SWF File compression strategy

Since SWF files are frequently delivered over a network connection, they should be as compact as possible. Several techniques are used to accomplish this, including the following items:

Reuse—The structure of the character dictionary makes it easy to reuse elements in a SWF file. For example, a shape, button, sound, font, or bitmap can be stored in a file once and referenced many times.

Compression—Shapes are compressed by using an efficient delta encoding scheme; often the first coordinate of a line is assumed to be the last coordinate of the previous line. Distances are also often expressed relative to the last position.

Default values—Some structures, like matrixes and color transforms, have common fields that are used more often than others. For example, for a matrix, the most common field is the translation field. Scaling and rotation are less common. Therefore, if the scaling field is not present, it is assumed to be 100%. If the rotation field is not present, it is assumed that there is no rotation. This use of default values helps to minimize file sizes.

Change Encoding—As a rule, SWF files only store the changes between states. This is reflected in shape data structures and in the place-move-remove model that the display list uses.

Shape Data Structure—The shape data structure uses a unique structure to minimize the size of shapes and to render anti-aliased shapes efficiently on the screen.