In the below image, A is prone on a rooftop, out of LoF. Enemies B and C are nearby. C is at a higher elevation than B. I have always thought that A can remain prone and move forward to reach the diagonal line in the image with the front of its base, slicing the pie to gain LoF against C but not B. To me this appears to be the same as pie slicing around a corner, but in the vertical dimension. But, an opponent yesterday told me that his TTS opponents have been playing it differently. Apparently they say that A has to move all the way to the edge of the building to gain LoF on C (I guess essentially treating A as having no height while prone). Consequently, A can't get LoF on C without also getting LoF on B. I may be misrepresenting the position since it was told to me second-hand. Anyway, opinions on this? To me it seems obvious, but basically I was warned that opponents outside my local meta might see this pie-slicing as an attempt to game the system, or at least as contrary to the intent of the prone rules. I'd like to avoid bad blood by sorting it out in advance :-)
I don’t see why not. Depending on how you do pie slicing in general. They’re probably mistaking the 3x3 thing as going both ways, which it doesn’t.
This is the error. Prone troopers have a Silhouette (S) value of 0, that is, the equivalent height of a base. http://infinitythewiki.com/en/Prone This is whocouldhaveguessed 3mm.
I sounds like there are some confusions here. Simple geometry dictates that since you're 3mm tall, then no matter how infinitely lower elevated your enemy is to you, it's still below 3mm thus no LOF and therefore you need to be on the edge to gain LOF. HOWEVER! This is only true if you want, say, a sniper on ARO-duty prone on a rooftop, getting LOF in his ARO phase, this NOT the case in active turn because, as @Bobman points out, the 3x3 requirement doesn't go both ways and this is where the rule 'if you see me, I see you' comes in, in order to avoid trivially getting normal rolls and literally any given target in your active turn. TL;DR: Yes, your post in OP is correct.
I think I know what's going on here. It's either one of two things and most likely the second. 1) Your opponent thinks S0 means 0mm height - which incidentally would mean a completely flat surface that can theoretically never "see" downwards without using Lean Out. 2) It is geometrically impossible to draw LOF to a 3x3mm area of a 3mm tall silhouette on a higher level than you. What your opponent missed is probably that this doesn't mean that it's impossible to draw LOF to a miniature on high, but that it means that theoretical slicing is harder against a unit on a roof because reciprocal LOF can only happen from the unit on the roof to the unit below it. Consequently, it also means that it's easier to slice the unit on a roof out of the equation if your opponent has set up two units vertically. Could be that they don't know about mutual awareness, but could also be that they wanted to weaponize a rules quirk. And before anyone says the i-word - no, this is just rules-technical-bullshittery-using-trigonometry-in-a-3D-space, so good luck finding anyone who actually wants to play with this.
I was going to say that the elevated cover rules say you have to be at the edge of the piece of terrain you're claiming cover from, but they don't. Maybe I'm remembering the 2nd edition phrasing? (I'm looking forward to 4th coming out soon, so that I can accidentally quote rules that are 2 editions out of date)
This. It means in practice it used to usually be easier to play "if next to the edge you have LOF if not next to the edge you don't; this allows you to claim cover when moving to the edge of the roof while prone".
