Spartan is the single best open-source C++ game engine that you probably shouldn't use!  This started life as a way to get a job in the game industry and after successfully doing that, the creator has continued to develop and refine the game engine and use it as a platform for cutting edge graphics testing and development.   While you may not want to use the Spartan Engine to develop your game, you can certainly use it, or parts of it, to create or improve your own game engine.

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Gamefromscratch

Spartan Engine is an advanced open-source game engine built by a single developer to showcase technical skills and secure industry positions at Sony, CodeMasters, and AMD. Written in C++, it features impressive capabilities like procedural grass rendering, large open worlds (68 km²), and advanced graphics techniques including temporal anti-aliasing. While technically impressive and MIT-licensed, it's designed for research and experimentation rather than production game development, lacking documentation and having an unpolished editor. The engine serves as an excellent reference for learning game engine architecture and graphics programming concepts.

Spartan Engine - The BEST Game Engine You SHOULDN'T Use!