How do portals work "under the hood"?

[Blastfrog]

I'm curious, how many changed needed to be made to the world renderer in order to achieve this, and what kind of changes were they?

[Quasar]

Sodaholic said:

I'm curious, how many changed needed to be made to the world renderer in order to achieve this, and what kind of changes were they?

SoM would be better qualified to answer than I am, but some key components required were the following:

• Ability to clip segs and visplanes against arbitrary clip buffers and not just the single clip buffer used by the normal engine.
  
• Various changes to seg clipping itself algorithm-wise, particularly having something to do with the ability to draw past lower or upper textures that DOOM normally would use to stop the rendering process.
  
• A stack for masked drawing rather than a single pool of masked entities (sprites and 2S line fragments).
  

There's a lot more to it. But again, I didn't write it so I'm foggy on the heavy technical details.