ACTIONSCRIPT 3 Developer’s Guide en

ACTIONSCRIPT 3.0 DEVELOPER’S GUIDE
Display programming
The DisplayObject class’s opaqueBackground property and scrollRect property are related to bitmap caching using
the cacheAsBitmap property. Although these three properties are independent of each other, the opaqueBackground
and scrollRect properties work best when an object is cached as a bitmap—you see performance benefits for the
opaqueBackground and scrollRect properties only when you set cacheAsBitmap to true. For more information
about scrolling display object content, see “Panning and scrolling display objects” on page 178. For more information
about setting an opaque background, see “Setting an opaque background color” on page 185.
For information on alpha channel masking, which requires you to set the cacheAsBitmap property to true, see
“Masking display objects” on page 190.
When to enable caching
Flash Player 9 and later, Adobe AIR 1.0 and later
Enabling caching for a display object creates a surface, which has several advantages, such as helping complex vector
animations to render fast. There are several scenarios in which you will want to enable caching. It might seem as
though you would always want to enable caching to improve the performance of your SWF files; however, there are
situations in which enabling caching does not improve performance, or can even decrease it. This section describes
scenarios in which caching should be used, and when to use regular display objects.
Overall performance of cached data depends on how complex the vector data of your instances are, how much of the
data you change, and whether or not you set the opaqueBackground property. If you are changing small regions, the
difference between using a surface and using vector data could be negligible. You might want to test both scenarios
with your work before you deploy the application.
When to use bitmap caching
The following are typical scenarios in which you might see significant benefits when you enable bitmap caching.
Complex background image: An application that contains a detailed and complex background image of vector data
(perhaps an image where you applied the trace bitmap command, or artwork that you created in Adobe
Illustrator®). You might animate characters over the background, which slows the animation because the
background needs to continuously regenerate the vector data. To improve performance, you can set the
opaqueBackground property of the background display object to true. The background is rendered as a bitmap
and can be redrawn quickly, so that your animation plays much faster.
Scrolling text field: An application that displays a large amount of text in a scrolling text field. You can place the
text field in a display object that you set as scrollable with scrolling bounds (the scrollRect property). This enables
fast pixel scrolling for the specified instance. When a user scrolls the display object instance, Flash Player or AIR
shifts the scrolled pixels up and generates the newly exposed region instead of regenerating the entire text field.
Windowing system: An application with a complex system of overlapping windows. Each window can be open or
closed (for example, web browser windows). If you mark each window as a surface (by setting the cacheAsBitmap
property to true), each window is isolated and cached. Users can drag the windows so that they overlap each other,
and each window doesn’t need to regenerate the vector content.
Alpha channel masking: When you are using alpha channel masking, you must set the cacheAsBitmap property
to true. For more information, see “Masking display objects” on page 190.
Enabling bitmap caching in all of these scenarios improves the responsiveness and interactivity of the application by
optimizing the vector graphics.
In addition, whenever you apply a filter to a display object, cacheAsBitmap is automatically set to true, even if you
explicitly set it to false. If you clear all the filters from the display object, the cacheAsBitmap property returns to the
value it was last set to.
Last updated 6/6/2012
183