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How does a radius effect deal with obstructions? Does the radius expand and simply affect every non blocked square? Does it go through walls? Does it go around corners?

Like the Cleric's Channel Energy for example. I just don't see how this works if there is anything other than a big open area.

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3 Answers 3

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A lot of this is from Aiming a Spell, but, in short, you should determine if the effect's a burst, spread or emanation; then determine the effect's shape (e.g. cone, cylinder, line, sphere); then mark up the battlefield. That's because

Most Areas of Effect Stop at Obstructions...

The key word is most. An area of effect--like a burst or an emanation--stops when it hits things:

A burst spell affects whatever it catches in its area, including creatures that you can't see. It can't affect creatures with total cover from its point of origin (in other words, its effects don't extend around corners). The default shape for a burst effect is a sphere, but some burst spells are specifically described as cone-shaped. a burst's area defines how far from the point of origin the spell's effect extends.

An emanation spell functions like a burst spell, except that the effect continues to radiate from the point of origin for the duration of the spell. Most emanations are cones or spheres.

...But There Are Exceptions

Spreads don't stop at obstructions. Instead a spread

spread[s] out from a point of origin, which must be a grid intersection. The effect can extend around corners and into areas that you can't see. Figure distance by actual distance traveled, taking into account turns the spell effect takes. When determining distance for spread effects, count around walls, not through them. As with movement, do not trace diagonals across corners. You must designate the point of origin for such an effect, but you need not have line of effect (see below) to all portions of the effect.

Also, cylinders don't stop at obstructions. Instead a cylinder has a

point of origin. This point is the center of a horizontal circle, and the spell shoots down from the circle, filling a cylinder. A cylinder-shaped spell ignores any obstructions within its area.

Emphasis, in all cases, mine.

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Most* area effects like that can be one of three different types: spreads, bursts, or emanations.

Spreads go around and behind obstacles (hence "spread"). Bursts and emanations don't.

A cleric's Channel Energy is a burst, so it is blocked by solid obstacles, affecting only squares you can draw line-of-effect to from the origin. (This is in 3D though, not 2D, so it can go over short obstructions.) It's not a matter of vision though: it affects squares that you cannot see, just so long as there isn't a solid barrier—i.e., fog and other concealing effects don't block it.

* There are exceptions, like cylinder-shaped area effects, which effect everything regardless of these types. See the full area effect spell rules.

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1  
Fog is the quintessential example of blocking line-of-sight but not line-of-effect, and may be a little better than bushes as an example since bushes are going to block some of the burst (which the game's mechanics don't really cover but the issue of whether or not to accept that abstraction potentially distracts from the answer). –  KRyan May 6 at 20:53
    
@KRyan Very good point, thanks. –  SevenSidedDie May 6 at 21:00
    
I think you should include Cylinders into the list of area effects, as they are neither of the three you mention, but ignore obstacles completely. –  MrLemon May 7 at 9:03

You need to have line of effect from the point of origin to target creatures within radius (whether it's a spell or channelled energy). So it goes around corners as long as you have line of sight to the creature behind the corner, but it does not go through walls.

Here are the rules:

Line of Effect: A line of effect is a straight, unblocked path that indicates what a spell can affect. A line of effect is canceled by a solid barrier. It's like line of sight for ranged weapons, except that it's not blocked by fog, darkness, and other factors that limit normal sight.

You must have a clear line of effect to any target that you cast a spell on or to any space in which you wish to create an effect. You must have a clear line of effect to the point of origin of any spell you cast.

A burst, cone, cylinder, or emanation spell affects only an area, creature, or object to which it has line of effect from its origin (a spherical burst's center point, a cone-shaped burst's starting point, a cylinder's circle, or an emanation's point of origin).

An otherwise solid barrier with a hole of at least 1 square foot through it does not block a spell's line of effect. Such an opening means that the 5-foot length of wall containing the hole is no longer considered a barrier for purposes of a spell's line of effect.

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