Thursday, July 5, 2012

Scripting BananaBread / Using Compiled C/C++ Code in JavaScript

In BananaBread we compile an entire 3D game engine from C++ into JavaScript. The simplest way to use it is to compile everything you need and just run it. However, it might be useful to let the compiled code be controlled from JavaScript, that way you can use normal JavaScript to control the game engine. Here are an examples of that: Fireworks Demo.

In that demo two APIs are provided from the compiled game engine: camera control and particle effect creation. The scripting API used in them begins here, and an example use can be seen here.

How does this work? There are 4 main steps to accessing compiled C/C++ from normal JavaScript:
  • Make a C API if the code is in C++. You can use C++, but then you need to deal with name mangling and this pointers in a manual way - C is easier. Example.
  • Use EMSCRIPTEN_KEEPALIVE to keep the code alive. EMSCRIPTEN_KEEPALIVE is a macro that uses compiler attributes to tell the compiler not to eliminate code as dead even if it isn't used (it will be used from JavaScript, but without this the compiler doesn't know that). Example.
  • Export the function through Closure Compiler. In -O2 closure compiler is used to minify and optimize the code. As a consequence, the original function names are unrecognizable, and closure will also remove code it sees is never used. The way to do this is to add the function to EXPORTED_FUNCTIONS when calling emcc to compile to JavaScript. The function will then show up on the Module object even after closure compiler runs (side note, all of the exports through closure are on the Module object, for example you can access memory through Module.HEAP8, etc.). Example (scroll to EXPORTED_FUNCTIONS).
  • Access the code through ccall or cwrap. ccall does a one-time call to a function, while cwrap returns a native JavaScript function that wraps the C function. Both take as arguments the return type and argument types (see docs). Example.
You can then access the code, and if you want, you can write a nice JavaScript API on top of the C-like interface ccall/cwrap give you.

Returning to BananaBread specifically, we now have the infrastructure to allow JavaScript access to all the compiled game engine's functionality. Right now as mentioned before we have camera and particle effect APIs (was very quick to start with those), but straightforward work can let us control the how characters move, the rules of the game (how you earn points, get ammo, etc.), how objects behave, how weapons work, etc. The underlying engine is mainly used for first person shooters, but it is easy to use it for other things, from visual demos like the fireworks from before to 2D games to non-game 3D virtual worlds and so forth, once you have the proper scripting APIs in place (in fact I did something very similar a few years ago using the same engine).

If that kind of thing is interesting to you please get in touch, ideas for how to design the JavaScript part of the API are welcome.

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