ZDoom

[quote="Graf Zahl"]Keep in mind that a cubemap is a spherical projection onto the cube's planes with the camera in the center position. Changing the viewpoint inside the box will inevitably reveal its true nature...[/quote]
I guess that answers the very last question in my original post.
[quote="Enjay"]Or maybe it just wouldn't work and moving the camera might distort the projection of the images?[/quote]

You cannot deny that the moon in there doesn't look right in game, can you? :P
For me it stands as a perfect example of how an incorrectly made cubemap skybox can break the illusion of looking at some real scenery.

I never said that the mod itself is bad - it's still by a wide margin the best horror themed Doom map ever.

Using my work as bad examples hurts my feelings :(

Keep in mind that a cubemap is a spherical projection onto the cube's planes with the camera in the center position. Changing the viewpoint inside the box will inevitably reveal its true nature.

If you want to see an example of what happens if the camera is not properly placed in relation to the projected image, check out Tormentor's City of the Damned 2. It has a moon in the skybox but it is perfectly circular in the image but not centered so when viewed with the center-based projection it will just jarringly look off. (This skybox looks to me like the moon was a retroactive addition and not a part of the original rendering.)

True, but then you have to construct the skybox in the map and that isn't always desirable or even viable.

If you use a skybox viewpoint actor you can do whatever the hell you like. And if I were any good at writing shaders, I wouldn't even need that.

When messing around with the skyboxes from "Rogue Tropper" I found that most of the skyboxes in that game only have a useful image from half way up the sides of the skybox. From the mid point down, the sides (and bottom) are just one solid colour.

https://forum.zdoom.org/viewtopic.php?f=37&t=61685

[spoiler][imgur]https://imgur.com/btUhHLR[/imgur][/spoiler]

If I was constructing a skybox in a map, I could chose what height I wanted to have the skybox camera. So I was wondering if such a thing was possible in an OpenGL GLDefs-style virtual skybox?

[wiki]GLDEFS#Skybox_definitions[/wiki]

I'm guessing that it isn't currently possible in GZDoom but I don't know if it would even be technically possible at all. If it was, then perhaps it might be possible to raise/lower the virtual camera, as appropriate, to make such skies more useful. Or maybe it just wouldn't work and moving the camera might distort the projection of the images?