Flash CS3 Documentation

Learning ActionScript 2.0 in Adobe Flash > Working with Movie Clips > About caching and scrolling movie clips with ActionScript > When to enable caching

When to enable caching

Enabling caching for a movie clip 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 will 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 even decrease it. This section describes scenarios in which caching should be used, and when to use regular movie clips.

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.

For information on alpha channel masking, which requires you to set the cacheAsBitmap property to true, see About alpha channel masking.

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 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 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 movie clip 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 movie clip 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 (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.

All of these scenarios improve the responsiveness and interactivity of the application by optimizing the vector graphics.

For samples about applying bitmap caching to an instance and to scrolling text, see the Flash Samples page at www.adobe.com/go/learn_fl_samples. The following samples are available:

• cacheBitmap.fla; Download the Samples zip file and navigate to the ActionScript2.0/Cachebitmap folder.
• aliasing.fla; Download the Samples zip file and navigate to the ActionScript2.0/Advanced Anti-Aliasing folder.

When to avoid using bitmap caching

Misusing this feature could 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, 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 movie clip instance. For example, if the movie clip on 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 (uncached) movie clip 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 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 as much as possible; for example, when you create windowing applications.

Flash CS3