Obscuring mist and concealment.

[Thomas Keller]

Can someone explain how you can see out of the Obscuring Mist spell without being seen? I'm not understanding the whole 'draw two lines from your square to the enemy square' business.

[Kayerloth]

CRB on Concealment wrote:

To determine whether your target has concealment from your ranged attack, choose a corner of your square. If any line from this corner to any corner of the target's square passes through a square or border that provides concealment, the target has concealment.

You pick any of the 8 corners of your space (5ft cubic square for a size M creature). Draw a line between that corner and all the corners of the targets space. That's going to be a total of 8 lines between your square and their square. Then answer the question of "do any of those lines pass through a square or border that provides concealment.

Now reverse the situation. You'll find that one guy (the one in the OM) has a better level of concealment vs the other when attacking. One has 20% (partial) concealment and the other none OR one has 50% concealment (total) and the other 20% (partial). The later will occur when you pick a square just inside the outer ring of squares of an Obscuring Mist spell for one of them and the other square is out in the open (no concealment or cover)

And boy do I hope I kept all that straight while typing this out. :D

[Quixote]

That sounds about right.

Consider this: when calculating distance, you don't count the square you start in, you count the first one after you, up to the one you end on.
So when you're on the edge of the mist, you skip it entirely, but your opponent has to count your square, which counts as 5ft of mist, which grants you concealment.
And when you're 5ft deep into the mist, you don't count the square you're on, which means there is 5ft of mist between you and your targets, so they have concealment against you. But your opponent counts both the square between you and the edge and the square you're in, which is more than 5ft, so you have total concealment.

[That Crazy Alchemist]

Try to imagine it like real life. If you and I are facing each other.
Can I throw you a ball without needing to move my feet? Of course! I could move my head and upper body around all I wanted and I'd be able to throw a ball at you no problem. That's what not having any cover is.
Now say there's a 1 foot pillar in the way, is it still possible for me to throw you a ball without first moving my feet? Of course, I can just peek around the pillar and throw it to you. That's regular cover. I can still see you and throw something at you without needing to move somewhere else but only if I move my upper body in just the right way.
No imagine there's a 10 feet wall between us. Can I still throw the ball to you without moving my feet? No, no matter how I bend and twist my upper body there's simply no possible way to throw the ball to you. I'd have to move my feet and get to a place where I'd be able to see you. That's Total Cover.

The lines you draw between the corners of you and your target are checking your sightlines. Seeing if there is a way to see your target while in the space you are currently in.

[Thomas Keller]

If you're in a square completely full of mist, how is your sight not impeded?

Oh, and where does the radius begin? On my square or an intersection of squares immediately around me?

[Magda Luckbender]

Cast Obscuring Mist at one of the 8 intersections of the cube you occupy. By default we simplify this to the 4 corners at ground level. There can be advantages to casting OM 5' or 10' up in the air, rather than at ground level. It spreads out in a 20' from there. Squares at the edge of OM are only partly full of mist.

@Thomas Keller: You've been asking good questions. Thanks for pushing us all to come up with clear explanation of a complicated situation.

[Wonderstell]

I'm not completely sold on the 20-50 situation. I understand how you'd avoid suffering a miss chance by standing at the edge of concealment, since you always pick the most favorable corner when drawing your lines for ranged attacks.
But if you have a full square of smoke between you and your target, both of you should have total concealment, right?

Obscuring Mist wrote:

A misty vapor arises around you. It is stationary. The vapor obscures all sight, including darkvision, beyond 5 feet. A creature 5 feet away has concealment (attacks have a 20% miss chance). Creatures farther away have total concealment (50% miss chance, and the attacker cannot use sight to locate the target).

'

A=Ally
E=Enemy
S=Smoke
O=Open space

O.O.O.O.O
.A.O.S.O.E
O.O.O.O.O

Ally A tries to fire an arrow through a single square of Obscuring Mist. What miss chance to they suffer? Can they see through that one square of smoke?

[Quixote]

Thomas Keller, your vision is only impaired if you have to count one or more squares within the mist as part of the distance between you and your target. You don't count your own square.

Wonderstell, there's 5ft of obscuring element between you and your target.

"Creatures farther (than 5ft) away have total concealment." --that's how it works *in the fog*, so both parties in your example have concealment against each other.

[Diego Rossi]

Another RL example that maybe is easier to get.

At the border of your property, there is a hedge made of several rows of Leyland cypress like in this image.
If you are behind 3 rows you see nothing on the other side e no one on the other side sees you. If you are behind the first row you can see perfectly what is on the other side, but you are partially hidden by the plants.
If some plant blocks your view you need only to move your head to see what is on the other side. When attacked from a distance you only need to duck back behind the plant to be totally hidden. That is well within what is "staying still" in the game.
In the magical fog created by the Obscuring mist, you are in the same situation. You only need to expose your head to see clearly, while most of your body is still hidden by the fog. When attacked you only need to duck it back to get full cover and 50% miss chance.

Personally, I would prefer it the text of the spell was:
"If there is a square of fog between two creatures, they have total concealment, if one creature is adjacent to one in the fog, the one in the fog has cover."
Clunky, but more realistic. But it is magic, so realism is always questionable.

[Thomas Keller]

Magda Luckbender wrote:

Cast Obscuring Mist at one of the 8 intersections of the cube you occupy. By default we simplify this to the 4 corners at ground level. There can be advantages to casting OM 5' or 10' up in the air, rather than at ground level. It spreads out in a 20' from there. Squares at the edge of OM are only partly full of mist.

@Thomas Keller: You've been asking good questions. Thanks for pushing us all to come up with clear explanation of a complicated situation.

Thank you, Magda. Very nice thing to say. May I return the compliment and say that your explanations in the Advice subforum, once I was able to wrap my stubborn brain into shape, are very concise and easy to understand (for a young person, old fogies like me need a while to absorb the knowledge!)