Decane: In short: foliage makes for non-existent cover in all three stalker games, unless you mod 'transparency_threshold' to a higher value.
I have a hard time believing this as I occasionally use bushes for cover (in SoC that is). That enemies do indeed lose track of me is pretty much officially confirmed by the attention meter going down when I do this, not to mention the enemies' behaviour that I observe. Of course it doesn't work with all foliage but for instance the area in the Dark Valley near the exit to the Garbage has numerous huge bushes that perfectly hinder the enemies' visibility.
I mean, I believe that your facts about the transparency system are correct but I think there's far more opaque foliage in the game than you'd think.
ekj7: Anyone know if foliage actually provides any useful coverage against enemy visibility? I sure have a hard time seeing anyone through bushes, but the enemy seems to have absolutely no trouble seeing, and shooting, me through it.
Seeing and shooting are two different things. Enemies frequently shoot at locations where they last saw you (even when you disappear behind a solid wall) but they shouldn't actually see you through the more dense foliage. Watch the attention meter to see what works as cover and what doesn't.