Thursday, October 1, 2009

GLES Quake - a port of Quake to the Android platform

I've just published the source code to a port of Quake to the Android platform. This is something I did a while ago as a internal test application.

It's not very useful as a game, because there hasn't been any attempt to optimize the controls for the mobile phone. It's also awkward to install because the end user has to supply the Quake data files on their own.

Still, I thought people might enjoy seeing a full-sized example of how to write a native code video game for the Android platform. So there it is, in all its retro glory.

(Porting Quake II or Quake III is left as an exercise for the reader. :-)

What's different about this particular port of Quake?

Converted the original application into a DLL

Android applications are written in Java, but they are allowed to call native languge DLLs, and the native language DLLs are allowed to call a limited selection of OS APIs, which include OpenGL ES and Linux File I/O.

I was able to make the game work by using Java for:
  • The Android activity life-cycle
  • Event input
  • View system integration
  • EGL initialization
  • Running the game in its own rendering loop, separate from the UI thread
And then I use C++ for the rest. The interface between Java and C is pretty simple. There are calls for:
  • initialize
  • step - which does a game step, and draws a new frame.
  • handle an input event (by setting variables that are read during the "step" call.)
  • quit
The Java-to-C API is a little hacky. At the time I developed this game I didn't know very much about how to use JNI, so I didn't implement any C-to-Java calls, preferring to pass status back in return values. For example the "step" function returns a trinary value indicating:
  1. That the game wants raw PC keyboard events, because it is in normal gameplay mode.
  2. That it wants ASCII character events, because it is in menu / console mode.
  3. That the user has chosen to Quit.
This might better have been handled by providing a series of Java methods that the C code could call back. (But using the return value was easier. :-))

Similarly, the "step" function takes arguments that give the current screen size. This is used to tell the game when the screen size has changed. It would be cleaner if this was communicated by a separate API call.

Converted the Quake code from C to C++

I did this early in development, because I'm more comfortable writing in C++ than in C. I like the ability to use classes occasionally. The game remains 99% pure C, since I didn't rewrite
very much of it. Also, as part of the general cleanup I fixed all gcc warnings. This was fairly easy to do, with the exception of aliasing warnings related to the "Quake C" interpreter FFI.

Converted the graphics from OpenGL to OpenGL ES

This was necessary to run on the Android platform. I did a straightforward port. The most difficult part of the port was implementing texture conversion routines that replicated OpenGL functionality not present in OpenGL ES. (Converting RGB textures to RGBA textures, synthesizing MIP-maps and so forth.)

Implemented a Texture Manager

The original glQuake game allocated textures as needed, never releasing old textures. It relied upon the OpenGL driver to manage the texture memory, which in turn relied on the Operating System's virtual memory to store all the textures. But Android does not have enough RAM to store all the game's textures at once, and the Android OS does not implement a swap system to offload RAM to the Flash or SD Card. I worked around this by implementing my own ad-hoc swap system. I wrote a texture manager that uses a memory mapped file (backed by a file on the SD Card) to store the textures, and maintained a limited size LRU cache of active textures in the OpenGL ES context.

Faking a PC Keyboard

Quake expects to talk to a PC keyboard as its primary input device. It handles raw key-down and key-up events, and handles the shift keys itself. This was a problem for Android because mobile phone devices have a much smaller keyboard. So a character like '[' might be unshifted on a PC keyboard, but is shifted on the Android keyboard.

I solved this by translating Android keys into PC keys, and by rewriting the config.cfg file to use a different set of default keys to play the game. But my solution is not perfect, because it is essentially hard-coded to a particular Android device. (The T-Mobile G1). As Android devices proliferate there are likely to be new Android devices with alternate keyboard layouts, and they will not necessarily be able to control the game effectively using a G1-optimized control scheme.

A better approach would be to redesign the game control scheme to avoid needing keyboard input.

A Few Words about Debugging

I did the initial bring-up of Android Quake on a Macintosh, using XCode and a special build of Android called "the simulator", which runs a limited version of the entire Android system as a native Mac OS application. This was helpful because I was able to use the XCode visual debugger to debug the game.

Once the game was limping along, I switched to debugging using "printf"-style log-based debugging and the emulator, and once T-Mobile G1 hardware became available I switched to using real hardware.

Using printf-style debugging was awkward, but sufficient for my needs. It would be very nice to have a source-level native code debugger that worked with Eclipse.

Note that the simulator and emulator use a software OpenGL ES implementation, which is slightly different than the hardware OpenGL ES implementation available on real Android devices. This required extra debugging and coding. It seems that every time Android Quake is ported to a new OpenGL ES implementation it exposes bugs in both the game's use of OpenGL ES and the device's OpenGL ES implementation.

Oh, and I sure wish I had a Microsoft PIX-style graphics debugger! It would have been very helpful during those head-scratching "why is the screen black" debugging sessions.





4 comments:

Andrex said...

"Oh, and I sure wish I had a Microsoft PIX-style graphics debugger! It would have been very helpful during those head-scratching 'why is the screen black' debugging sessions."

Hrm, wouldn't DDMS do the trick? Or does that not work with OpenGL or the NDK?

Jack Palevich said...

DDMS when used with a front end like Eclipse is a great Java code debugger.

But PIX is a graphics debugger, that lets you debug the graphics calls and how they affect the frame buffer.

Jared said...

Now do Wagic =D

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