Line of Sight (EU2012)

While the aesthetics of the in-game fog/vision appears vague, the only requirement for having line of sight of a voxel seems to be whether a unit can see the center point of that voxel. Units are two voxels high with the center point of the top voxel being where their origin of line of sight comes from. The requirement for line of sight of a unit seems to be whether there is line of sight of the top voxel of that unit, bottom is ignored. The game's cover mechanic extends a unit's origin/destination of line of sight.

Obstacles

Similar to the cover concept, most obstacles provide visual full/half/no cover and usually match with the defence counterpart. The entire cube/side acts as a blocker for vision, excluding the edges/corners if they coincide on the line of sight. Model shape does not seem to matter.

Some non transparent obstacles seem to provide no visual cover. Not sure if it's a bug or a feature. Ramps and the service truck with a crane seem to be see-through. It is unclear whether these objects only provides one voxel of visual cover (like half cover) or none at all. While there may be logical explanations for these, there also exist objects with the incorrect visual cover as well (half cover with full visual cover, full cover with half/no cover).

Transparent cover can exist, but usually only discovered through explosions. Exploded vehicles that still provide high cover may end up useless as vision blockers. In certain cases, an exploded wall may provide full cover (and visually transparent) due to hanging objects being intact (paintings/pictures, ladders), despite the wall behind being destroyed.

Lastly, there are some exceptions that seem intended, such has the diagonal walls of UFOs, lampposts, flag poles, etc, which provide no defence cover and may have particular visual cover geometry.

Estimating with the camera

Use of the in-game camera in certain cases can make quick estimates of whether two points/areas will have line of sight of each other. Try to get vertical on the camera the line from the tile to an obstacle corner. Unfortunately, it's not exactly easy or possible if it requires the third dimension, or if the distances are too far away to create a vertical line.

• Off cover vision trick
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  Any enemies using the car will not have sight of the soldier behind the alien object
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  Similarly, the left corner of the truck is not visible

Peeking cover

Attacker and target units are within each other's vision range, but the tile used to attack is outside of the target's vision range.

Peeking tiles do not expand the vision range, only make parts of the vision sphere visible as additional source of line of sight. The origin/destination of line of sight are extended by peeking tiles, but limited to the original vision sphere centered on the unit. This is why when the unit steps out due to an action, they can sometimes trigger another pod, since the vision center/sphere actually shifts then.

While peeking, the entire segment from the unit's center point to the peeking tile center point is included, whether as attacker or target, not just the center point of the peeking tile.

Non visible attackers

This can explain weird circumstances like being targeted by non visible units. Let's say without any obstacles, both the attacker and the target are in vision range of each other at a diagonal in such a way that if there is full cover in front of the attacker, their outer/furthest peeking tile is outside the target's vision sphere. If the inner/closer peeking tile is also blocked by visual cover, then the attacker would appear to be non visible to the target.

The same idea applies when the attacker and target are just outside of each other's vision range, but the target is using cover and therefore their peeking tile is within the attacker's vision sphere. This was the case for the Sectoid in the cover visibility example.