Okay, before I start, I have to say that I'm not a complete idiot, and I usually have the intellectual capacity to turn sufficient data into usable information, however, this problem (including this "solution") has cost me hours. Many of them. I've been reading this thread and struggling to understand it since it started.
Okay, now, (and for the love of God, put this in the first post so others don't suffer the same brainache I have)
THIS IS THE SOLUTION
note: this solution assumes you're using sky_day01_05, if you're using a different skybox texture, change the "day01_05" where appropriate.
For example (sky_night_01_01 , sky_day03_05).
1) Download GCFScape http://nemesis.thewavelength.net/index.php?p=1&c=76&o=150
2) In GCFScape, open the file
\Steam\SteamApps\source materials.gcf
3) Using GCFScape to browse the file you just opened, click through to
hl2\materials\skybox\
4) Open sky_day01_05dn.vmt and Add the line
-----
"$hdrbasetexture" "skybox/sky_day01_05dn"
-----
Ignore the dashes.
It should now read:
-----
"UnlitGeneric"
{
// Original shader: BaseTexture
"$hdrbasetexture" "skybox/sky_day01_05dn"
"$basetexture" "skybox/sky_day01_05dn"
"$nofog" "1"
"$ignorez" "1"
}
-----
5) Repeat this for all the .vmt files associated with your texture (in this case sky_day01_05).
You should therefore edit 6 vmt files in total: sky_day01_05dn
sky_day01_05up
sky_day01_05lt
sky_day01_05rt
sky_day01_05ft
sky_day01_05bk
Remember when inserting the new line to change the last two letters (directional indicators) to correspond with the file you're editing.
6) Make sure your map is using the texture that you've edited, or edit all the .vmts that you will be using.
To change your skybox texture go to Map -> Map Properties -> Skybox Texture.
Leave out the directional indicators (dn, rt, etc.).
In this example the skybox texture in my map properties reads: sky_day01_05
Note: I still get the error, but now I have sky instead of Hall of Mirrors.
I hope this helps some people avoid the trouble I've been having.
Sorry if I sound rude... just trying to help the internet. What am I saying? Fluck the internet.




