Cover (EU2012)

In combat, units can take cover behind objects in the environment to lower their chance of being hit by enemy fire. This mechanic functions automatically as units moves throughout the environment. Whilst in cover the unit gains a defence bonus against attackers unless the attacker is flanking the unit. Cover is divided into two categories: Low Cover and High Cover.

Hovering the cursor over tiles within the map will reveal whether they offer any state of cover from any given direction. If the cover icons appear as red instead of the usual blue, then the unit will be moving into Flanked Cover, and won't gain any bonus against some or all enemies firing on it.

Types Of Cover

Uncovered

Any grounded unit without adjacent cover will show no cover shield on the tactical HUD at all. This grants a +50% critical chance bonus for the enemy firing at the target (100% with Second Wave's "Absolutely Critical" option).

Low / Half Cover (and Flying)

Represented by a half shield symbol on the tactical HUD. This conveys a -20% hit chance penalty for the enemy firing at the target.

High / Full Cover

Represented by a full shield symbol on the tactical HUD. This conveys a -40% hit chance penalty for the enemy firing at the target.

Flanked Cover

When there is at least one attacker flanking a target, the target's cover icon will change from its usual colour (light blue for friendlies, red for enemies) to yellow. When firing from a flanking position, there will be no cover defence bonus applied.

Note: Red cover indicator logic might inaccurately include the cover's peeking tiles in its calculation even though a unit can not peek behind the cover line, nor would it make sense to count it as flanked cover if it did.

Dual Cover

If a unit is adjacent to two pieces of ground cover to where it is ambiguous for the attacker which one would count (for instance, low cover in the north and high cover on the west, with the attacker at the north west quadrant), only the cover closest to the attacker applies for their shot, or whichever side of the 45 degree line that the attacker lies on. This can lead to seemingly odd situations where destroying low cover next to your target allows it to take on a high cover bonus instead.

Visibility

When in cover all peeking tiles count towards the unit's visibility, as if they are simultaneously on all two or three of their initial/peeking tiles, ONLY IF the cover counts towards the target/source of fire. The following images are from Enemy Within and are a complicated demonstration of how cover works, with a possible XCOM bug/exception to boot because it wouldn't be XCOM without an edge case.

Left soldier blocked from peeking and out of range
Right soldier has LoS albiet incorrectly considered a flank

In the right image, magenta highlights the peeking tiles of both the Sectoid and right soldier and black highlights where those units actually lie. With vision being a 17 tile radius and the Sectoid being 18 tiles away, there would be no line of sight (LoS) on the Sectoid directly. However, with the soldier being on the right side of the Sectoid's low cover line, he has LoS of the Sectoid's peeking tile since it's within 17 tiles and therefore has a shot.

The left image shows that the left soldier is on the left side of the cover line, therefore the Sectoid's peeking tile is not activated, making the Sectoid effectively 18 tiles away. Since this is EW, he can not utilize the right tile to peek and activate the Sectoid's cover, since the right soldier is occupying it.

The bug here has to do with the Sectoid shot of the right soldier counting as a flank. The soldier gains LoS from where he is standing, but neither this nor the right tile is a flanking shot. It is possible that the LoS rules and the flanking rules are calculated separately. If so, the game is including the left tile in the flanking determination erroneously. OR, ignoring that a flank would not include the peeking tile, this is actually a demonstration of the shallow angle flank, with the soldier being at max vision with only one tile offset from flanking. In either case, it is likely still a bug in the sense that the peeking tile should not count for flanks.

If this had been Enemy Unknown, it is likely the left soldier would gain LoS because in EU there is no limitation on other units occupying a peeking tile. Therefore, the left soldier peeks on the right where the other soldier already is, activating the Sectoid cover and getting a shot.

A unit will peek for visibility onto an adjacent tile only if it contains low or no cover, but not high cover.

Flanking

Cover only grants a defence bonus if a unit is standing directly next to it and the edge line/plane divides the unit from another (with exceptions, see second infographic). Stepping away will immediately loses this cover bonus even though technically cover is still between the two of them.

It's not possible to bypass cover by firing over it. For example, if an enemy is taking cover at the bottom of a steep cliff edge, putting a unit on the top of that cliff won't grant a flank (even though there's clearly nothing between your gun and the target) - the alien will still be treated as being in high cover. Fortunately the range bonus will typically make up for it. It is possible, however, to flank from a shallow angle (possibly demonstrated in the example above) at side cover, though it is rare considering it usually requires only one tile offset left or right at near max vision range or more, though the exact explanation is not known.

An additional quirk of the system relates to Alloy S.H.I.V.s, which provide low cover to adjacent units. When it involves enemies, the low cover the S.H.I.V. creates inevitably creates an edge line/plane dividing the two. Intuition would suggest this situation should be treated as a point blank uncovered or flanking shot considering the adversarial relationship, but sadly that is not the case!

Be aware that the game can sometimes fail, granting a cover bonus when the target is clearly flanked (e.g. you've just had your Assault unit move directly adjacent to your target). Saving the game and reloading it prior to taking the shot may rectify this in some cases.

Destroying Cover

Missed Shots

Any missed shot has some chance to damage/destroy cover. Ballistic pistols/assault/sniper rifles are basically useless (and according to game values do 10/20/30 environmental damage respectively), while Shotguns(40) and SMGs(60) can occasionally destroy rocks/logs and small flimsy suburban objects like building walls, benches, etc.

All Laser and Plasma weapons (both 125) can begin to destroy most objects and ignite vehicles but not detonate them. Watch out for friendly fire vehicle ignition! Grenades (250) are similar except they can also detonate vehicles.

The S.H.I.V Minigun (80), Laser and Plasma upgrades (both 125) also deal damage.

The chance of a missed shot to damage an object seems to be based on whether the shot lands on the actual 3d model itself. This is why certain small objects like the fire hydrant are notorious for being "indestructible", they just have a lower chance of being hit.

Large objects

Large objects such as tall trees, statues, mausoleums require two hits to be completely destroyed. Unlike tall trees, the others often degrade from high cover to lower cover after the first hit, destroyed on the second. At precise points, tall trees can be destroyed with one hit using a Grenade.

Rockets

Most objects are immediately destroyed by Rockets (500), and all vehicles will detonate. Some large objects may still require a second hit to destroy.

Fire

Advanced weapons destroying cover and explosives have the chance to light the environment on fire. Fire will spread and any objects caught on fire will slowly disintegrate, even detonating vehicles/gas pumps without warning.

Indestructible

The green alien objects are indestructible. Vehicles require two hits to detonate and can also degrade from high cover to lower cover, but don't get destroyed after detonating. Platforms, stairs, ramps and certain pillars can not be destroyed. As well as large liquid tanks can not.

Units That Ignore Ground Cover

Some units lack the ability to use cover at all.

Zombies

With the exceptions of the Civilian and Zombie, all of the above have the Hardened property, which grants them a passive 60% defence against critical hits (100% with Second Wave's "Absolutely Critical" option). Cyberdiscs only have this bonus when in disc form.

Oddly enough, with the same exceptions, they - and even some others - also have innate defence scores, lowering their chance to be hit regardless of their inability to use of cover. In some cases, such as for the agile Chryssalid, this may be expected; for others, such as the bulky Sectopod, not so much.