This tutorial is specific to the BOOMQTEX resource, but could be adapted to any Boom map.
The basic principles are based on a 1998 map by Nicholai Main, that demostrates sliding doors in Boom format:
https://www.doomworld.com/vb/doom-editing/41089-trying-to-duplicate-sliding-doors-in-prboom/
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We'll start with a simple room with a doorway:
We'll add some definition with a door frame, and we'll model how the door would look fully