I was dissappointed with the quality of the code and the final demo game. Also, it uses DirectX.

Here's an alternative: Dive into the GamePlay engine (https://github.com/blackberry/GamePlay/) and go through all the samples (https://github.com/blackberry/GamePlay/tree/master/samples), especially this collection of small feature demos (https://github.com/blackberry/GamePlay/tree/master/samples/b...). You'll get exposed to core concepts like Lighting, Terrain, Particles, 2D GUIs and Lua Scripting, all implemented from the ground up in C++ and OpenGL.