How does vanilla doom render its walls and floors?

[Breadbunbun]

I can't seem to find more information on this beyond that it draws its walls with lines from the texture file. But i was curious if the doom engine renders in tris, quads, or maybe even it being an arbitrary polygon rendering system. If anyone could give some more info on this it would be much appreciated.

[DavidN]

This is a great summary if you haven't seen it already: https://doomwiki.org/wiki/Doom_rendering_engine The whole thing works in a very unorthodox way, if you're used to more modern 3d engines where surfaces, polygons, tris/quads are definite things... it's more like taking instructions from the map and the player's current position and then painting an impression of that. It is a ball of absolutely ingenious cheats to get around the way that 1993's home computers had no chance of computing and rendering a true 3D representation fast enough for a game to run.

 

This video also touches on some of the techniques: 

 

Edited March 27, 2021 by DavidN

[Aaron Blain]

Are you aware of this?

 

Game Engine Black Book Doom

[Breadbunbun]

On 3/26/2021 at 7:31 PM, DavidN said:

This is a great summary if you haven't seen it already: https://doomwiki.org/wiki/Doom_rendering_engine The whole thing works in a very unorthodox way, if you're used to more modern 3d engines where surfaces, polygons, tris/quads are definite things... it's more like taking instructions from the map and the player's current position and then painting an impression of that. It is a ball of absolutely ingenious cheats to get around the way that 1993's home computers had no chance of computing and rendering a true 3D representation fast enough for a game to run.

 

This video also touches on some of the techniques: 

 

This was very useful. Thank you for showing me this. 

[Breadbunbun]

On 3/27/2021 at 5:11 AM, Aaron Blain said:

Are you aware of this?

 

Game Engine Black Book Doom

I'll certainly check it out when i can. Though im not sure when that will be.

[Aaron Blain]

I'm still working on Sanglard's Game Engine Black Book: Wolfenstein 3d, so I've barely looked at this one.

 

I think Bisqwit uses a Doom-like approach in this video:

 

 

"Similar effect, not exact same technique." He says. I don't know what the differences are, though.

[Gez]

That video describes a portal rendering engine, so it's the principle of the Build engine (Duke 3D, Blood, Shadow Warrior, etc.) and Jedi Engine (Dark Forces, Outlaw); but not the Doom engine.

 

Portal engines and the Doom engines are very similar superficially, they both have an approach where geometry is divided in sectors that are marked with lines that can be one-sided or two-sided, and two-sided lines can have upper and lower textures to account for differences in sector heights. The differences is how they handle the rendering of it. For portal engine, as explained in Bisqwit's video, you draw the sector the camera is in, and then you recursively draw other sectors visible through two-sided openings. For the Doom engine, the approach is different, based on the BSP tree.

 

The advantage of portal renderer is that they allow impossible geometry, because there's no technical requirement an opening as to go to the sector that is physically connected to the two-sided line. This makes room-above-room much easier to implement, with caveat. Doom cannot do that since the BSP tree is statically precomputed. Doom's advantage, however, is increased performances. Since the BSP tree is precomputed, a lot of expensive computations don't have to be done during the game. Even on modern systems, this remains somewhat true -- the Doom engine is more scalable for mega-maps than a portal engine is. I mean the kind of maps that require a strong limit-removing port, like The Given or Sunder, would be hard to do with acceptable performances in Build.

Edited April 8, 2021 by Gez

[Aaron Blain]

Oh, is this how the original Build editor was able to let you edit geometry on the fly, and edit heights in 3d mode?