I've finally got OpenGL rendering Banished with identical output to both the DX9 and DX11 renderers. I've still got a little bit of work to support the boring parts of a rendering engine, such as handling window resizes, switches to fullscreen, and handling renderer changes at runtime. But all that's platform specific code. What really matters here is I now have a working GL implementation that should (hopefully) seamlessly work on OSX and Linux.
So for better or worse, I've designed my own shading language, written a compiler that parses it, it gives syntax and semantic errors, and if there are no errros, outputs the program in several other shading languages. This way, I can write a shader once and target multiple platforms. And plus, now I own the Dragon Book!
Getting a new graphics API running in an existing game engine is somewhat painful - there's always some kind of issue or architectural oversight that doesn't meld well with new APIs. But the game engine for Banished is written in such a way to abstract all the graphics functionality behind a common API, so porting it isn't as awful as it could be. The game can already switch between DirectX 9 and 11 at runtime, so adding OpenGL to the mix shouldn't be too bad.
With the mod kit and steam workshop out there, I've been working on porting the game to OSX and Linux, cleaning up code, and writing some new code. I'll still be fixing bugs and making small changes, but my focus now is going to be on ports, and building prototypes for new games.
