ZDoom

Well, shit. I need this for a universal mod that overrides the sky so a mapinfo option does me no good, sadly.

This could only be done as a MAPINFO flag because some skies can tile and others cannot be mirrored without looking weird.

Won't work in my case because it's not one sky but 240 frames that make up a sky, and the fact that the engine fucks up anything that isn't 1024x256. I can't keep that resolution if I mirror it myself. Cropping the image even more to make space is completely unacceptable.

IDK if this feature request will make it but if it doesn't you could try making your sky seamless yourself as in this sky from Doom 1 that I easily made:

[imgur]https://imgur.com/MD3VTI6[/imgur]

...Sorry, I meant the X axis since the idea is to mirror it vertically. Mirroring it on Y wouldn't do much.

Skies look like this right now.
[imgur]https://imgur.com/u8kI3kv[/imgur]

That looks bad. Awful, even. What if instead of tiling, the sky is flipped on the Y axis for drawing the bottom part? That would make it seamless since there is only one point the sky wraps around and that's the horizon, as both ends fade into infinity and don't wrap around again.