Useful open source projects

https://github.com/diasurgical/devilutionX - Diablo 1 Engine. Uses circle CI. Crossplatform: Linux 32bit & 64bit, Windows 32bit & 64bit, Nintendo Switch, macOS, *BSD, Haiku.

https://github.com/FWGS/xash3d-fwgs - Half-Life 1 Engine. Uses cirrus CI. Crossplatform: supported x86 and ARM on Windows/Linux/BSD/Android.

https://github.com/REGoth-project/REGoth - Gothic 1 and 2 Engine. Uses bgfx rendering library and shaderc shader compiler (inside bgfx). Crossplatform: Windows, Linux, BSD, macOS, Android.

https://github.com/Ziple/kT - A little set of libraries to help developing 3D applications with D3D and OGL.

https://github.com/blueshogun96/KunaiEngine - An open source engine consisting of open source and highly portable code libraries written in C/C++.

https://alain.xyz/blog/comparison-of-modern-graphics-apis - basic functions for DX/OGL/VK

https://github.com/Gocnak/sdk-2013-hammer - Hammer from 2007 brought up to 2013 standards, with added features

https://dev.ds-servers.com/sip/engine/-/tree/branchX/ - stalker like engine. MIT -> AGPLv3

https://github.com/wwwtyro/glsl-atmosphere - Renders sky colors with Rayleigh and Mie scattering.