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Engine Review on Apr 15th, 2013 - 6 people agree 1 person doesn't
I am updating my review. (Oct 2014)
As an engine, and programming interface, C4 is superb. It's been built and continually refined by Dr Eric Lengyel, a highly trained mathematician and computer scientist. C4's engineering is probably, and I do mean probably, the most academically sound game engine ever constructed. Sometimes I feel like I'm reading Shakespeare reading C4 code.
If your programming toolbelt is highly diverse, it will take adjusting to C4's environment. It's highly self-contained. The C++ STL isn't involved anywhere, and essentially all its components were implemented from scratch by Eric. There are clearly advantages and disadvantages with that.
As an artistic tool for translating the pictures in your head to what's onscreen easily, it's a bit of a friction generator. Every release improves this aspect some, and Terathon has expressed an increased focus to its artistic toolset / chain for the next few versions.