For this article, we look deeper into the intricacies of forest terrain and how it interacts with line of sight.
Determining line of sight is one of the most basic elements in the game. When you are trying to determine whether one of your creatures can see an enemy, you need to draw a line that starts on the edge of your creature's square and ends inside your enemy's space. To have line of sight, that line needs to do more than touch a corner of the target's space or run along the edge of that space -- it needs to actually enter the target's space.
One common type of terrain that blocks line of sight is forest. In most situations, if the two creatures aren't in forest squares, you can treat forests like walls when determining line of sight.
Line of sight is blocked if it touches two edges of the same forest square. Because the corner of a square is the intersection of two edges, touching the corner of a forest square blocks a line of sight from passing through (but not into).
There's one important caveat to this. When determining line of sight from a creature in a forest square, consider a corner where the line of sight originates to be only one edge of a forest square. (Line of sight does not need to originate from a corner, although cover determination does.)
What makes forest different from walls is that a creature can stand inside forest.
Here are a few examples of applying these rules to specific situations.
There is no line of sight between these two creatures. No line of sight can be drawn that does not get stopped by a forest corner or edge.
There is line of sight between these two creatures. When tracing this line of sight, the first corner counts as one only edge.
No line of sight exists between these creatures. No line can be traced without crossing two edges of a forest square.
In this example, the Elf Archer is able to trace line of sight to the Warforged Infiltrator. The Elf can trace line of sight from his upper left corner as a starting point, and it doesn't count as two edges because it is the starting corner.Line of sight does not exist in the reverse case.
Because all lines from the Infiltrator need to go through that forest corner, the Warforged can't see the Elf.
If the Elf were to attack the Warforged, the Elf would have combat advantage. Whenever an attacker can trace line of sight to a target but the target cannot trace line of sight in return, the attacker is invisible to the target and benefits from combat advantage accordingly.