34.2. 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. 34.2.1. Lossless Versus Lossy Compression Compression can be lossless, which means no information is lost and the final file is identical to the original. Most compression schemes use forms of lossy compression. Lossy compression sacrifices some data from the file to achieve much higher compression rates. Lossy compression schemes, such as MPEG, use complicated algorithms that toss out data for sound and image detail that is not discernible to the human ear or eye. The decompressed file is extremely similar in character to the original, yet is not identical. This is similar to the way JPEG handles still images. 34.2.2. Spatial Versus Temporal Compression Spatial (or intraframe) compression takes place on each individual frame of the video, compressing the pixel information as though it were a still image. Temporal (or interframe) compression happens over a series of frames and takes advantage of areas of the image that remain unchanged from frame to frame, throwing out data for repeated pixels. Temporal compression relies on the placement of key frames interspersed throughout the frames sequence. The key frames are used as masters against which the following frames (called delta frames) are compared. It is recommended that a key frame be placed once every 3 to 10 seconds. Videos without a lot of motion, such as talking head clips, take the best advantage of temporal compression . Videos with pans and other motion are compressed less efficiently. 34.2.3. 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. This list focuses on just those that are relevant to video intended for web delivery.
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. Because it uses complicated compression algorithms, it requires a lot of processing power and may not run smoothly on older machines.
H.264/AVC This codec (also known as MPEG-4, Part 10) was designed to provide good video quality at drastically reduced bit rates, making it an exciting new codec for web video. It was created by ITU-T in collaboration with the group that developed MPEG (together known as the Joint Video Team). The H.264 standard and MPEG-4, Part 10 standard are technically identical, and the technology is also known as AVC, for Advanced Video Coding. In addition to being useful for Internet transmission, it may accommodate higher bit rates for broadcast, DVD, and telephony. Apple Computer integrated support for H.264 compression into its OS X 10.4 (Tiger) operating system as well as QuickTime 7, both released in 2005.
Windows Media Video Codec Microsoft uses its own proprietary video and audio codecs for use with the Windows Media System (in Version 9 as of this writing). It serves as the basis for the video codec VC-1, which is in the process of being standardized. For more information on Windows Media video codecs , see www.microsoft.com/windows/windowsmedia/howto/articles/codecs.aspx.
MPEG The MPEG codec can be used only 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. Some other codecs (H.264 and Windows Media Video) are based on MPEG compression.
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.
Radius Cinepak Cinepak provides decent compression/decompression rates. It employs both spatial and temporal compression and a lossy compression scheme at lower quality levels. Cinepak is well supported, but due to grainy video quality and the availability of alternative compression schemes, it is outdated and seldom used.
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 . |
