## Determining cover for area attacks

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Oneiric86
Joined Apr 2012
17 Posts
Hi guys,

Me and my players are slowly adapting to DnD. We've been playing for a year and a half or so but as we evolve (and start new campains), we get to the point where classes get more interesting, but more complex. We have a hard time guessing cover. We understood recently that a corner grants cover to melee attacks but not ranged (because you choose the square that's better for you), but we have 2 specific questions which will probably be easy for you:

1.
If I create an area attack, the PHB seems to imply I have to choose a single corner for every line of effect I want to trace. Is it so? Can I, for example, trace my lines from corner 1 to the corners of target 1 and my lines from corner 2 to target 2, or I have to use the same corner to check my lines to every square?

2.
If I'm in the following situation:
1
XXXX
XXXX

Where X = obstacle (let's say a wall) and:
A: 1 = an archer and 2 = it's target
Do I have line of effect and line of sight?
Considering I can pick a corner and trace 2 lines to different corners (the two left corners) I'd say yes, with cover     (-2), as I see it like someone who extends to fire a shot (like in the movies when people shoot around corners).

B: 1 = the start of an area 3 and 2 = an enemy
Do I have line of effect?
Considering I can pick a corner and trace 2 lines to different corners, the book would say yes (with cover (-2)). But     it seems quite illogical, cause that would mean the spell goes through the wall.

C: 1 = the start of an area 5 and 2 = an enemy
Do I have line of effect?
Considering I can pick a corner and trace 2 lines to different corners, the book would say yes (with cover (-2)). But     it seems quite illogical, cause that would mean the spell circles around the wall.

Thank you!
Mand12
Joined Jun 2010
17444 Posts
1)  Cover is determined by the corners that are the least favorable for the defender.  It doesn't matter if the dragon is mostly hiding in his cave, if even one corner's LoE trace to even one square in the dragon's space is clear, then he doesn't have cover.  Technically speaking you should trace from all four corners of the origin square to each and every square of the target's space, but this is a lot of work and usually you can just eyeball the limiting case.  It's usually pretty clear which this should be, but until you get used to it it's worth doing multiple traces.

2A)  No LoS, no LoE.  Lines parallel to edges of blocking terrain don't count as traceable lines.  Reasoning is given in PHB 273:  "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."  In the case you've given, the lines going from the bottom left corner of 1 to the top right and bottom right corners of 2 go through the X's, but the lines to the top left and bottom left corners of 2 touch the X's.  All four lines are invalid, so there is no LoE.

2B)  No LoE, same reason.

2C)  No LoE, same reason.  Size of AoE is irrelevant, it does not "spill around" obstacles.

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Oneiric86
Joined Apr 2012
17 Posts
But in the DMG, p. 43, at Getting Technical (Determining cover for melee attacks) and at Choose a corner (Determining cover for ranged attacks), it says that "A line that runs parallel right along a wall isn't blocked."

So what do they mean by this then?
Mand12
Joined Jun 2010
17444 Posts
They mean that in the following:

XX
XX
XX

where

green = attacker
red = defender
black = wall
blue = open square

The target does not have cover just because the open square borders a wall.

I do agree that this seems contradictory between PHB and DMG, so now I wonder if this has been covered in an official FAQ or errata.  Will check.

Edit:  Hrm.  No mention of errata.  Now I'm just as confused as you are.  PHB and DMG seem to disagree - but I'd give precedence to PHB, if for nothing other than that the DMG says "the rules in the PHB explain cover."
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Noctaem
Joined Mar 2010
5128 Posts
I would love to have this settled and detailed because it's an issue that has come up repeatedly for groups I've been in and groups I've DM'ed.  Including the weirdness between the PHB and the DMG.

EDIT:  And now that I think about it, this comes up pretty often on the boards as well.
"Non nobis Domine Sed nomini tuo da gloriam" "I wish for death not because I want to die, but because I seek the war eternal"
Alcestis
Joined Oct 2009
8509 Posts
RC uses the PHB wording and supercedes all previous material.
Mand12
Joined Jun 2010
17444 Posts
RC uses the PHB wording and supercedes all previous material.

Thanks, Alcestis.  That answers things.
D&D Next = D&D: Quantum Edition
Noctaem
Joined Mar 2010
5128 Posts
"Non nobis Domine Sed nomini tuo da gloriam" "I wish for death not because I want to die, but because I seek the war eternal"
WibbleNZ
Joined May 2009
99 Posts
The contradiction is not between the PHB and DMG rules for cover, but between the rules for LoE (PHB 273) and cover (PHB 280). When tracing LoE, a line parallel to and touching an object is blocked, but when tracing for cover it is not...
Oneiric86
Joined Apr 2012
17 Posts
Excuse me for being dumb, but I guess RC = Rules Compendium and then what is applied is what Mand12 answered in the first place, letting aside this note in the DMG (except for the situation given by Mand12 after my reply)?

Thanks guys!
Mand12
Joined Jun 2010
17444 Posts
Yes, what I described in my first reply, and the answers I gave, is unequivocally correct as of the Rules Compendium.
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RisingZan
Joined Aug 2003
917 Posts
The contradiction is not between the PHB and DMG rules for cover, but between the rules for LoE (PHB 273) and cover (PHB 280). When tracing LoE, a line parallel to and touching an object is blocked, but when tracing for cover it is not...

The difference is that for Line of effect, as long as at least one line doesn't run against an object, you have LoE.
The cover rules don't count an object they run along otherwise two creatures fighting near a wall would always have cover, which would get weird.

