You could make some weird impossible geometries by just superimposing sections. Two sections could occupy the exact same coordinates, but movement and rendering were done across edges shared by sections and did not care about if there was some other section in the same space. As long as there was never a way to see those sections at the same time.

That's what they just said? They didn't mention the mirror effect? The mirror effect was done with duplicate geometry? It occupied the overlapping space of what was behind it? The build engine also didn't have vertical levels.

> You could make some weird impossible geometries by just superimposing sections. Two sections could occupy the exact same coordinates, but movement and rendering were done across edges shared by sections and did not care about if there was some other section in the same space. As long as there was never a way to see those sections at the same time.

This is exactly what portals are.

Portals for rendering date back to the dawn of 3d graphics and many early FPS engines used the concept.

But these portals were only there as a rendering optimisation. While you could abuse them to make non-euclidean maps, the tools were intended to make proper maps, and the portals would be invisible to players.

I know nothing about this stuff, but when I saw in-game I was really impressed. You can even break the screens into shards, and the little pieces still maintain the effect when they’re lying on the floor or flying through the air.

The designer needs to confine the player so they can't break the illusion (where Prey lets Morgan Yu shatter that 4th wall), or you can do things like put the player on a speeding train traveling through a scenery and falling off is fatal

I miss the customizability of that game. Early bungie works were up there with early blizzard or early valve. They sold a tool for fun instead of casting a bait for profit.

Then, later in development, they ran a tool to stitch together all that geometry in a more-normal way, and they only had one case of impossible-overlap, which continued to use the non-euclidean link.

https://gaming.stackexchange.com/questions/20601/where-is-th...

DF used BSP for culling. But, I made a mod level long ago playing with features and made a stairway in the middle of a room that you could only see from in front of it. It was some kind of free-standing portal with no surrounding support.

Fun fact: the creator of the DF engine told me he based it on a reverse engineering of a beta release of DOOM. Apparently, the final release of DOOM cut a lot of engine features to gain speed. But, DF shipped with them and maybe a few more.

Can you provide a source? As I understand it DF is based on the (pre release?) Tie Fighter engine, and DF began production before Doom released. That's why for example DF has some non-textured 3D models like the turrets and mouse droids. I follow The Force Engine (reverse engineered fan port) and Dev Game Club (hosts worked at LA).

Supposedly DF devs did see Doom (perhaps even a pre release version), but already had their own tech working by that point.

> Apparently, the final release of DOOM cut a lot of engine features to gain speed.

Doubt. There's a lot of interviews with the ID guys, and I don't recall anyone saying they cut significant technical features. They made Doom in only 13 months, minus almost a month to port Wolf3D to SNES. Maybe Carmack -- or the released alphas and betas -- can clarify? Or perhaps you are confusing some of the Doom porting efforts, which did cut down levels and sometimes features significantly.

All of this the DF engineer told me face to face. Might have been based on an alpha version of DOOM. He might have already had some tech for DF before studying DOOM. It was a casual conversation over ten years ago.

I don't recall what the changes were. Something about being less flexible to be faster. Only way to know would be to redo the reverse engineering of the BSP format from one of http://toastytech.com/dooma/

But certainly the game Portal (2007) hyped using portals for rendering prior to Prey.

They were referring to Prey (2006).