It doesn't need to be open source. Minetest (an Irrlicht C++ clone) is open-source, as is Infiniminer. Also, Voxel.js. All the relevent code is available for

It doesn't need to be open source. Minetest (an Irrlicht C++ clone) is open-source, as is Infiniminer. Also, Voxel.js. All the relevent code is available for future learning...