DescentBB

1.How do you make an invisible wall?

2.How do you make it "rain" outside?

1.) Find a texture which has a Alpha of zero but does not have the pass through flag checked in the gam file.
You'll probably have to make a custom texture.

2.) As far as I know, the only way to do that is with scripting. And it will only rain 'outside' or any rooms you have marked external.

Jon the Great: there's a texture like that already, I believe it's called InvisibleTrainWall (it's the texture used to keep the player from going into the train tunnels in Level 4).

Ah, then just use that.

I thought that allowed the player to pass through it though. Oh well I guess I was wrong.

InvisibleTrainWall does show up on the Automap though, if that matters. I think it might also tint the face slightly. I remember Dark Wolf showed me something in her Chocobo racing level where there was a very visible "box" around the level formed by faces which had InvisibleTrainWall on them.

Weather is usually acccomplished in DALLAS scripting. There are ways to cheat it, though. For more on weather, you can check this:
phpbb/viewtopic.php?t=3963

Use the texture 'InvisibleTrainBarrier', which is a modified version of 'Shtail' (aka starhawk tail). It is at the very bottom of the Mine textures filter in d3edit.

And as for the tint, it is not actually tinted, the only instances where you can see it is when there is fog in a room. It reveals it because of the way D3 generates fog. Though it does look stupid when a flare sticks against a huge invisible wall, so it is better to warp the player out of a 'dead end' room than use an invisible barrier.

It would probably decal too, wouldn't it?

Yeah, another prob with the invisible wall is that when you shoot it the scorch marks are left on the wall in mid air, which looks pretty awkward.

Now that i think of it, another way would be to have the room blow the player away with wind, or have an elongated cube with portals on all sides and wind for use on terrain/large areas, to act as a barrier.