Upon import, embedded video is compressed according to settings you specify. A basic understanding of video compression will help you use the compression options available to you in Macromedia Flash MX.

Digital video is typically compressed using two methods: spatial and temporal.

Spatial compression is applied to a single frame of video, and compresses the video image much like single JPEG images are compressed. The degree of spatial compression affects the overall video quality.

Temporal compression takes advantage of the fact that consecutive frames of video often contain much of the same pixel data. By identifying differences between consecutive frames of video, and storing just those frame differences in the compressed file, temporal compression can dramatically decrease video data size. Frames compressed with temporal compression are called interframes.

A video keyframe is a complete frame of video—not just the computed differences between two frames. Keyframes are used as reference points for subsequent interframes.

The Sorenson Spark codec included in Macromedia Flash MX uses a combination of spacial and temporal compression techniques. For more information on that codec, see Additional information about Sorenson Spark.