When to enable caching

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 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 Alpha channel masking.

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 by Flash Player, 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.

When to avoid using bitmap caching

Misusing this feature could negatively affect 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 sometimes the computer) needs to do. Group surfaces together as much as possible--for example, when you create windowing applications.

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