Yeah, but Half Life 2's Source engine was itself a continuation of Goldsrc which was itself a continuation of the Quake 1 engine. The lineage is there but beyond a certain point it's not really Quake anymore.
GZDoom/UZDoom is a similar grey area, it is built on the original Doom codebase but they've added so many features that it's practically its own distinct engine now. Those forks can even render arbitrary 3D models, which OG idTech couldn't do until Quake.
We'd have to come up with definition of quake :-) FTE has a lot bolted on it but the focus us in Quake, quake mods, lifting some limitations and making mod dev convenient.
But it is the same overall code structure, the same game, etc.
All these oss quake engines, are they quake? Ironwail, quakespasm vkQuake?
There already is a term: Quake Source Port
https://quake.fandom.com/wiki/Source_port
I think the litmus test is weather they are backwards compatible with old maps/campaigns from the original engine/game.
Half-Life 2 sure won't play quake maps nor will it play hl1 maps.
> Half-Life 2 sure won't play quake maps nor will it play hl1 maps.
Not without modifications but Half-Life: Source is essentially a tech demo to show that they can be ported easily (if you are OK with dropping some pesky features like randomized wall textures).
AFAIK hl1 maps needs to be open in hammer, tweaked a bit and then recompiled to function in hl2. You also better have those originals .rmfs rather than a .map or a even worse, a .bsp :)
Qames/quake from 9front =). It can run LibreQuake with Malice as a MODs, and that's it. Quake, Quakeworld and everything for vanilla, no modern changes like QuakeSpasm or worse, DarkPlaces. If someone backported HL2 to the original Quake with reduced physics and still run under a Pentium III fast enough, it would be something astonishing.
I see impressive stuff with reimplementations such as Surreal Engine, but they will require far more powerful machines.
If Surreal had a software renderer (not requiring AVX or similar) running under an SSE2 machine, that would yield even more respect, because if your reimplemented engine runs in legacy machines the portability would explode. Just have a look on Scummvm on how many platforms and OSes can it run. Or the Super Mario port for PC, where some fork supports even 3DFX under DOS, and GL 1.2. Thus runnable under TinyGL with no 3D accelerators and even under Plan9/9front with custom tweaks.