Bitmap caching and SWF file performance

Bitmap caching helps you enhance the performance of nonchanging movie clips in your applications. When you set the MovieClip.cacheAsBitmap or Button.cacheAsBitmap property to true, Flash Player caches an internal bitmap representation of the movie clip or button instance. This can improve performance for movie clips that contain complex vector content. All of the vector data for a movie clip that has a cached bitmap is drawn to the bitmap, instead of to the main Stage.

Note: The bitmap is copied to the main Stage as unstretched, unrotated pixels snapped to the nearest pixel boundaries. Pixels are mapped one-to-one with the parent object. If the bounds of the bitmap change, the bitmap is re‑created instead of being stretched.

For detailed information on caching button or movie clip instances see the following topics:

  • About caching and scrolling movie clips with ActionScript in Learning ActionScript 2.0 in Adobe Flash

  • Caching a movie clip in Learning ActionScript 2.0 in Adobe Flash

    Use the cacheAsBitmap property with movie clips with mostly static content and that do not scale and rotate frequently. With such movie clips, using the cacheAsBitmap property can lead to performance improvements when the movie clip is translated (when its x and y position is changed).

    Enabling caching for a movie clip creates a surface, which has several advantages, such as helping complex vector animations to render fast. In some situations, enabling caching does not improve performance, or even decrease it.

    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 might be negligible. 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 by optimizing vector graphics.

Complex background image

An application that contains a detailed and complex background image of vector data. To improve performance, select the content, store it in a movie clip, and set the opaqueBackground property to true. The background is rendered as a bitmap and can be redrawn quickly, so that your animation plays faster.


Scrolling text field

An application that displays a large amount of text in a scrolling text field. Place the text field in a movie clip that you set as scrollable with scrolling bounds (the scrollRect property), enabling fast pixel scrolling for the specified instance. When a user scrolls the movie clip instance, the scrolled pixels shift up and generate 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 (set 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.


When to avoid using bitmap caching

Misusing bitmap caching can negatively affect your SWF file. When you develop a FLA file that uses surfaces, remember the following guidelines:

  • Do not overuse surfaces (movie clips with caching enabled). Each surface uses more memory than a regular movie clip; only enable surfaces to improve rendering performance.

  • A cached bitmap can use significantly more memory than a regular movie clip instance. For example, if the movie clip on the Stage is 250 pixels by 250 pixels, when cached it might use 250 KB instead of 1 KB when it's a regular (uncached) movie clip instance.

  • Avoid zooming in on 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 movie clip instances that are largely static (nonanimating). You can drag or move the instance, but the contents of the instance should not animate or change a lot. 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; for example, when you create windowing applications.

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