问题
I'm developing an engine and a game at the same time in C++ and I'm using box2D for the physics back end. I'm testing on different android devices and on 2 out of 3 devices, the game runs fine and so do the physics. However, on my galaxy tab 10.1 I'm sporadically getting a sort of "stutter". Here is a youtube video demonstrating:
http://www.youtube.com/watch?v=DSbd8vX9FC0
The first device the game is running on is an Xperia Play... the second device is a Galaxy Tab 10.1. Needless to say the Galaxy tab has much better hardware than the Xperia Play, yet Box2D is lagging at random intervals for random lengths of time. The code for both machines is exactly the same. Also, the rest of the engine/game is not actually lagging. The entire time, it's running at solid 60 fps. So this "stuttering" seems to be some kind of delay or glitch in actually reading values from box2D.
The sprites you see moving check to see if they have an attached physical body at render time and set their positional values based on the world position of the physical body. So it seems to be in this specific process that box2D is seemingly out of sync with the rest of the application. Quite odd. I realize it's a long shot but I figured I'd post it here anyway to see if anyone had ideas... since I'm totally stumped. Thanks for any input in advance!
Oh, P.S. I am using a fixed time step since that seems to be the most commonly suggested solution for things like this. I moved to a fixed time step while developing this on my desktop, I ran into a similar issue just more severe and the fixed step was the solution. Also like I said the game is running steady at 60 fps, which is controlled by a low latency timer so I doubt simple lag is the issue. Thanks again!
回答1:
As I mentioned in the comments here, this came down to being a timer resolution issue. I was using a timer class which was supposed to access the highest resolution system timer, cross platform. Everything worked great, except when it came to Android, some versions worked and some versions it did not. The galaxy tab 10.1 was one such case.
I ended up re-writing my getSystemTime() method to use a new addition to C++11 called std::chrono::high_resolution_clock. This also worked great (everywhere but Android)... except it has yet to be implemented in any NDK for android. It is supposed to be implemented in version 5 of the crystax NDK R7, which at the time of this post is 80% complete.
I did some research into various methods of accessing the system time or something by which I could base a reliable timer on the NDK side, but what it comes down to is that these various methods are not supported on all platforms. I've went through the painful process of writing my own engine from scratch simply so that I could support every version of android, so betting on methods that are inconsistently implemented is nonsensical.
The only sensible solution for anyone facing this problem, in my opinion, is to simply abandon the idea of implementing such code on the NDK side. I'm going to do this on the Java end instead, since thus far in all my tests this has been sufficiently reliable across all devices that I've tested on. More on that here:
http://www.codeproject.com/Articles/189515/Androng-a-Pong-clone-for-Android#Gettinghigh-resolutiontimingfromAndroid7
Update
I have now implemented my proposed solution, to do timing on the java side and it has worked. I also discovered that handling any relatively large number, regardless of data type (a number such as the nano seconds from calling the monotonic clock) in the NDK side also results in serious lagging on some versions of android. As such I've optimized this as much as possible by passing around a pointer to the system time, to ensure we're not passing-by-copy.
One last thing too, my statement that calling the monotonic clock from the NDK side is unreliable is however, it would seem, false. From the Android docks on System.nanoTime(),
...and System.nanoTime(). This clock is guaranteed to be monotonic, and is the recommended basis for the general purpose interval timing of user interface events, performance measurements, and anything else that does not need to measure elapsed time during device sleep.
So it would seem, if this can be trusted, that calling the clock is reliable, but as mentioned there are other issues that then arise, like handling allocating and dumping the massive number that results which alone nearly cut my framerate in half on the Galaxy Tab 10.1 with Android 3.2. Ultimate conclusion: supporting all android devices equally is either damn near or flat out impossible and using native code seems to make it worse.
回答2:
I am very new to game development, and you seem a lot more experienced and it may be silly to ask, but are you using delta time to update your world? Altough you say you have a constant frame rate of 60 fps, maybe your frame counter calculates something wrong, and you should use delta time to skip some frames when the FPS is low, or your world seem to "stay behind". I am pretty sure that you are familiar with this, but I think a good example is here : DeltaTimeExample altough it is a C implementation. If you need I can paste some code from my Android Projects of how I use delta time, that I've developed following this book : Beginning Android Games.
来源：https://stackoverflow.com/questions/9274932/strange-stutter-in-box2d-on-different-android-devices