Decal Thing
From ZDoom Wiki
The Decal Thing (9200) is a useful tool to add graphics to walls without requiring a custom texture. See Decals for more information on importing custom images as decals.
Specifying the decal in DECALDEF
Set up your decals the way you want them using the DECALDEF lump. Make sure you set the Decal ID, you will need it to reference your Decal in the map.
Setting up the Decal Thing (9200)
Place the decal thing close to a wall (no more than 64 units). Set the angle of the thing to face away from the wall. Also remember to set the Z-height accordingly. To select the decal you want to place, use the Decal ID number that you wrote into your DECALDEF lump. Place this number in the first special argument of the Decal thing.
A good way to think about how this works is the decal thing "shoots" the decal away from it. This means you can use it on upper and lower segments of 2-sided walls, provided you set the z-height accordingly.
