Line of Sight and Line of Effect

From the PHB
Pg. 274 the picture of blocked line of sight that should not be blocked.
Pg. 280 Cover statement and picture: “A line isn’t blocked if it runs along the edge of an obstacle’s or an enemy’s square.”
Pg. 273 Under line of sight “You can see the target if at least on line doesn’t pass through or touch an object or effect-such as a wall, a thick curtain, or a cloud of fog- that blocks your vision.”

My question: For line of sight does passing an obstacles corner, block line of sight.

First there is the discrepancy of the text about cover on page 273 that says a wall will block line of sight even if the line just touches it. On page 280 it specifically states that the line of sight is not blocked if it runs along the edge of an obstacle or enemy.
Assuming the very specific statement on 280 means the line of sight can touch a wall so long as its along the edge then the picture of line of sight on page 274 is wrong, stating that the goblin is Blocked by the line of sight running past the walls corner. (Unless you are supposed to treat the corners differently than an edge, which I doubt since any line going along an edge, technically goes past/touches 2 corners).

My thoughts on this are that the picture on page 274 is actually supposed to be Titled Line of Effect, because for line of effect there must be an unobstructed line from any part of the firing square to any part of the target square (per page 273). That page 280 is the correct way to determine cover, and that passing a corner does not block line of sight. And that the line “You can see the target if at least on line doesn’t pass through or touch an object or effect-such as a wall, a thick curtain, or a cloud of fog- that blocks your vision.” Is an error, and would more aptly describe line of effect. I came to this conclusion because of the two following circumstances.

Down a hall: X’s are the wall, 0’s are unoccupied space, A shooter and B target.

XBX
X0X
X0X
XAX

If we use the rules for cover as stated on page 273, A would have Superior Cover (-5) from B, since the line of sight would always be touching a wall since the corners of B all touch the wall. I assume this is not what is intended and the rule for cover on 280 definitely supports that A should be able to see B without any cover. If this is the case then the picture on 273 is wrongly stating that line of sight is blocked to the goblin, though the picture would be a perfect example of line of effect.

Two alcoves in a wall: X’s are the wall, 0’s are unoccupied space, A shooter and B Target.

XX00
XB00
XX00
XX00
XA00

If we follow the cover rules on page 280 , A has line of sight to target B and B only has cover (-2) since A can draw a line to B’s upper right and bottom right corners along the wall from A’s top right corner. The reason I see this not to be a valid shot is that Line of effect does not have the stipulation that sight does on page 280. So the lines drawn for line of effect in this case would still be along the edge of the wall to B, and so while the target is seen, he is not targetable.

Anyway I am very interested in finding out what others have found or determined about line of sight and line of effect and the discrepancies noted above.

Simple: The rules for LOS/LOE and cover aren't quite the same. The former counts touches, the latter doesn't. This can be seen in the differential language for the two terms, if you read carefully. So in your first diagram, there is no cover (some lines touch obstacles but do not pass through them) and they do have LOE (at least one line connects them without touching an obstacle). In your second, the two targets do technically have cover from one another (for the reasons you stated), but they lack LOE, so an attack is impossible--in other words, I agree with your reading.

It should also be noted that Line of Sight is broken as its asymmetric nature allows for edge cases of one person seeing another but not being seen. This can be fixed by tracing a line connecting any part of your square to a target square.

Cover also breaks in the case of:

A-
XB

Where apparently B can attack A with no cover, yet cannot actually move there. I suggest this is fixed by having sharp corners block reach, else you have strange situations allowing you to say, swap places without hitting a trap (-) even though you wouldn't be able to move around that corner without hitting the trap normally. Or worse, A can hit B when the - is a wall X with a slim gap granting line of sight. I believe there may be another edge case for cover but I forget it now.

Chris_Nightwing said:

It should also be noted that Line of Sight is broken as its asymmetric nature allows for edge cases of one person seeing another but not being seen.

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How do you figure that happens? If I can trace a line from one corner of my square to the corner of another square and it's not interrupted, then the line can be traced exactly in reverse too.

baberg said:

How do you figure that happens? If I can trace a line from one corner of my square to the corner of another square and it's not interrupted, then the line can be traced exactly in reverse too.

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The rule is 'from a corner of your square to any part of the target square' - this does lead to a problem, which took some mathematicians quite some time to clarify with me!

Chris_Nightwing said:

The rule is 'from a corner of your square to any part of the target square' - this does lead to a problem, which took some mathematicians quite some time to clarify with me!

Click to expand...

Actually the rule is specifically (pg. 280 PHB): "To determine if a target has cover, choose a corner of a square you occupy (or a corner of your attack's origin square) and trace imaginary lines from that corner to every corner of any square the target occupies."

In summary, choose the optimal corner of a square you occupy, and draw 4 lines (one to each corner of the enemies optimal sqaure for you to target - most cases the single square they occupy).

Now Line of effect is the one that does not have to use corners, just a line from your space to anyware on their space.

Now the situation you brough up Chris is an interesting one and certainly creates a strange movement.

Minwanderer I totally agree with you as well, but that means the text for Line of sight on page 273 and the picture for line of sight on 274 are both wrong, which I also agree seems to be the case. Once we get a few more agree responses I will post in the possible errata thread.

While that's technically true, how often do you expect those specific corner cases (ie where one person has cover from another but not vice versa) to come up in play? And how often do you actually trace the lines to determine line of sight/effect in the middle of a game. I would say just eyeball it with the definitions of both in mind, and you'll be perfectly fine 99.99% of the time.

Glitch13 said:

Minwanderer I totally agree with you as well, but that means the text for Line of sight on page 273 and the picture for line of sight on 274 are both wrong, which I also agree seems to be the case. Once we get a few more agree responses I will post in the possible errata thread.

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I don't see how.

Every piece of evidence referring to LOS includes corners and edges as "blocking." This is true for both the text and the diagram on pp. 273-274.

Every piece of evidence referring to cover omits corners and edges as "blocking." This is true for the reference to cover on P. 273 as well as the text and illustration on page 280.

Chris_Nightwing said:

It should also be noted that Line of Sight is broken as its asymmetric nature allows for edge cases of one person seeing another but not being seen.

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I can't think of such a case offhand. Do you have an example?

Even assuming you're right, that wouldn't necessarily be broken. In real life it's quite possible to see another person without them being able to see you-- even ignoring facing. Think of putting your eye to a crack, or the very edge of a wall's corner, and peering at a distant person. You can see them clearly but you are nearly entirely blocked from their sight.

If LOS were always perfectly symmetrical, you could hide from enemies by wrapping a towel around your head.

MindWanderer said:

I don't see how.

Every piece of evidence referring to LOS includes corners and edges as "blocking." This is true for both the text and the diagram on pp. 273-274.

Every piece of evidence referring to cover omits corners and edges as "blocking." This is true for the reference to cover on P. 273 as well as the text and illustration on page 280.

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Aha, you found what I was missing. I kept lumping the cover in with Line of sight and they treat them differently. So that fixes the Alcoves in the wall case, but then that also means that the hallway situation means that while the two creatures in the hall have no cover, they do not have line of sight since every corner "touches" a wall. So you can't even see them so they are invisible (total concealment -5)? Seems a bit absurd.

Thanks for the responses.