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25.2. Compression

Digital video wouldn't be possible without methods for compressing the vast amounts of data necessary to describe sound and frame images. Video files can be compressed in a number of ways. This section looks at a variety of compression schemes and introduces the methods they use for achieving compression rates. Understanding your options can help you make better decisions for optimizing your video files.

25.2.3. Video Codecs

There are a number of codecs (compression/decompression algorithms) that can be used to compress video files for the Web. Many of these codecs can be applied to several different file formats (discussed in the next section of this chapter).

Video-editing software packages often offer a long list of codecs in their compressor list options. Here we focus on just those that are relevant to video intended for web delivery.

Radius Cinepak

Cinepak provides decent compression/decompression rates and is compatible with both QuickTime or AVI formats. It employs both spatial and temporal compression and a lossy compression scheme at lower quality levels. Low to medium quality settings will produce acceptable quality video. It is also well supported, so if you want your video to be viewable by the widest possible audience, choose Cinepak.

Sorenson

The Sorenson Video codec was designed for low-bandwidth applications and is capable of producing files with lower data rates (if you select the Limit Data Rate option) than Cinepak while maintaining excellent quality. It is the ideal codec for web delivery, with some concessions. First, because it uses complicated compression algorithms, it requires a lot of processing power and may not run smoothly on older machines. It also requires that users install QuickTime Version 3 or higher. While growing in popularity, it will be a while before it is completely risk-free.

Intel Indeo

The Indeo codec provides compression rates similar to Cinepak by the use of spatial and temporal compression, with lossy compression at low quality levels. Its drawbacks are that it does not maintain quality at data rates as low as Cinepak, and it requires high-end machines to perform at its best.

Animation

If your video clip is all computer-generated graphical imagery (i.e., not sourced from videotape), you may want to try the Animation compressor. Depending on the type of image, the Cinepak codec may work just as well (or better) for these types of files.

MPEG

The MPEG codec can only be used when the final video file will be in MPEG format (it is not compatible with other file types). It uses a lossy compression scheme (although it may be lossless at high-quality settings) and spatial and temporal compression. MPEG offers the best compression possible, but MPEGs are not yet as widely supported on the Web as other video formats.



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