From time to time a call for help about making a new sky for DOOM2 is being asked in the forums. Like this one recently:
The texture in question is a blue sky by Fuzzball with dimensions of 1024x200.
Okay, so we make a new map and import this image as RSKY1
with this result
Ugh, that does not look good. It appears that only part of the sky is displayed and that part is repeated 4 times.
The reason is that, we have run into a limit of how large a sky image can be. The largest that the sky image can be is only 256 wide. But we have 4 directions, north, south, east and west. From that we could deduce that, if we chopped up the 1024 image into 4 256 wide images we could assign each to a direction. And that is exactly how it was done for some maps in FINAL DOOM.
Okay, we opened Slade3 and imported the 4 images. Next we have to make the DOOM engine recognize those images to be the new sky. This is done with the combination of the TEXTURE1 and PNAMES lumps.
Highlight the 4 images and rightclick to open the following context popup windows
Because the DOOM engine reads only the last TEXTURE1 lump loaded, we have to include the 4 sky images with the base lump.
Click on Yes.
Click on Import from Base Resource archive and then OK.
Now Slade3 generated the two lumps TEXTURE1 and PNAMES.
Highlight TEXTURE1 and click on Edit Textures
Then replace the default sky RSKY1 with the 4 images as in the videoclip below:
Once done, all that is left to do is to save the map.
Yes, that is much better.
A bit of an update:
While it is well and good to present this tutorial in the above form, I just recently found a video which I had made some ago which might explain the procedure more clearly