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I believe they augmented the engine instead whenever they found a feature they needed that it did not support. That is what makes sense.
While they certainly did that too, they spent 2002 re-writing tech. They really did do the quite dramatical changes that you suggest that didn't do. Yes the editor and tools have also been heavily modified. Rendering? Re-written. Animation? Re-written twice since the E3 2001 version. Indeed in August 2006 they started re-writing both the animation system AND associated tools. Lighting? re-written. Visibility determination/hidden surface removal? Re-written. AI? Re-written. Physics? Replaced with different middleware than what shipped with their UE license. Terrain support? Added by 3D Realms. Multi-threading support? Added by 3D Realms somewhere between August 2006 and April 2007.
Edit:
Some quotes to back the above up:
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Yeah, but just the overall structure. Editor, scripting language, networking etc. We've completely gutted and written our own AI system, rendering, particles, skeletal animation and more so it won't look/feel like an Unreal game at all I don't think. In hindsight, I don't think licensing an engine was a smart move for us. We're pretty uncompromising in what we want to do, so we don't like having limitations. What killed us was not having the programming staff to do what we wanted to do effectively and not recognizing that for a long time. Chalk that up to inexperience.
(Emphasis mine) - George Broussard, January 7, 2004.
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Yes, in hindsight, writing our own engine would have been the way to go, but that wasn't an option once we were so deep into things. We basically stepped back in early 2002, said "This just isn't going to work or be what we want" and spent most of 2002 re-writing things to get us where we needed to be, once and for all. Most of 2003 was spent on content creation and hring new people. Once we were able to make progress, content creation bottlenecks emerged that needed to be dealt with. So, it's been an interesting journey, but one I do not recommend be repeated, by anyone, ever Haha...
Emphasis mine) - George Broussard, January 7, 2004.
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Everything is different. Our visibility, rendering, everything. You could bring in raw geometry, but there's no guarantee it would ever run, as any level made in Unreal may be apples to oranges to what you would do, or how you would do it with out stuff.
- George Broussard, April 13, 2004.
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We have nothing to do with Unreal, 2k3/4 rendering and we've 100% written our own rendering, lighting and visibility for DNF. Apples and Oranges. We will be visually competitive.
- George Broussard, May 14, 2004.
For the next set of quotes keep in mind that Brian Lawson was hired in August 2006, these are the recommendations he got on linkedin.com from George Broussard and Brian Hook:
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Brian L.
Tech Programmer at 3D Realms
To sum up Brian, let me put it this way -- we had two engine programmers responsible for 360 and PC graphics, multithreading, streaming, memory management, animation, performance optimizations, and all the other tasks associated with building core engine tech. And he was one of them. Most companies have eight or more engine programmers, yet we only had two. He built our entire core animation pipeline, both in-game and the associated tools. He worked with the animators on technical and workflow issues, and when he was done with animation he went straight onto some of the most difficult tasks in engine coding -- retrofitting an inherently single threaded engine to have multithreading support, and then getting a predominantly PC engine running on the 360. Brian's a great programmer that never shied away from the hard technical tasks, yet he was never afraid to get the input from others. May 9, 2009
brian (Producer at 3D Realms ) managed Brian at 3D Realms
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Brian Lawson
Tech Programmer at 3D Realms
I can't tell you what a pleasure it was to work with Brian. He's a great programmer who can do anything from the small and mundane to the most complex. I saw Brian replace a bloated, horrible animation system with a streamlined, optimized version that was 1/10th the lines of code. He gets results and produces solid, dependable, shippable work. Add in a great attitude and personality and you have a guy you really need to have on your team. I'd work with Brian again without hesitation. May 7, 2009
George (Co-Owner / Founder at 3D Realms ) worked with Brian at 3D Realms
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Multi-core support is in and quite nice. We all run Core Duo 6600's and 7950/8800, ati 1900 level cards.
Multi-core is the future and the game is pretty much going to require it. You really have to, to make a game competitive with modern consoles, or beyond. One cpu isn't enough anymore.
64 bit will come, but is lower priorty. Vista/64 bit isn't a priority for us at the moment.
- George Broussard, April 19, 2007.
So in other words at this point Brian Lawson had already re-written the animation system
and the tools to go with it
and had gotten multi-core support up and running
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It's not worth arguing over.
There was a root of Unreal a long, long time ago.
3D Realms rewrote, replaced, rewrote again, replaced again, iterated, modified, rewrote yet again most of the engine over the years.
When we got our hands on it, entire sub-systems were yet again rewritten or otherwise replaced.
It still has the Unreal architecture and the editor (also modified heavily through the years), but there's very little Unreal code left in there.
Duke Nukem Forever is to Unreal Engine what Half-Life 2 or Modern Warfare 2 is to Quake engine.
We've not really worried about labeling it. In the trenches, among developers, it's kind of pointless and, frankly, amateurish to try to talk about the technology in the terms that I'm seeing discussed here.
It's probably most accurate to just call it "The Duke Nukem Forever Engine" and leave it at that.
If someone wants to get in a semantics argument about what bits are Unreal or what bits are other middleware, you better be a programmer who is very familiar with the source code and who is currently working on the project or you will be better off to just STFU.
---------- Post added at 10:07 PM ---------- Previous post was at 10:04 PM ----------
BTW - The fact that is was rooted in Unreal and has the memory of the Unreal architecture, tool path and formats for everything has been VERY beneficial to our development. Our expertise with modern Unreal has been VERY helpful and the quality of Epic's software there and today deserves some credit for the fact that the game is coming out.
Do not interpret the fact that most of the software has been rewritten as anything other than just a natural result of how long the game has been in development and how committed the team has been over the years at 3D Realms to keep attending to and modernizing things and, in some cases (like interactivity), actually pushing things far ahead of the most modern game engines.
(Emphasis mine) - Randy Pitchford, March 11, 2011
Think of it this way every single line of code written(or for that matter re-written) for this game in the last 10 years or so has been original to 3DR(later Triptych, Gearbox and Piranha as well) or has been middleware completely unrelated to the Unreal Engine. Every single line of code! For 10 years!. 10 years of code is a lot and all of it is completely unrelated to the Unreal Engine.