Cover and Blocking Terrain Discussion

[tiornys]

From the Q&A thread:

Q55: I found this image in a blog, but I think it is worng, isn't it? The bottom-middle square, and the ones in left-middle, should have just "cover", since the bluish line hits 2 of the square's vertices. Also, the green square hit by the pink line should have "Superior Cover" (only one vertex is hit). Or am I wrong here? Thanks.

A55:
I am assuming here that the black lines on the diagram represent blocking terrain. In that case, the diagram is correct. For both squares in question, there is simply no line of effect to the square. Touching a corner or side of the square is not sufficient to give line of effect: "Line of Effect: A clear line from one point to another point in an encounter that doesn't pass through or touch blocking terrain. Unless noted otherwise, there must be line of effect between the origin square of an effect and its intended target for that target to be affected."

The confusion might be caused by this statement in the rules for Cover: "A line isn't blocked if it runs along the edge of an obstacle's or an enemy's square." It's best to bring this line into consideration only if you otherwise have line of effect to the square in question. For example, this clarification applies to the square to the left of the bottom square you were asking about. The blocking terrain near the bottom does not block the two corners in question because you can easily draw line of effect into that square from the origin square.

Then is it even possible to have three lines blocked and yet have line of effect?
And then, shouldn't the leftest yellow square near the pink line also be safe?

Yes, three lines can be blocked and you can have line of effect. In the diagram, imagine that the left blocking terrain extends another 3/4ths of a square downward. Now you clearly have line of effect into the first yellow square on that side, but you can only contact the one corner.

For the leftmost yellow square near the pink line, you use the bottom right corner of the origin square to determine cover. From that corner, you can see both of the bottom corners of the leftmost yellow square, thus that square only has cover.

[Marcloure]

OK, now I think I understand the rules. To have line of effect, there must be at least one line from the origin square that hits the target square without blockage (lets ignore arrow slits for now), and that line cannot just run along the edge of an obstacle. However, to determine cover, one need to count those lines, and for this purpose they can run along the edge. Did I got it right?

Just to clarify another point. To determine cover and line of effect, do the lines need to start at the same corner of the origin square for every target, or can each of them start at a different vertex? In this image from PHB1, they all start at the same vertex, but I am not sure if this is how it works better. Is there any later clarification?



And thank you for your help.

[tiornys]

Just to clarify another point. To determine cover and line of effect, do the lines need to start at the same corner of the origin square for every target, or can each of them start at a different vertex? In this image from PHB1, they all start at the same vertex, but I am not sure if this is how it works better. Is there any later clarification?



And thank you for your help.

I'm not sure if there's any clarification anywhere, but you do need to pick a specific corner of the origin square for each target square, and then draw lines from that corner to each corner of the target square. However, you can pick different corners for different target squares. Typically, one corner is the "best" choice for a given target square, and that's the one you should use to determine cover.

For example, in your new diagram, if you pretend it's a burst 3 and that there's a target in the square to the right of Attacker, then you would use the bottom left corner of the origin square to determine cover for that target.

[Marcloure]

Yeah, as I thought. Thanks again for all your help.