How did Alien Vendetta make the BIWALL textures animated?

[peach freak]

I know that Doom has a list of textures that are hardcoded to be animated. But I noticed that Alien Vendetta has BIWALL1-10 textures. I was messing around with one of their maps and replaced a middle texture with a BIWALL texture, even though BIWALL isn't a harded animated texture in Vanilla Doom. I believe the AV dehacked file only changes the automap names and story texts, so I don't think it was done through dehacked. I noticed the BIWALL had a string of textures so that you could offset the x values to get your desired texture and get it to animate.

How did they make BIWALL an animated wall?

EDIT: I was looking at the TEXTURE1 lump in Alien Vendetta and noticed that all the BIWALL textures were put in between the GSTFONT2 and 3 textures (and GSTFONT1-3 are animated hardcoded textures). Could that havve anything to do with it?

[esselfortium]

Doom's texture and flat animations are defined by the first and last frame. You can insert as many frames as you want in between them, and they'll be part of the animation.

[Foxpup]

Additionally, all Doom 1 animations work in Doom 2, and vice versa for versions 1.666 and up of Doom 1, and there is also one animated wall (WFALL1-WFALL4) and one animated flat (SWATER1-SWATER4) that are defined in all versions of Doom, but have no graphics in the IWADs (though Final Doom does use the WFALL animation). Any of these "unused" animations can also be used to create extra animations.

The x-offset trick can also be used to create additional switch textures, which won't break the original textures since (in properly constructed maps) they're never tiled beyond their original width.