@
Aaronotm 
this is possible thanks a great program called
uMod, v.2 r49. Is famous for not manipulating files (so not against TOS of any kind) and replacing textures while the game runs

Was used in Mass effect 1 2 3 to increase quality of texture and fixing some crap looking one (is possible also here, by a program called
Benvista PhotoZoom Pro 5.0.8 to get high size textures resolution and put them OVER the original (maybe crappy) one)
This whole post is a tutorial
After other successful edits, maybe i will post a whole new one tutorial

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RGraegaergrwagargwrg Yes, i was looking into IR scope, but i think it depends on game and not from textures :/
Replacing a scope is only replacing reticle. There are 2 different textures after the reticle:
One that looks like a black hole (or a black square) in a white area, representing the visual area of some scopes.
And a 3rd one that seems the real visibility tube of IR scopes. So would be a nice try replacing the normal scopes black texture with the IR tube texture.
Trying harder XD i was still working on (well, experimenting) some textures to make invisible also the orange and green plastic houses. Still no luck.
Is harder because the game uses some color filters itself, so not all maps act the same whit a particular color. for no curtains, i was not lucky, because textures remains (only in trench was possible...and the 2 minicurtains in the underpassage. The rest is a strange texture used randomly around :/ but no curtains is important and i achieved it...not so proud of this)
Now i try with scopes

, is enough the fix in visibility that i've done ( and there are some bonus invisible area

profit!)
Im also looking for the total invisibility of the reticles of scopes, this is off course related to the scope skin (look for it while in customization, scopes)