Wednesday, August 13, 2014

Test Notes on Emscripten

Recently, I tried the Emscripten compiler, which can compile C/C++ to HTML5/JavaScript. Also based on LLVM, it is a similar tool as the Adobe CrossBridge, but targeting a different platform. With Emscripten, it's possible not to write even a single line of JS for creating an HTML5 application. But for CrossBridge, you may still need to write some AS3 to make things work. Emscripten has very good SDL and OpenGL support, so you can compile your SDL/OpenGL application into JS without getting hands dirty of JavaScript.

Emscripten is similar to CrossBridge, in may aspects. For Emscripten, like the CrossBridge, you also need to break the C/C++ main infinite loop and use "emscripten_set_main_loop()" to run the loop content frame by frame. However, CrossBridge supports multi-threads and background workers, which is actually an advantage of AS3 over current JS. Both use similar inline asm (AT&T style) for interlope with the targeting languages. Their ways of dealing with the file system are similar - both use a virtual system to simulate C/C++ reading & writing processes and both provide tools for packaging assets. Besides, both CrossBridge and Emscripten can be used as code obfuscators, Emscripten even use the closure compiler for optimizing and obfuscating the generated JavaScript.

I thought the Emscripten SDK is mature enough. Even Unity3D and Unreal engine are using it for publishing HTML5, after they abandoning FlasCC and the flash platform. Also see the Porting Examples and Demos, especially the amazing Cube2 engine demo (The CrossBridge Cube2 port is not yet rendering correctly). However, in my test, for the latest Emscripten SDK (emsdk-1.21.0-web-64bit.exe), if you want to port your SDL applications or games, very likely, you will fail. That's because the support of SDL, although is working for many cases, is far from complete. What's more, the officially supported SDL version is SDL 1.3, while most old games/application are based on SDL 1.2 and most new ones are based on the latest SDL 2 (There're many API differences among these versions). Many SDL functions are not supported or implemented, such as very commonly used "SDL_DisplayFormat", "SDL_ConvertSurface". So when you compile, you may get "unresolved symbols" warnings and a black screen - the compiled application will just not run. Although there is an unofficial SDL port for Emscripten, I'm not sure whether it is worthy of trying.

Fortunately, Emscripten SDK is under very active development, unlike the almost abandoned CrossBridge. I'm looking forward to a future version with more features and great improvements. By the way, there is an unofficial fork of the CrossBridge SDK, which pulled many things together and collected lots of  examples:, I'm also watching on the project, hope it will continue to involve.

I think, the same problem for both CrossBridge and Emscripten is, there are not many games/applications written in C/C++ there waiting to be ported to the browser platform, so only a few people are actually using these cross language compilers. The reason is most games written in C/C++ are not designed for the browser platform. So if there exists a tool can compile AS3 to JavaScript, I believe it will be very, very popular. And one factor that Emscripten will be more popular than CrossBridge is that JavaScript is not a suitable language for programming large scale games and applications, but AS3 itself is good enough to handle many large projects.

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