How to avoid uncompressing bitmapDatas multiple times

Question

Hi,
Not sure if this should go here (performance) or IOS section, since I mainly have this issue on iOS but I think its on all air players.
When a bitmap (for exemple a bitmap in a fla) is added, you can see with Adobe SCOUT (great tool btw!) that it is uncompressed when added. Allright.
The bad thing is, if it is removed from stage, even if you keep it in memory for reuse, it is very often re-uncompressed when redisplayed. I mean 'often' because it seems erratic. If it stay removed for just a second, most of the time it won't be uncompressed again, but after 3 or more seconds, most of the time it will be (some kind of garbage collector in the "video" memory ?)
Setting its quality to lossless in the fla helps the decompression goes a bit faster (30-40 % I would say), on the detriment of swf size, but does not solve the main question : how can I force an animation (made of bitmaps for example) to never be uncompressed again as long as I dont tell it to to be "disposed" from wherever this is stored ?
Having it permanently on screen hidden somewhere is just too much a performance killer to be considered a solution when you have many. CacheAsBitmapMatrix could be usefull for vector data  on this issue (not sure), but drawing a bitmap seems a bit stupid if you do already have bitmaps.
It might not seem a big issue but in a game with effects etc. most of those will only show up once every 2 to 10 seconds, but if you loose 5ms/frame for 40 frames each time one does, and you have many, it really impact performance on devices like ios etc, whereas memory doesnt seem to be the problem here, it happens with low or high memory usage both.
If anyone knows a tip, thanks a lot!
EDIT : Tests made with gpu mode, flash builder 4.7, air 3.5, iPad3 (or other), release build.

Answer 1

In my experience, the second solution here: actionscript 3 - Bitmaps Being Cached As Bitmaps - Stack Overflow is the one that gave me best results thus far, even when loading a swf (as opposed to a bitmap) which contains bitmaps inside. To avoid this decompressing on the run (on demand) you can instead request flash to decompress it on load. This will increase the load time but improve the perceived performance of the app since a separate thread takes care of the decompressing. This of course means no jitter or skip when the image hits the screen:
var fileLocation:URLRequest = new URLRequest(levelToLoad);
levelToLoad = "testLevel.swf";
var levelLoader:Loader = new Loader();
var loaderContext:LoaderContext = new LoaderContext();
loaderContext.imageDecodingPolicy = ImageDecodingPolicy.ON_LOAD;
levelLoader.load(fileLocation, loaderContext);
Embedded images (via code) are not supposed to be compressed, so this problem shouldn't occur with applications that do so. However, if you are like me, and you use the Flash Pro IDE to import PNGs or other bitmap assets, Flash Pro compresses them as JPG by default, using the setting provided by the image itself.
To fix this and avoid these decompression issues, simply go to your Flash Pro asset library, right click on the image you want to change, click on properties and select "lossless / PNG/GIF" as compression. This will increase largely the load time but you will have the much better perceived performance discussed here. This should be done in addition to implementing the code above, or it will make no difference.
In my experience the frame rate average dropped only by one frame per second, but it's much more consistent without any skips or lags which is essential for games.
See: http://help.adobe.com/en_US/as3/dev/WS52621785137562065a8e668112d98c8c4df-8000.html
And
http://www.bytearray.org/?p=2931