U3D (https://urho3d.io/) is a free lightweight, cross-platform 2D and 3D game engine implemented in C++ and released under the MIT license. It utilizes either Direct3D9 or OpenGL for rendering (Shader Model 2 or OpenGL 2.0 required as minimum.)

For getting started, see:

Building Urho3D library
Using Urho3D library
Running Urho3D player application
Examples
Overall structure
Conventions
Editor instructions
Miscellaneous how-to's

For further reference, see:

Container types
Object types and factories
Subsystems
Events
Engine initialization and main loop
Scene model
Resources
Localization
Scripting
Lua scripting
Rendering
Input
Audio
Physics
Navigation
Inverse Kinematics
User interface
Urho2D and Physics2D
Serialization
Networking
Database
Multithreading
Attribute animation
Tools
Unicode support
Custom file formats
Coding conventions
Contribution checklist
Scripting API (generated)
Lua scripting API (generated)
Event list (generated)
Attribute list (generated)

For U3D related links and projects, see External links.

For credits, copyright and licensing information, see Credits & License.

For release history and major changes, see History.

Also see Porting notes for necessary changes when moving to a newer U3D version.