OpenGL on Android tips

I’ve been working on an 2d rendering engine for OpenGL on the Android platform using the standard Java API. In my ramblings around the internet there seems to be a misunderstanding about how some portions of the API were intended to be used. In particular the use of vertex, texture and colour arrays and the draw arrays function seems to be commonly misused from what I have read on forum posts around the net. Here I will list a few short tips about making your OpenGL apps on the Android platform run a bit smoother.

The first mistake that seems to happen a lot is the constant creation and destruction of objects in the rendering thread. Android devices and other mobile or embedded systems are very limited in their nature, they do not have much RAM and the processor is not as powerful as your standard PC or mac. Because of this it is important to keep the number of objects pretty constant within your games, otherwise the garbage collector will slow things down constantly recycling objects that you create and destroy on a regular basis.

The second mistake I’ve seen has to do with how the buffers are mis-used. It seems many people are regularly changing the contents of the buffers to move or change the location, size or even rotation on-screen. My mind boggles as to why you would do this over using the standard GL model view matrix operations. It’s my belief that you should create the buffers that hold the actual vertex information once, then in your renderer use glTranslate, glRotate and glScale to achieve any transforms required. The reasons for this are two-fold, firstly if you modify your buffers all the time you will need to modify all the vertices to achieve simple things like translate. Some people have reported that functions on the buffer objects can be slow on some devices, so it is best to use them sparingly. Secondly using the GL functions and matrix operations should be optimised in native code for each device, and so they should be faster. This should also apply to using the OpenGL library on other platforms with Java, mostly because of its interpreted nature. This would be different in some cases if you were writing code for a native language, as I’m told state changes to the GL matrices can be slower than changing the vertices, but this is only true for a small number of vertices where the computation time to change them is trivial.

The final tip I have for you is to make use of multiple threads for your app. You have a very limited amount of processing power so taking advantage of the fact many of the more modern devices are dual core is imperative. The simplest way to do this is to create a separate thread for the main logic of your app, leaving the main rendering thread to perform the main task you want it to, updating the display. Writing multithreaded applications is more complicated in some instances, but java provides some nice mechanisms that you should take advantage of. Declaring objects or just some methods on an object synchronized is a good way to achieve this. Java only allows one thread to execute synchronized methods on objects at a time, but the lock is done at the object level and not the method level. This means you can effectively make an interface object between both threads that is what we call thread safe. This means you are less likely to run into race condition bugs, and hopefully you’ll also avoid deadlock. Please note you should try to keep synchronized methods short to minimise the amount of time another thread is held up. This will help improve the responsiveness of your app. For more information on Java synchronization click here.

I hope this information helps some of you out there trying to build your own apps for Android. Many of these tips are also pretty good advice for building Applications using Java and OpenGL on any platform. If you still run into performance problems you should run a debugging tool called a profiler to find out which code is causing the most slow down. It’s quite important to remember the limited nature of resources on mobile devices, and that disposing of objects can come at quite a price in processing power when the garbage collector kicks in.

2 Responses to “OpenGL on Android tips”

  1. April 9, 2012 at 3:57 am

    Thanks for the tips.

  2. April 11, 2012 at 12:21 am

    I’ve discovered something else that may help a few people. I was having troubles with my transforms not working, inparticular the orthographic projection and translation. I discovered the reason was because I was using the integer version of the transform instead of the float one. For instance I was using orthox(…) instead of orthof(…). After changing all my transforms over to floats everything works fine. It was a pain of a bug to find!

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