ACTIONSCRIPT 3 Developer’s Guide en

ACTIONSCRIPT 3.0 DEVELOPER’S GUIDE
Display programming
When to avoid using bitmap caching
Using this feature in the wrong circumstances can negatively affect the performance of your SWF file. When you use
bitmap caching, remember the following guidelines:
Do not overuse surfaces (display objects with caching enabled). Each surface uses more memory than a regular
display object, which means that you should only enable surfaces when you need to improve rendering
performance.
A cached bitmap can use significantly more memory than a regular display object. For example, if a Sprite instance
on the Stage is 250 pixels by 250 pixels in size, when cached it might use 250 KB instead of 1 KB when it’s a regular
(un-cached) Sprite instance.
Avoid zooming into cached surfaces. If you overuse bitmap caching, a large amount of memory is consumed (see
previous bullet), especially if you zoom in on the content.
Use surfaces for display object instances that are largely static (non-animating). You can drag or move the instance,
but the contents of the instance should not animate or change a lot. (Animation or changing content are more likely
with a MovieClip instance containing animation or a Video instance.) For example, if you rotate or transform an
instance, the instance changes between the surface and vector data, which is difficult to process and negatively
affects your SWF file.
If you mix surfaces with vector data, it increases the amount of processing that Flash Player and AIR (and
sometimes the computer) need to do. Group surfaces together as much as possible—for example, when you create
windowing applications.
Do not cache objects whose graphics change frequently. Every time you scale, skew, rotate the display object,
change the alpha or color transform, move child display objects, or draw using the graphics property, the bitmap
cache is redrawn. If this happens every frame, the runtime must draw the object into a bitmap and then copy that
bitmap onto the stage—which results in extra work compared to just drawing the uncached object to the stage. The
performance tradeoff of caching versus update frequency depends on the complexity and size of the display object
and can only be determined by testing the specific content.
Enabling bitmap caching
Flash Player 9 and later, Adobe AIR 1.0 and later
To enable bitmap caching for a display object, you set its cacheAsBitmap property to true:
mySprite.cacheAsBitmap = true;
After you set the cacheAsBitmap property to true, you might notice that the display object automatically pixel-snaps
to whole coordinates. When you test the SWF file, you should notice that any animation performed on a complex
vector image renders much faster.
A surface (cached bitmap) is not created, even if cacheAsBitmap is set to true, if one or more of the following occurs:
The bitmap is greater than 2880 pixels in height or width.
The bitmap fails to allocate (because of an out-of-memory error).
Cached bitmap transform matrices
Adobe AIR 2.0 and later (mobile profile)
In AIR applications for mobile devices, you should set the cacheAsBitmapMatrix property whenever you set the
cacheAsBitmap property. Setting this property allows you to apply a wider range of transformations to the display
object without triggering rerendering.
Last updated 6/6/2012
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