Perl Graphics Programming

The Format and Function of SWF Files

Like PNG or MNG, SWF is a binary tagged file format. Each movie begins with a header that encodes most of the global information that a player needs to play back the file, such as the frame rate, the dimensions of the movie, etc. The header is followed by one or more tags, and finally by a single end tag. Tags are one of two types: definition tags or control tags.

As the player software (typically a browser plug-in) reads the definition tags, the objects that they describe are placed in a structure called the Dictionary. Control tags add and remove the objects from a structure called the display list, which represents the layers of the movie.

Definition Tags

A definition tag describes a reusable object that is to appear in the movie. Each of these objects has a unique ID associated with it that identifies it as a character in the movie. Control tags (or other definition tags) that appear after a character has been defined can refer to that character by its ID. The defined object can be a shape, a sound, a button, or any of the other types of objects listed below. Almost all of the elements of an SWF file have a full alpha channel, allowing you to control the transparency of any objects placed in the movie.

Shapes

A shape is defined by one of three tags: DefineShape, DefineShape2, or DefineShape3. The latter two tags are variations of the DefineShape tag that were introduced in successive revisions of the SWF specification (several tags have multiple ...