaps I am just very, very wrong in my blanket statement.


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## infocynic (Oct 17, 2008)

raven_dark64 said:


> And here I thought that everybody knew that the DMG rules for cover were messed up, in need of errata, and shouldn't be used.
> 
> Guess that's just true on the WotC forums.
> 
> Or perhaps I am just very, very wrong in my blanket statement.



I'm curious what in particular is messed up about the DMG rules, except for weird cases with partial-square obstructions (which I think I'll just avoid on principle for now).

Also re: Arrow Slits. The DMG specifically says (p 67)


			
				DMG p. 67 said:
			
		

> An arrow slit grants a ranged attacker superior cover while granting him or her a clear view of the battlefield. The firer determines the target’s cover from the square just outside the slit.




So yeah, I'd go back to a -5 penalty (and I did say -5 or -10) for superior cover unless you have some special power to bypass that. I don't think there's any problem with this then.

Slightly related: how does a condition like blinded impact area/close burst/blast attacks? My current understanding is that it doesn't, which seems odd, but the PHB says you just need LOE to the origin square and then determine cover from there, and blinded just says your targets have total concealment, and concealment doesn't help against a burst or blast. Would you rule any differently for powers written as area 1 square within 10?


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## WalterKovacs (Oct 17, 2008)

The thing with burst/blast effects is, the attack roll is often not a case of directling multiple single attacks [although some are, especially ones for martial classes]. Still, even in cases of melee weapons used for close burst or a cloud of literal daggers being thrown by the rogue, it is the matter of just attacking an area and seeing what you can hit. So, being unable to see what you are trying to hit in those cases doesn't seem to be a big problem. Although as a blind character they would need to somehow come up with a "guess the square you are targetting" mechanic as if they were attacking a bunch of invisible things.

It makes sense, in a way, that if you can't see, throwing an AoE means you are bound to hit something.


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## Kordeth (Oct 17, 2008)

infocynic said:


> Slightly related: how does a condition like blinded impact area/close burst/blast attacks? My current understanding is that it doesn't, which seems odd, but the PHB says you just need LOE to the origin square and then determine cover from there, and blinded just says your targets have total concealment, and concealment doesn't help against a burst or blast. Would you rule any differently for powers written as area 1 square within 10?




No, I wouldn't. The fact that it's an area attack means, by its very nature, means that it hits _everything_ within that square. If the blinded character can guess a square an enemy is in, using the rules on p. 281, and he drops an area 1 square spell on that square, it hits everything in that square without the penalty. If it was meant to take the penalty, it wouldn't be an area attack.


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## Nikosandros (Oct 18, 2008)

It would seem that the best thing is simply to ignore the part in the DMG about the four lines blocked from every corner. We can just use the PHB ruling that you need LOE in order to target the square.


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## infocynic (Oct 19, 2008)

The problem with "guessing" a square is that they want to argue "my non-blinded allies tell me '10 feet in front of you and 20 feet to the left'"... and that seems... well, on the one hand, I admire the teamwork, but on the other hand, it seems like it's defeating the purpose of blinding the wizard at all. Maybe I could have the wizard roll 1d10, on a 0 or 9 you hit the correct square, else you're off by 1 in the direction indicated starting from the top of the map / north (whichever is better defined at the moment) and continuing clockwise) to simulate the guesswork involved in measuring distances with your eyes closed.


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## Majoru Oakheart (Nov 22, 2008)

raven_dark64 said:


> Thought this might help clear a few things up.




Doesn't number one in that picture only have cover since two lines still touch corners of the enemies square?  Since lines that pass along the side of an obstacle like that count as "visible"?


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## Kordeth (Nov 22, 2008)

Majoru Oakheart said:


> Doesn't number one in that picture only have cover since two lines still touch corners of the enemies square?  Since lines that pass along the side of an obstacle like that count as "visible"?




Assuming you mean PC #1 in the big illustration at the bottom, no. You're confusing cover with line of sight. A line that touches an obstacle _blocks_ line of sight.


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