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Sage Advice February 2016

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Campaign Extra Date 1996-09-13
Last Updated 2020-10-03 08:19:40 - Update

Line of Sight vs. Line of Effect

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Hey guys,

So there were some interesting situations that popped up last nightregarding these issues and I made a boo-boo on a couple. I felt I mightnot be right (hence the waving of the retreat, though I am glad we played itout) but I feel maybe we should get something more concrete down.

The PHB says to determine Line of Sight (LoS) one needs to "pick acorner of your space and draw an imaginary line from that corner to any part ofthe target's space (emphasis mine). You can see the target if atleast one line doesn't pass through or touch (again, emphasis mine)an object or effect that blocks your vision. If you can see your target,but at least one line does pass through an obstruction, the targethas concealment (-2 to the attack roll)."

Having no LoS doesn't stop the attack from happening (unless you also lackLine of Effect), but the target benefits from Total Concealment (-5 to theattack roll).

Ok, so Dave correctly quoted the 'corner-to-corner' rule, but I have sincefound out that was only used in the Start-Up rules included in Keep on theShadowfell. The above rule takes precedence. The difference is thebolded word 'space'...that does not include the person's corner; their spacehas to be within their square (or squares, for Largercreatures). Often won't make a huge difference, but it will sometimes,like below.

In the following diagram (A and B are characters, W represents a wall):


A cannot draw LoS to B. A's bottom right corner does go to B's topright corner but runs along the wall, touching it. This stopsLoS from being established (and it makes sense to us from a 'real-life'perspective, no?)

Note that A may still know B is there...B is only hidden from A ifhe makes a stealth check that beats A's passive perception check. 

To become hidden, one must: 1) Break LoS, and 2) Make a stealthcheck at the end of a movement.

To maintain hidden, one must 1)Have some sort of concealment, and2) Make a stealth check at the end of a movement.

That's for LoS.

Line of Effect (LoE) works almost exactly the same: "If every imaginaryline you trace to the target passes through or touches a solid obstacle, youdon't have line of effect."

Mordekai should have been able (IIRC) to place his power where heoriginally wanted to since "You don't need to be able to see the originsquare" for an area attack. Mea Culpa, but it all worked out.

Again, IIRC, Garrick shouldn't have been able to target the dragonwith his Dimensional Vortex power since he drew a line to thedragon's corner, but not within its space (their bottom leftcorners respectively, if I am correct). I no longer have the map where itwas, but I remember thinking "got him on the corner!" which is whatDave quoted.

Second error I made (but that I sorta stand by):

When Kashatri hit the group with his breath weapon attack (a close blastfive), Garrick and Izak were at ground zero for it, while Odus and Erevas werearound corners. Kashatri needed LoE to Odus and Erevas for it to hit them,regardless of the nature of the attack. I applied a bit of real-worldphysics regarding the nature of air molecules (they fill up the availablespace) although I kept it within the original area of the blast. Blasts ofa more physical nature (like flame) I wouldn't do that, which is whythe dragon's breath didn't react that way yesterday, sparing at least onehero.

Honestly, the rules say I ruled the dragon correctly (obeying LoE strictly)but not Kashatri...I have to be honest and say the rules as written don't makea lot of sense for things that are gaseous or pouring water. For thingsthat are actual gases, I sort of think they'd go around corners...withoutanswering from a strictly player perspective, what do you guys think?

Sorry for the errors, but they went both ways...

Looking forward to replies.


Thanks for doing all of this, Eric!

After reading this, I definitely feel like I am more comfortable with therules. There's always been a battle between real-world physics and the magic ofD&D, so uncertainty is nothing new and doesn't really even spoil things forme anymore; I've come to expect a 'lil chaos!

The one big uncertainty for me last night was regarding the wording of thepower I used where it said, "the zone is heavily obscured and blocks lineof sight". I now realize that even thought "blocks line ofsight" makes a person think "I'm invisible!", we all use muchmore than just sight to gauge our surroundings. Touch, and certainly sound arehuge factors. When I moved to a square, Maldrick, Mekkalath and the kobolds sawwhere I stopped moving. Then I cast the darkness, and line of sight wasblocked, but they all knew I was still there. Even if I had rolled a Stealthcheck at the end of that movement, BEFORE I ended the move there was NOobscuring or blocking line of sight, so I COULDN'T have become"Hidden". When the void blast went up, the term "HeavilyObscured" imposed that nice -5 penalty to ranged attacks, and what shouldhave been only a -2 penalty to (adjacent) melee attacks (an error in my favour,yay!), but I now understand that everyone knew WHERE I was, because I wasstanding there waving to them when I stopped my movement! =D

If I then spent an action point to buy another move action AFTER theobscuring effect was up, and rolled a sufficient stealth check to beat opposingPerceptions, then I'd be officially "Hidden" and they would have noidea where I was. "Hidden" is, as Eric described in his e-mail,devilishly difficult to achieve and arguably harder to keep!

As for line of sight, I think I got everything except one point.

When the text says "draw a line from your corner to the target'sspace", what does that mean? Does it mean it can't be "just acorner". In Diagram 1, this is what I thought Dave did to hit the dragonwith that power that saved Erevas. So by what you wrote Eric this shouldn'thave worked? Does that mean that the text saying "from your corner to partof the target's space" can be ANY point of ANY square? What I mean by thatis indicated in diagram 2, below:

If we can target any part of the target's occupying space (made much easierif the creature is large) then that means we have LoS, correct? So in the caselast night where there was a wall blocking Garrick from seeing part of thedragon's space, that would prevent us from targeting the dragon as "TheTriggering Enemy" because you don't have line of sight to the dragon,correct?

Looking at adding now a hard corner (as I think we had last night), LoS(red) is blocked by "crossing" blocking terrain, so it's anon-starter. However, we don't even have Line of Effect because LoE (blue)"touches" the hard corner as indicated in Diagram 3, therebyviolating that rule.

Did I get it right?


I think i get the line of sight thing but...how much of the target's 'space'needs to be seen or targetable for it to be considered LoS? From what Iremember from the line of sight blockers I had up during the fight, Garrickcould see about 20-25% of the lower left square occupied by thedragon...similar to Diagram 2 in Joe's email below (the bottom most arrow). Iknow I quoted 'corner to corner' because I honestly thought that's how thingsworked...wasn't trying to cheat or anything. But if I targeted 'corner tospace', I think i could have still had an arrow point to a very small portionof the dragon's space but again i'm going by memory and as you said, withouthaving the map it's impossible to determine. At worst though, wouldn't thedragon have just had concealment (-2 penalty) from Garrick?

Also...another rule which I wanted to clarify is how we're going to beplaying with range and z-plane (i.e. height). This came up when Maldrickblasted us while flying. His power had a range of 10, I believe. So, for Range10...is it only counted on the x-y plane or does the z plane factor in?Traditionally, we counted the z-plane for example, if Maldrick we're flying 20ft up, we would subtract 4 squares and he would only affect a Range 6 on thex-y plane. Last night you seemed to indicate that was wrong, which is fine. Ijust want to clarify if that is indeed the rule we will be playing with goingforward. I guess this would only affect 'Range' powers as 'Bursts' and 'Blasts'extend in all directions, right?

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