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Wed, 20 Jun 2012

Android, and porting C++ and OpenGL via the JNI

I've been interested in the idea of porting free software to Android since I started working with Android. The first free software programs I considered doing an Android port of were written in Java. The reason I looked at Java programs first is Android seems to have a slight preference for Java over C and C++.

When investigating various Java programs for potential ports, I realized that porting the UI portions of the programs over, particularly ones that used Java graphical libraries such as awt or swing, would be difficult. Android does not implement these graphical libraries.

So then I began investigating free software Java libraries. One popular one which caught my eye was Jackcess, which could read Microsoft Access database files. I wrote a little Android UI wrapper around the library, and within a few days was able to release Panacea Database. Since its release, I have added more functionality to the program. I still have not tapped all of the library's functionality, such as for database creation.


The idea of porting C and C++ free software programs to Linux, especially ones using "OpenGL" family graphics, has been in the back of my mind for a while. An informative conversation I had with Katie from Golden Hammer Software at the 2011 Android Developer Labs pushed me along this route as well, not just in learning about the technical aspects of porting C++ apps to Android, but seeing how it was feasible.

When you're looking at doing OpenGL work on Android, one of the important things to know is that Android does not do OpenGL. Android handles OpenGL ES, which is a library which only handles a subset of what OpenGL does. OpenGL ES does not have all of the features that OpenGL does. For example, OpenGL ES does not handle OpenGL begin and end commands. You can not directly specify rectangles on OpenGL ES like you can on OpenGL. And so on.

Apple iOS uses an implementation of OpenGL ES as well. Porting C or C++ code which uses OpenGL ES from iOS to Android (or vice versa) is not that hard. This in fact is what Golden Hammer Software did. Porting Windows or Linux code that uses a full OpenGL library to Android is a much more difficult enterprise.


Porting a C or C++ program that directly links to a full OpenGL library to Android is going to be a little bit of work. This brings us to the Simple DirectMediaLayer (SDL) library. The Simple DirectMedia Layer is a cross-platform multimedia library designed to provide low level access to UI elements of a program (audio, keyboard, mouse, joystick, 3D hardware via OpenGL, and 2D video framebuffer).

Many programs that directly depend on the SDL library have no direct dependencies on OpenGL - the programs use SDL to mediate access to the needed lower-level backend libraries.

Most programs that depend on SDL were written to depend on the SDL 1.2 or lower library. SDL has being rewritten since version 1.3, and is not backward compatible with 1.2. Here, we are only concerned with SDL 1.2 and lower, which is what the majority of the software out there uses. There is a unofficial port of SDL 1.2 to Android, which was mostly done by Sergii Pylypkeno (pelya).

Pelya has ported 13 SDL games to Android and put them up on Google Play. One of the apps, OpenTTD, has had over 100,000 downloads so far, and another, Free Heroes 2, also has had over 100,000 downloads. FH2 currently has a rating of 4.2 out of 5, so people seem to be happy with the port. With these games done, pelya has said he is finished porting any more games, but he is still maintaining the SDL 1.2 library for Android.

His library has its own unique little build system. I am developing on an Ubuntu GNU/Linux desktop, and am comfortable with using the command line if need be, so it is fine with me.

The way he sets things up with his build system, he has the jni directory with various libraries a lot of sdl applications will need, such as of course sdl itself, the freetype library, the jpg library, the physfs library, and other such libraries. Among these he has an application sub-directory named application. Within it is a link called src which points to the application being ported within it - such as OpenTTD, or Free Heroes 2, or whatever.

I started off by trying to build every application he had within that application directory. He suggested to try ballfield first, and that is easy to compile and test. Grafx2, Jooleem, Kobodeluxe, Milkytracker, Openal-demo, Opentyrian, Regression, Simplemixer, Test32bpp and Testmultitouch all worked OK. Others failed before compiling for various reasons, or did launch but were still broken - perhaps I needed to tweak the settings more.

He published then unpublished Jooleem. I thought it was pretty cool and e-mailed him saying I wanted to release it, but was there some unknown reason he unpublished it to Google Play. He said there wasn't, so I did some work on it, then published it. He may have been right - the game does not have a high download rate, nor does it have a high retention rate compared to other SDL ports I did later.

Having some experience with working with the stuff he ported, especially Jooleem (which I now call Bubble Boxem), I decided to try porting a game that pelya had not tried yet. Circus Linux was a small and simple program that used the SDL library, so I decided to port that. I succeeded in porting it as well.

Much of what is needed is in pelya's instructions. First you want to compile the program. The instructions explain how to do that. If there is a data subdirectory, it should be zipped up, moved to AndroidData as the instructions explain, split if necessary and if split, the original data.zip removed. You want an icon.png file for the program icon. Then once you get it compiling, you want it to run. If nothing appears on the screen, __android_log_write and __android_log_print can help. Start at the beginning of main(), looking for output in logcat, then continue until you find the first problem. Then the second one. At some point, hopefully, the program will load.

Why SDL programs won't compile or run can differ from program to program, but I've found common themes. The first four listed are the most important to remember.

  • The C++ code says to run in hardware mode instead of software mode when Android can not do so.
  • Looking for directories with configuration files and graphics can be another problem, you have to set it up properly.
  • Check if it is looking for defines in a config.h file. These defines will have to be set properly for Android. Also look for similar defines not in the config.h files, like a define for LOCALE or the like.
  • The SDL_SetVideoMode call might have parameters Android can not handle
  • Pelya's framework script does not compile C++ files with a suffix of cc instead of cpp.
  • Stuff from iostream like cout and cerr do not work out of the box. Neither do XM audio files

The above list covers every problem I've had so far with compiling or getting a screen to come up on Android.

Now that something is coming up on the screen, you may want to consider replacing SDL_UpdateRect calls with SDL_Flip calls, or you may get some gibberish on the screen. The SDL port does not currently handle SDL_UpdateRect calls well.

You also want to make sure the volume button is working when in the SDL app. if you want to use it, make sure it is not redefined as a key. Explicitly listening to KeyEvent.KEYCODE_VOLUME_DOWN or KeyEvent.KEYCODE_VOLUME_UP and manually implementing adjustVolume also works.

Another consideration is the keyboard, and seeing visible text on the screen. With pelya's framework, text appears in an EditText (which I sometimes move around on the screen, change colors of etc.) You can have a keyboard pop up on the screen and so forth. It is something to think about

Sometimes the game just needs the arrow keys, and maybe a few more keys. Pelya's framework has mechanisms to deal with this. I use one such mechanism in my Ice Blocker game, when a player wants to switch from horizontal to vertical (or vice versa).

Future plans

So far I have ported six games to Android using pelya's Android SDL library. I am looking to see if there are any more good free software SDL apps to port over. Most of the games I've ported were primarily mouse-based games - they are now touch-based games. So the aesthetics have not changed that much for those particular games. In addition to this, most of the games I've ported have had a fairly simple graphical library dependency - on SDL. In the future I might port games with more of a keyboard (or arrow key) dependency. I also might port games which have more of a direct OpenGL dependency.

I also am interested in expanding the existing games I have. I am interested in doing more work through the Java/C++ JNI bridge in the games I have already done. I also am thinking about how to handle different languages and internationalization. Android's bionic library can not handle locale. This means gettext and it's portable object (po) and message object (mo) files do not work out of the box. Garen Torikian has been nice enough to give me some advice about this, and I might do translations in something along the lines of how he did it in Neverball ME

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