Cover (EU2012)

Click here to enlarge.

There is a notable exception to the cover/flanking rules of the game, which could possibly render the last example of the prior infographic incorrect regarding the red enemy. If the soldier is in the same column/row as the enemy with only side cover as shown, and line of sight rules would imply it is not flanking due to blocking cover, the game says otherwise.

In combat, soldiers and aliens can take cover behind objects in the environment to lower their chance of being hit by enemy fire. This mechanic functions automatically as the soldier moves throughout the environment. Whilst in cover the soldier gains a defensive bonus against enemy attacks unless the enemy has flanked the soldier. Cover is divided into two categories: Low Cover and High Cover.

Highlighting tiles within the map when plotting a move will reveal whether they offer either (or 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

• No Cover
  

A completely uncovered unit on the ground shows 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)
  

Low Cover is 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
  

High Cover is represented by a full shield symbol on the tactical HUD. This conveys a -40% hit chance penalty for the enemy firing at the target. Note that the base aim scores of ALL alien units exceed this by some margin - high cover alone is NEVER enough to ensure you won't get hit. If you can fire at your target, it can fire at you.

At Classic and Impossible difficulties, it's better to try to stay out of line-of-sight of aliens whenever possible (assuming you are not going on Overwatch yourself). When overwatching be aware of what accuracy percentage you are likely to get - it may be better to use the hunker down ability and double your defence bonus rather than risk a 20% shot, especially if can already see the aliens (as they're unlikely to move and trigger overwatch fire if they can just shoot at you instead!). Especially early on when running with rookies sporting low accuracy and carrying weak weapons, it may not be worth the risk.

• Flanked Cover
  

When flanked, the unit's cover icon will change from its usual colour (light blue for friendlies, red for enemies) to yellow. Enemies only appear flanked when your flanking unit is selected. When firing from a flanking position, the shot is treated as if the enemy has no cover.

• Dual Cover
  

If a unit is occupying 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 it comes to LoS, the peeking tiles of cover count towards the unit's visibility, as if they are simultaneously on all two or three of their initial/peeking tiles, but 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 tile of the Sectoid and right soldier and the black highlights where the units actually lie. With vision being a 17 tile radius and the Sectoid being 18 tiles away, there would be no 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 this is not a bug and is actually a demonstration of the shallow angle flank, with the soldier being at max vision with only one tile offset from flanking.

Finally, 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.

Flanking

When a unit is in cover and when it is flanked can be a bit confusing or unintuitive at first, and unfamiliarity can lead to situations where targets remain covered when you thought they'd be caught flat-footed (even putting aside the bugs that occasionally allow this anyway).

When a unit is adjacent to an object that provides cover, a plane is drawn between the unit and the object that extends from the sides to infinity in each direction, both horizontally and vertically. If a second unit fires at the first, and the line of fire connecting the two units intersects the plane, then the shot's accuracy will be affected by that cover - even if the cover itself isn't intersected.

Note that when an aiming unit is in high cover, it can also be treated as having a line of sight as if it was firing from the squares to either side, assuming they're not also blocked by high cover; in other words, your soldiers have the ability to lean. This is why taking cover against a corner of a building and firing around it at an enemy there will be considered a flank, even when the soldier is actually taking a position that will technically put the cover "plane" between himself and the enemy.

Cover does not grant any bonus to your defence score unless you're standing directly next to it (though high cover may break line of sight completely, preventing you from being shot at in the first place). This can lead to some very odd situations - for example, if a unit is standing next to low cover and steps away from an enemy on the other side, he immediately loses his cover bonus even though it's still between the two of them. This means that what looks like a complex shot (due to various obstacles being in the way) is really treated as having the same hit percentage as a shot with only the final barrier between you and the target, and it also means that otherwise solid objects can potentially be shot through in many situations. While you can often gain additional accuracy by closing the distance, the cover bonus can only be affected by a flank around the target's directly adjacent cover, so don't waste actions on moves that don't address one of those two factors.

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, usually 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 units standing directly next to them. This includes enemies - the "cover plane" being between the two, said enemy gains a cover bonus from the S.H.I.V. firing upon it, whereas intuition would suggest this situation should be treated as a "no cover" or a "flanked" situation!

Finally, be aware that the game will sometimes fail to realise that an enemy unit is flanked, and grant it its cover bonus even though there's no cover between the shooter and the victim (eg, 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.

Removing Cover

Any missed shot from a Laser or Plasma weapon has some chance of destroying cover (conventional bullet firearms are almost useless), but explosive weapons, ie Grenades and Rockets, are the reliable method of removing cover. Mid to late game when the aliens begin fielding stronger troops, grenades are unable to kill them in a single blast and the strength of grenades then becomes their ability to destroy the nearby environment and remove the target's cover bonus.

Another possibility is using the Assault's Flush ability to force the enemy to leave its cover, though a Flushed enemy will only stand in the open if no other cover is nearby to move in to.

Weapons that do terrain damage can also cause volatile objects like vehicles and gas pumps to explode. This usually does not cause them to stop functioning as cover, but it will do considerable damage to anything using it as cover. Normally the object will visibly catch fire and then explode on the next turn, but if enough damage is done to it it can explode immediately.

Units That Ignore Cover

Some units lack the ability to use cover at all.

Civilians Chryssalids

Cyberdiscs Drones

Ethereals Muton Berserkers

Sectopods SHIVs

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.