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this post was submitted on 29 Mar 2024
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Interesting, learned something new from my silly comment!
Also, this'll blow your mind too, Doom wasn't actually 3D. It was a clever trick involving the lack of the ability to look up and down. They used some sort of algorithm (I forget how it works exactly) to turn the 2D walls, doors, and platforms that appear from the top-down view in the map into vertical stacks of lines that "look" like 3D objects in front of you. The sprites are also all just 2D projections overlayed onto the game.
This system introduced all kinds of wierd quirks in the game, like the trippy effect you get when you activate no-clipping and clip through the edge of the map.
Like for instance, monsters and other sprite objects in the original incarnation of the Doom engine have infinite height. So you can't step on top of, or over, any monsters if e.g. you are on a ledge high above them. That's because they're 2D objects, and their vertical position on the screen is largely only cosmetic. This is why you can't run under a Cacodemon, for instance.
"Actors" (monsters, etc.) in Doom do have defined heights, but presumably for speed purposes the engine ignores this except for a small subset of checks, namely for projectile collision and checking whether a monster can enter a sector or if the ceiling height is too low, and for crush damage.
This was rectified in later versions of the Doom engine as well as most source ports. By the time Heretic came out (which is just chock-a-block full of flying enemies and also allows the player to fly with a powerup) monsters no longer had infinite height.
Most notably perspective only gets calculated on the horizontal axis, vertically there is no perspective projection. Playing the OG graphics with mouse gets trippy fast because of that. Doom doesn't use much verticality to hide it. Duke Nukem level design uses it more and it's noticeable but still tolerable. Modern level design with that kind of funk, forget it.
I learned recently that the Jedi Engine for the original Dark Forces had an additional trick. You could have a hallway over another hallway--which Doom cannot--but you can't see both hallways at the same time. So there might be a bridge over a gorge, but the level design forces it so it's a covered bridge, and you wouldn't have an angle where you could see inside the bridge and down into the gorge.
Duke Nukem can do that, too, both it and Dark Forces use portal engines while Doom is a BSP engine. With a portal engine you're not bound to a single global coordinate system, you can make things pass through each other.
Not actually a feature of the renderer you can do the same using modern rendering tech, though I can't off the top of my head think of a game that uses it. Certainly none of the big game engines support it out of the box. You can still do it by changing levels and it wouldn't be hard to do something half-way convincing in the Source engine (Half-Life, Portal, etc, the Valve thing), quick level loading by mere movement is one of its core features, but it isn't quite as seamless as a true portal engine would be.
Doom64 accomplished this by adding a silent elevator sector type, so it could have bridges that appear to be floating "over" an underpass that you could walk through but you could also cross over the top. This, of course, immediately got turned into marketing bullshit trying very hard to imply that "Doom64 was fully truly 3D, and the Doom engine could now do room-over-room."
Which it can't. These weren't bridges, they were cleverly disguised elevators.
What you eventually notice is that you can never look at one of these bridges from below and then from above, or vise-versa, without first passing through a tunnel or building that completely obscures your view of it. When your view is obstructed, you cross over a trigger somewhere that causes the elevator to, without making any sound (because elevator sounds were hard coded into original Doom), zip up to its cross-over-the-top position or its walk-under-the-bottom position. It could only ever be in one state at a time, never both.
Here's a video that explains the limitations of the DOOM engine and with it also briefly how the rendering part of it works (from 4:08 onward) in a very accessible manner:
https://youtu.be/ZYGJQqhMN1U
If you want a more in-depth explanation with a history lesson on top (still accessible, but much heavier), there's this excellent video:
https://youtu.be/hYMZsMMlubg
Here is an alternative Piped link(s):
https://piped.video/ZYGJQqhMN1U
https://piped.video/hYMZsMMlubg
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Ooooo I absolutely want these—thank you!