Mistwell said:
Except, he just gave Goblin 3 cover by moving behind that altar. Cover is a -2 attack bonus. Which means his combat advantage bonus is negated by the cover that help gain the combat advantage to begin with.
The goblin is in the open, he isn't behind the cover: his body is exposed. This form of cover provides it to someone crouching behind it, but not someone 5 feet away. I believe there's room here for DM interpretation as to which characters have cover or not.
If there is a full, solid wall between two characters, neither have line of sight, both have absolute cover. If there is an archer's slit in the wall with one character immediately behind it, and the other character is on the other side, in an empty space 3 squares away, I think it's obvious which of the two has cover, and which does not?
Consider it this way: the line of attack from the ranged attacker's eye and weapon can reach any portion of the target's body. The target is not covered in any way. But behind the altar or arrow slit, a large portion or even all of the attacker's body is covered, meaning it's harder to hit them, and they have a way to hide making it uncertain when or whom they're attacking.
This happened quite frequently as we tested it in last night's game. It was easy to gain new cover or concealment to make a stealth check, and it was easy to beat the foes perception check, but pretty much every time the taking of that cover or concealment was also granting the foe the same cover or concealment, negating the bonus (though making the Ranger harder to hit as well).
I believe you were doing this all correctly, except treating cover as working equally in both directions. The sniper perched in a window has cover, the person walking in the middle of the street does not.
Am I interpreting this correctly? Does most cover and concealment tend to grant cover/concealment to the target as well, thus negating the normal +2 attack bonus from combat advantage?
If cover always works equally in both directions, it's meaningless, and there's no reason to make use of it. There can be situations in which both sides have cover: two sides of a room with barriers to hide behind, or ships which have pulled up alongside. In such a case, both sides would be getting attack penalties. But where one side is making use of terrain cover, and the other side has none, the advantage goes one way, in my opinion.
I believe the easiest way to resolve cover or concealment is to consider who cannot be easily hit or seen. Someone in 1 square of bushes has partial concealment to anyone outside; but the person inside can see others just fine. 2 squares of bushes between two people? They both have concealment. One person in darkness and one in light? The one in darkness is concealed. One person pressed up against a wall and firing around a corner, versus another in the middle of the room? The one at the corner has cover.