How does GZdoom manage floors above floors?

[darbyjack]

Is it some kind of trickery? Or is it modified beyond the point where it is not really the Doom engine anymore? Excuse the ignorance, I know nothing of the matter, but a bit fascinated by it. Thanks!

[Gez]

Well it's the Doom engine, but with Heretic and Hexen additions to get better over-and-under physics, and with bugfixes.

Then some handling for collision with 3D floor and passage through portals.

Now for the rendering side, yeah, it's been heavily modified, especially if you use the OpenGL or softpoly renderers which are basically completely new renderers rather than modifications of Carmack's original rendering algorithms.

[darbyjack]

Interesting. I wasn't aware Hexen and Heretic could pull it off. My understanding was that the engine essentially converted 2D to give the illusion of 3D, and floors over floors were not possible at all.

[Gez]

Heretic and Hexen didn't have true 3D floors like GZDoom. But they allowed actors to be over and under each other, instead of being blocked by 2D horizontal collisions regardless of their relative heights like in Doom. This is what allowed Hexen to have its glitter bridges.

ZDoom built on that, fixed some bugs, and then added geometry-based 3D floors rather than faking bridges with actors.

[darbyjack]

I see. What about cacodemons flying above other monsters or the player in doom?

Also how was Gzdoom able to add geometric floors? I thought it would have been impossible with the original engine. Is it basically a new engine with elements of the old one?

[Graf Zahl]

I see. What about cacodemons flying above other monsters or the player in doom?

You know, limits and restrictions can be removed, and that's just what happened here.

[Gez]

I see. What about cacodemons flying above other monsters or the player in doom?

In vanilla? They don't. They collide with the actors on the floor. You can make a running jump between two platforms, and then collide with a demon roaming on the floor below and fall.

3D collisions only exist for projectiles -- there it's possible for a fireball to fly above (or below) a target without colliding with it. But for players and monsters, collisions are only checked in 2D in vanilla. That's what was changed in Heretic and Hexen, and therefore in ZDoom.

Also how was Gzdoom able to add geometric floors?

Like everything else, by adding more code.

[Chris]

Is it basically a new engine with elements of the old one?

Depends on what you call a "new engine". (G)ZDoom started with the original engine then added more and more things to it to do more things, including custom actor behaviors, hi-res textures, sloped floors, etc. It's not exactly the same as the original engine, but it's not completely different, either.