Having just finished building the UI for the YouView set top box, I thought I’d share some of my insights into best practices when building applications for such resource constrained devices. The YouView UI is AIR based, written in AS3 and runs in Stagecraft 2, also known as ‘AIR for TV’. As the name suggests, AIR for TV is a special version of the Flash player for embedded systems, such as set top boxes. The first incarnation of the YouView UI (back when it was just codenamed ‘canvas’) was for Stagecraft version 1, which means coding in AS2 and suffering the abysmal performance that comes with running on AVM1 (ActionScript Virtual Machine 1).
Despite the delays and the need to code the UI from scratch in AS3, I think it was ultimately the right decision. Stagecraft 2 is a much better platform – Stagecraft 2.5.1 to be precise. It was a great opportunity to learn how to write optimal code and use hardware acceleration effectively on resource constrained devices. I’ll be doing some tutorials on this in the near future, but here’s the key points to observe when developing for such platforms:
Limit the complexity of your display list heirarchy
This may sound obvious, but ensure you nest as few things as possible, keeping the display list as shallow as possible. Stagecraft needs to traverse through the display list, working out which areas of the screen to redraw. This is similar to how the desktop Flash Player handles redraws, but with some key differences to how it decides what needs redrawing, how it tackles moving display objects and how it delegates the work of updating the frame buffer – a subject for another time. Mostly importantly, if you’re developing for a resource constrained device (such as mobile or set top box), you’ll have very limited CPU power, even if the device’s GPU (graphics processing unit) affords you great hardware acceleration capabilities. So, before Stagecraft can delegate any work to hardware, it enumerates changes in the display list in software. Complex display list heirarchies are a headache for some of the low-powered CPUs found in mobiles and set top boxes and this’ll show up as rocketing CPU usage, low framerates and few spare ‘DoPlays’ in Stagecraft (spare work cycles). By keeping your display list shallow, with only the bare minimum of display objects on stage at any one time, you’ll be making life easier for Stagecraft by doing less work on the CPU – whether or not graphics are drawn in software or hardware.
When building an application for a resource constrained device, you should be able to run each component in isolation, to assess its drain on CPU and system/video memory. There’s no point optimising the hell out of one component, when it’s actually another one that is the source of your performance bottleneck.
Know thine hardware acceleration capabilities
There’s no point blindly using cacheAsBitmap and cacheAsBitmapMatrix everywhere, if it’s not going to speed things up on the target device. Worse still, too many cacheAsBitmaps and you may be just wasting valuable video memory, or causing unnecessary redraws (again, the subject of a future article). A lot of platforms will accelerate bitmaps, even if stretched, but not necessarily if flipped or rotated. Alpha on bitmaps (or anything cached as bitmap) will usually be accelerated too, but this is not necessarily the case with all colour transforms. Benchmarking any component you’re building will quickly tell you where you might have pushed it too far, but you should also have a way of verifying that a particular set of transforms is indeed hardware accelerated. Stagecraft provides this when using its –showblit command line parameter. I’ll be going into more detail about this in another post.
Mind your memory
When using various hardware acceleration tricks, especially on resource constrained devices, video memory is at a premium and usually in limited supply. You will need to know the limits and have a way of seeing how much video memory your application is using at any one time – ensuring you dispose and dereference any bitmaps you’re finished with too. If your platform uses DirectFB for its rendering, as YouView does, the executable ‘dfdump’ can show you just where your video memory is going. This is something else I’ll get into in another article.
Blit blit blit
This refers to blitting, where blocks of pixels are copied from one bitmap to another. This technique is used a lot in games, where graphics performance is critical, you should arm yourself with the basics of how old video games used blitting of multiple things to a single bitmap for performance and video memory efficiency.
I’ll probably go into more depth on each of these things in forthcoming posts. Stay tuned.
It gets scary out there sometimes. During my freelance career I’ve worked a lot of different companies and have seen such coding horrors as you cannot imagine. So I thought I’d start immortalising some of them – so that we can all learn better coding practices, by looking at the bad.
Starter for 10 – What’s wrong with this picture?
public function set someProperty(value:String):void
var newPixels:BitmapData = new BitmapData(someUint, someUint, true, 0);
someBitmap.bitmapData = newPixels;
If it’s not immediately obvious, the fubar here is the potential replacement of a Bitmap’s BitmapData, without explictly disposing any existing BitmapData. I see this kind of thing quite often and it’s the source of many a memory leak. AVM2 isn’t that great and dealing with this kind of situation and there’s a crucial difference between GC cleaning up out-of-scope objects for you and things like BitmapData: GC will reclaim the memory associated with objects ‘when it feels like it’, whereas explicitly calling the ‘dispose’ method of a BitmapData will immediately give you back that memory. In the case of hardware accelerated setups, the memory associated with the pixel data itself (video memory) will be reclaimed immediately. You shouldn’t make more work for GC when you can avoid it – defensive programming is always best!
Having just got back from an awesome break in Thailand and Taiwan, I just updated my object pooling utility. Thanks to Ostra Ceruzka for raising an issue with its strict mode functionality with me – all fixed now! Serves me right for not writing enough tests, huh?
A couple of years ago, I created an AS3 object pooling utility for a games project I was building. Since then, I’ve used it quite a few times, in order to speed up apps and improve resource management, easing the load on the garbage collector by reusing objects instead of recreating them.
While object pooling isn’t a magic bullet to speed up every use case, it works especially well with custom classes that are heavy to continually contruct and destroy. A good example is my History of the World project, which uses an object pool for item renderers, instead of creating and destroying them as you navigate around – press ALT+CTRL to bring up the resource debugger, which shows a little information on its usage.
I recently updated the utility, improving its performance, adding features and putting loads of unit tests around it. It’s now hosted it over at GoogleCode. Using it is a simple as:
var pool:LoanShark = new LoanShark(SomeClass);
var someInstance:SomeClass = pool.borrowObject();
// Instead of nullifying an object, check it back into the pool
In a simple situation, where you wish to add many elements to an Array or Vector, you might just do:
However, the sizes of both Arrays are manipulated for each loop, which will have an adverse impact on speed and memory usage. So, we could cache the length of the input Array and not manipulate it:
var inputLength:uint = input.length;
for (var i:int = 0; i < inputLength; i++)
But we’re still growing the size of the output Array incrementally, which is very bad. Since we know input.length in advance, we could grow the output Array to its new size just once, before the loop:
var inputLength:uint = input.length;
var outputLength:uint = output.length;
var newOutputLength:uint = outputLength + inputLength;
output.length = newOutputLength;
for (var i:int = 0; i < inputLength; i++)
output[i + outputLength] = input[i]);
This is OK, but still involves a loop. If only we could push multiple elements into the push method in one go. Well, we can – enter the apply method. Since Array.push accepts multiple arguments (something rarely used) and apply allows us to pass an Array of arguments to any Function, one line and we’re done:
This works out faster and more memory efficient than the other methods. It works nicely for Vectors, too. If anyone has a faster method of doing this, do let me know.
I created this simple utility, called SWFIdle, to enable the Flash Player to lower its CPU usage while the user is not interacting with it. Since it’s possible to have multiple Flash instances embedded in one page (for example, a game and a couple of banners), I recommend that everyone uses this in their projects, so that SWFs needn’t fight for CPU and give Flash a bad name.