Intense, arcade-style tower defense for Android

The Story

HexDefense started as a class project for a mobile prototyping lab I took while at Carnegie Mellon. The lab required that apps be written in Java on the Android platform, and I figured it’d be a good opportunity to try writing a game. I’m a big fan of the tower defense genre and I’ve been heavily influenced by games on the iPhone like Field Runners and GeoDefense Swarm. From the outset, I wanted the game to have arcade style graphics reminiscent of Geometry Wars. That way, I figured, I wouldn’t have to find an artist to create the sprites, and I could focus on explosive OpenGL particle effects and blend-based bloom.

During the fall semester, I collaborated with Paul Caravelli and Tony Zhang on the first iteration of the game. I had the strongest graphics and animation background, so I focused on the gameplay and wrote all of the OpenGL code behind the game. I also created most of the game model, implementing the towers and creeps and creating actions with game logic for tower targeting, attacks, projectile motion, explosions, implosions and other effects. Paul contributed path finding code for the creeps based on breadth-first-search and created interfaces for implementing in-game actions based on the command pattern. He also contributed the original implementation of the grid model and worked on abstract base classes in the game model. Tony created the app’s settings screen and linked together activities for the different screens of the application.

At the end of the fall semester, the game was functional but unrefined. There were no sounds, no levels, and I’d only created one type of enemy. After the class ended, I talked with Paul and decided to finish it over my Christmas break. Paul was too busy to continue working on the app, so I continued development independently. I worked full-time for four weeks to deliver the level of polish I was accustomed to on the iPhone. I refined the graphics, tested the app across a variety of phones and added fifteen levels. I also added 3D directional sound, boss creeps and wrapped everything in a completely new look and feel. People say that the last 10% is the 90% of the work, and I think that’s particularly true on Android – there are minor differences across devices that make writing a solid game a lot more work than I expected.

The game was released at the end of January and has been well received so far. I created a lot of promotional art and setup a website with gameplay footage and press resources, and the game has garnered quite a bit of attention. It’s been featured on the front page of the Android marketplace and has a 4 1/2 stars. It’s rising in the “Paid Apps” rankings and is currently the #16th most popular game on the Android platform!

Lessons Learned:

I’ve learned a lot about the Android platform developing HexDefense. A couple of tips and takeaways:

  1. Let the OpenGL view run in CONTINUOUS mode. Nothing else (timers, threads that trigger redraws) will give performance close to this.
  2. Write all of the game logic so that it can advance the model by an arbitrary number of milliseconds. Because multitasking can cause hiccups in the game framerate, this is _really_ important for a smooth game.
  3. OpenGL textures are not numbered sequentially on all devices. The original DROID will choose random integer values each time you call glGenTexture.
  4. There are numerous drawbacks to using the Java OpenGL API. If your game needs to modify vertex or texcoord buffers every frame, you’ll have to accept a performance hit. The deformation of the grid in HexDefense is achieved by modifying the texcoords on a sub-segmented plane, and passing the data through a ByteBuffer to OpenGL is not cool.
  5. The iPhone’s OpenGL implementation is at least 2.5x faster, even on devices with half the processor speed. An iOS port of HexDefense is in progress, and the game runs twice as fast on an original iPod Touch as it does on a Nexus One. There are a lot of reasons for this, but it seems that drawing large textured quads has greater speed implications on Android devices.