XCOM Cover system and Chance to hit calculation for dummies:

[GeistHeller]

[s]How do I know if a cover blocks LoS ?[/s]

A simple to remember rule: Light covers never break LoS, Heavy Covers only break LoS if there is also a piece of Heavy cover on both sides of the soldier. What "your eyes" see does not matter, what matters is the size of the cover on each tile. That's why you can "shoot through walls".

Exemple:

_ Soldier A stands behind a tree, there is "empty air" on each of his flanks = Any enemy in front of the tree has LoS on Soldier A

_ Soldier B stands behind a three tiles wide Heavy Cover, he is leaning against the "middle" section of said Heavy Cover = Enemies on the other side of this cover cannot see him, unless they have a sufficiently high ground advantage or move to the point of flanking him.

_ Soldier C stands beside Soldier B, on the left section of the Heavy Cover = Enemies located on the other side of the Cover can see and target Soldier C if the axis of their position coincides with the cover's middle section or is located past it on the left side.

D E F

_ _ _
CB

Mutons D and E can shoot Soldier C, Muton F cannot, Muton F will likely try to flank on the right side. Soldier B is chillin'

[s]How are my Chances to hit calculated, is it different for the enemies ?[/s]

Base accuracy
+ Scope bonus: 10%
+ Light Plasma Rifle: 10%
+ Higher Ground: 20%
+ Holo Targeting: 10%
+ Damn Good Ground: 10%

- Target's Defense, 10 Defense means 10% less chance to hit.
- Light Cover 20%
- High Cover 40%
- Smoke 20%
- Dense Smoke 40%
- Lower Ground 20%
- Being suppressed 30%
- Target is in flight 20%
- Overwatch aiming penalty 20%
- Psi Barrier 40%

Exemple:

Rookie in Coverall with Scoped Rifle in Heavy Cover versus Muton in light Cover with Light Plasma:

Rookie has 65 base accuracy,
+10 from scope,
-20 from the Muton's Defense
-20 from Light Cover

= 35% chance to hit, not good at all.

The rookie's rifle has 10% chance to crit, regardless of cover, flanking grants +50%

Basic Mutons have 70, 80 or 90 base accuracy depending of difficulty,
+10 from Light Plasma Rifle
-40 from Heavy Cover

= 40, 50 or 60% chance to hit, quite likely to hit.

Plasma weapons have 20% chance to crit, regardless of cover, it means our dear muton may statistically crit once every five shots.

Morale of the story: Don't try to fight X-rays in a straigth firefight early on, or you will get brutalized so hard that your brain will start making up excuses like "the AI is cheating", "Accuracy is rigged against me".

[Red Phantom]

Great post. It ain't exactly rocket science.

[Sentient_Toaster]

There's also a +20% defense from flying, IIRC, and there's a Psi ability which gives +40% defense to everybody in an area.

Damn Good Ground is +10% attack/defense.

Overwatch penalty might be -20%.

[GeistHeller]

Thank you Toaster, adding them you are right, I just wrote this in one go.

[Cloaked Figure]

Fantasic post, I wish there more threads like this on the forums. Tips and strats, instead of the nonstop whining.

[Red Phantom]

Also, line of sight is blocked by any full cover object in between the shooter and the target that neither of them are touching.

[MASTAXPLODER]

http://youtu.be/nr2GeWiDrdY

[Cloaked Figure]

Originally posted by MASTAXPLODER:

http://youtu.be/nr2GeWiDrdY

Haha, Might have to watch this movie now.

[Grungi]

Good post; might be worth adding that your "hit roll" is calculated from a random seed stored in the save file rather than an on-the-fly RNG system, meaning that saving every turn and reloading on a Miss won't work.

[[E] Caladus]

+1 Internets to OP!

[Red Phantom]

You could turn this into the "XCOM: EU Rulebook 1st Edition"

If you had the play area and the pieces you could play these rules as a table top wargame.

[GeistHeller]

Pretty much. XCom:EU gameplay is very TTesque.

[Niteshade]

Originally posted by Concealed:

Fantasic post, I wish there more threads like this on the forums. Tips and strats, instead of the nonstop whining.

^
I'm new to the XCOM name, just DLed EU lastnight, been getting stomped within a few missions on classic Ironman and restarting for assorted reasons, and loving it still.
I agree there should be more strategy/tip threads then the numerous complaints. Yea dont get me wrong I see and understand where complaints are coming from what what I've been reading, but its almost like run on threads of the same overall subjects, which then proceed to flood out the good/helpful threads with solid advice. Props to u OP. Helpful post for anyone to reference

[Zenuf]

I found this post informative. The LoS frustraits me when it seems like people take shots at me through a battlefield of cover. Should frustrait me less with more understanding of how the mechanics work.

There is still a few things that that confuse me though, perhaps someone can help me understand.

http://img.photobucket.com/albums/v609/Fox269/2012-10-16_00002.jpg

In a situation like this, i was moving 3 of my squad around the back of a firefight to flank an enemy position, had my other man hunkering down under good cover drawing fire.

I was confused about two things in this encounter, how the aliens knew i was moving to flank, the alien in the pic disengaged the attack on my hunkered man and moved to counter my flank. Confused as to how he knew i was comming that way in the first place when they had never laid eyes on those 3 squad members.

I could chalk that up to the aliens just being good at tactics and covering an exposed weakness, but he actually moved from out of my LoS and took a shot at my men instead of just overwatching their flank. The other thing that confuses me is the shot itself, my eyes tell me he shouldn't really be close to LoS.

[Sentient_Toaster]

AI might get sound contacts. I've noticed that explosions often seem to cause sound contacts in fairly far locations suggesting that something heard the noise.

[Red Phantom]

Originally posted by Sentient_Toaster:

AI might get sound contacts. I've noticed that explosions often seem to cause sound contacts in fairly far locations suggesting that something heard the noise.

I'm pretty sure this is the case. Crashing through doors and windows also sems to attract them.