ZDoom clipping differences?

[Memfis]

Why sometimes monsters are stuck in each other or in something in PrBoom-plus but can move normally in ZDoom? Or it can be the other way around. I'd like to understand these differences so that I could avoid all these problems.

Here is an example map:
prboom-plus: demons are stuck, revenant isn't
zdoom: demons aren't stuck, revenant is

[Graf Zahl]

Doom originally never considered the case possible where the distance between two monster was less than their radius so any attempt of such a monster to move still ended up in the other one it was stuck in and the move was deemed illegal.

If this happens in ZDoom it does a few more checks, allowing the monsters to move, as long as the distance between them increases.

[Memfis]

Graf Zahl said:

allowing the monsters to move, as long as the distance between them increases

Hmm, I found this difficult to understand: so they can move as long as the distance increases, and for the distance to increase they need to move I guess... It's like the chicken and the egg situation. :)

And what about that stuck revenant? For some reason he seems to dislike the little rised floor behind him (he can move freely if you sink it into the ground in the editor).

[plums]

Memfis said:

Hmm, I found this difficult to understand: so they can move as long as the distance increases, and for the distance to increase they need to move I guess... It's like the chicken and the egg situation. :)

Another way to put it: while they are overlapping, they can move away from each other but not towards each other.

[GreyGhost]

Memfis said:

And what about that stuck revenant? For some reason he seems to dislike the little rised floor behind him (he can move freely if you sink it into the ground in the editor).

I suppose his first move is to attempt to climb that step, and since being 8 points higher doesn't get him any further away from the obstacle in front of him - he stays put.