Line of sight (LOS) is a unit's ability to see another unit or a location. Line of fire (LOF) is a unblocked line between two points, such that fire may fly unimpeded from one to the other.
Line of fire between two points is computed using only the ground itself, and terrain which offers cover. LOF is required but not sufficient for LOS: LOS is blocked by smoke and many kinds of terrain which offer concealment, particularly foliage.
Note that both LOF and LOS are affected by a unit's type and stance. Tanks are taller than men, and can typically see more from any given location. Similarly, prone men can see less than kneeling or standing men. A unit hiding behind a hedgerow cannot see through it, because when hiding they are considered prone and the hedgerow includes an elevation ridge in it that blocks LOF. Thus, the unit will have to unhide for a turn before it can target anything on the other side of the hedgerow.
LOS is extended slightly for mortar units, reflecting their ability to fire indirectly. The actual mortar tube cannot see a target, but the men firing it can (by kneeling or standing higher than the tube). LOS is similarly extended when spotting for artillery fire.
LOS is not affected by vehicles. Thus, although infantry can cover behind a vehicle, it will be spotted there just as if the vehicle was not there.
While the Target (or Target Light) command is being issued, the command line extending from the selected unit to the mouse cursor assumes the function of a line of sight tool. Different shades of color are used to depict LOS graphically:
- Light blue -- LOS is clear
- Gray -- LOS exists for some but not all soldiers in a team. For vehicles, you see grey if some weapons can see but not all. For example, a tank commander firing an antiaircraft MG may be able to see a target that the other weapons systems in the tank cannot.
- Dark blue and red -- LOS is blocked. The parts of the line which are out of sight are shown in red.
In addition to the command-line color coding, text notes may be displayed at times above the target, alerting the player to special conditions, such as Hull Down or Partially Obscured or plain Out of Sight targets. The Target command uses the unit's current stance when computing LOS. Thus, if a unit is crawling it will see less than it might if it stops; if it is standing it may see more than it would if it stops to fire.
You can use Target from both the current location of a unit, and from waypoints you have plotted for it. This allows you to check LOS from any location you can move the unit to. Unfortunately, as of CMBN 1.01, when you use Target from a waypoint, the command line is drawn from the current location of the unit, not from the waypoint. This makes it rather hard to interpret. Nonetheless, the color coding still works, allowing you to determine fairly well what the unit will be able to see from an action spot before it moves there. Note that as mentioned above, the current stance of the unit is used to compute LOS in all situations, including at waypoints.
Effects of LOS
A unit must have line of sight to fully spot another unit. Note that you can get contacts via sound, without LOS from any unit. But you can never see a unit without LOS.
Generally, a unit must have LOS to a target to initiate fire against it. This is always true for targeted direct fire. It is only partly true for area fire: it is possible to fire upon a unit you cannot see by targeting an area that is close to it. However, the actual action spot you fire at must be in LOS.
A unit needs only line of fire to maintain area fire once it is begun. Thus, you can start area fire then fire a smoke screen, and maintain the area fire. Units doing targeted fire will continue fire for only a short while after losing sight of the target unit.
Artillery observers require LOS to initiate an artillery request, except for during setup or to a TRP. After initiating the request they do not need LOS to the targeted action spot, but they do need LOS to the locations of spotting rounds.
Line of sight is also required between friendly units for one to exert visual command and control upon another.