Now suppose you have the three following situations
OX               OX               XO
OX               OX               OX
OX               OX               OX
OX               OX               OX
OX               XO               OX
1                   2                  3
green = attacker/Origin square
red = defender
black = wall/solid object
blue O = open square

The rules for ranged cover state that you pick any one corner of the attacker/origin square and draw lines to all four corners of any square of the target. If 1 or 2 lines are obstructed its cover, if 3 lines are obstruced its superior cover and if 4 lines are obstructed there's no shot.

In scenario 1 above, there is only cover (and not superior cover) betweeen the attacker and defender, because the two lines along the wall are not obstructed under the cover rules.  However, because lines along a wall don't give Line of Effect, there is no LoE and the attack can not be made.

In scenario 2 above, the attacker does have line of effect, because a line can be drawn from the top left of the attacker's square to the top left of the defender's square without touching any obstructions.  The defender does have cover, because the lines drawn from any of the attackers corners to the four corners of the defender pass through the wall to reach at least two of those corners.
(I will admit here's the one point I'm not 100% clear on is whether the defender has superior cover or just cover. The one front corner (bottom left) is touching a solid object that isn't really passed, so the line, corner and object all meet, then making 3 lines that cross/touch obstructions to reach corners of the defender. However, I believe the correct answer is the defender has Superior Cover because the rules under ranged cover only give a free pass specifically to lines parallel to a wall.)

In scenario 3 above, the defender does not have any cover and the attacker has LoE.

On a side note, in the 3 pictures above, if the attacker were making a melee attack with a reach of 4 (perhaps with a Blade of the Eldritch Knight), then the defender would have cover in all three scenarios (and in scenario 1, still no attack could be made due to lack of LoE).

(minor edits made to clarify and fix typos)
Noctaem
Joined Mar 2010
5128 Posts
thanks RisingZan !
"Non nobis Domine Sed nomini tuo da gloriam" "I wish for death not because I want to die, but because I seek the war eternal"
mvincent
Joined Jun 2004
8513 Posts
From the forum FAQ, if desired:
'What's the difference between cover and line of effect? Per the cover rules, “a line that runs parallel right along a wall isn’t blocked”. However, 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 two medium opponents separated by a 5’ wide square column technically don't have superior cover against each other, even though line of effect is blocked.'

peteincary
Joined Jan 2013
108 Posts
Now suppose you have the three following situations
OX               OX               XO
OX               OX               OX
OX               OX               OX
OX               OX               OX
OX               XO               OX
1                   2                  3
green = attacker/Origin square
red = defender
black = wall/solid object
blue O = open square

...

In scenario 2 above, the attacker does have line of effect, because a line can be drawn from the top left of the attacker's square to the top left of the defender's square without touching any obstructions.  The defender does have cover, because the lines drawn from any of the attackers corners to the four corners of the defender pass through the wall to reach at least two of those corners.
(I will admit here's the one point I'm not 100% clear on is whether the defender has superior cover or just cover. The one front corner (bottom left) is touching a solid object that isn't really passed, so the line, corner and object all meet, then making 3 lines that cross/touch obstructions to reach corners of the defender. However, I believe the correct answer is the defender has Superior Cover because the rules under ranged cover only give a free pass specifically to lines parallel to a wall.)

In scenario 2, if the attacker picks the northeast corner of his square, then he can draw a line to 2 corners of the defenders square (the NW and SW corners) that only are passing parallel along a wall. Thus the target has only cover, correct?

RisingZan
Joined Aug 2003
917 Posts
Its paradoxical, but I think you're right.  The weird thing is, if the archer moved left one more square, then the target would have superior cover even though the archer really should have a clearer shot from that position.....

This is a weird quirk of the cover rules and I wouldn't begrudge any DM who ruled scenario 2 to be superior cover (which I would certainly consider being completely around the corner to be.)

The "parallel to a wall" rule is really meant to prevent cover in scenarios like the one Mand drew, or for example, when shooting back and forth in a 5 foot wide hallway.  But as a side effect, it does add weird cases like this where moving closer to the wall makes your shot easier.
Mand12
Joined Jun 2010
17444 Posts
Its paradoxical, but I think you're right.  The weird thing is, if the archer moved left one more square, then the target would have superior cover even though the archer really should have a clearer shot from that position.....

Er...what?  No.

In number 2, the cover trace is from the left corners of the attacker.  The traces from that corner to the left corners of the defender are unhindered, the traces from that corner to the right corners of the defender are blocked.  Two lines means cover.  The trace to the left front corner of the defender is not blocked because the corner of the wall happens to be there.  Lines that touch a corner are not blocked.

The attacker moving left improves the angles, but does not change the cover result until you're shooting down the diagonal.
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RisingZan
Joined Aug 2003
917 Posts
Its paradoxical, but I think you're right.  The weird thing is, if the archer moved left one more square, then the target would have superior cover even though the archer really should have a clearer shot from that position.....

Er...what?  No.

In number 2, the cover trace is from the left corners of the attacker.  The traces from that corner to the left corners of the defender are unhindered, the traces from that corner to the right corners of the defender are blocked.  Two lines means cover.  The trace to the left front corner of the defender is not blocked because the corner of the wall happens to be there.  Lines that touch a corner are not blocked.

The attacker moving left improves the angles, but does not change the cover result until you're shooting down the diagonal.

Well the one unclear thing about the second scenario, is that the line from the attacker's left corner to the defenders bottom left corner does actually touch the wall without being parallel to it, meaning it would be a third line that crosses a wall.
The case of a line being tangent to a corner is not well defined as "crossing" or "not crossing" for the purposes of cover.