Ok, Eriochrome and Ryjak have been working me over on visibility. So here's another go at it. With what should be a simpler set of concepts to explain.
1) Visible Targets, Hidden Targets
If you can draw a line from any part of the observing/firing model's base to any part of the target model's base, without crossing blocking terrain, then that model is 'visible'.
If you can't draw a line to the target's base without crossing blocking terrain (Hills, Intact Buildings), then the target is Hidden. Hidden models can't be targeted.
Visible models can (almost) always be shot at. Though you still only have to run from units within your Awareness range.
As for elevated positions...
Infantry LoS is determined by whether or not you can see their base.
For Large Targets, we'll determine LoS by whether you can see the heads, body/hull, arms, or legs of the target. More on that later though.
2) Visibility & Awareness
You can shoot at any visible target, but what you'll need to hit is determined by the model/unit's visibility. There are 3 levels of visibility:
- Exposed (24" Awareness, +1 Ev if Difficult)
- Obscured (12" Awareness, +2 Ev if Difficult)
- Concealed (6" Awareness, +3 Ev if Difficult)
Easy Targets are hit on a straight BS vs. Evasion. Difficult Targets have standardized 'Soft Cover' modifiers applied.
So for Guard vs. Guard (BS6 vs. Ev6):
Easy Exposed Target (24): 4+
Difficult Exposed Target: 5+
Easy Obscured Target (12"): 4+
Difficult Obscured Target: 6+
Easy Concealed Target (6"): 4+
Difficult Concealed Target: 7+
3) Going To Ground
Going To Ground makes easy targets difficult, and difficult targets Hidden. That's the only effect.
Recon units can attempt to lift GtG from a unit.
Units with stealth can GtG after moving at an advance, instead of shooting.
6) Acute Senses
Doubles a unit's Awareness when attempting to target GtG units.
7) Hard Cover
Provided by Hills, Ruins, and Walls. Units/Models get +1 Toughness.
Models must be in contact with low walls to get bonus, high walls and ruins always provide hard cover to models behind them.
8) Blast & Flame Weapons
Blast Weapons ignore soft cover, but not hard cover.
Flame weapons ignore both soft and hard cover.
9) Indirect Fire Weapons
Treat hidden targets as concealed instead. If there's a model with the 'Observer' rule, then indirect fire weapons may use the visibility of the target to the observer, instead of their own. At the cost of the observer not being able to perform an action that turn.
That's the basic concepts.