With increasing bandwidth, video has become an important part of the Internet. Video sharing sites like YouTube are immensely popular and many other sites incorporate video footage. Unfortunately, video takes up a lot of space. Uncompressed footage takes up about 17MB per second, so it needs to be compressed before uploading. As with other forms of compression, both lossy and lossless techniques are available. DVDs are compressed using the lossy MPEG-2 format, which can reduce file size 15 to 30 times while retaining a high-quality picture.
Video compression software examines the pixels in each frame of video, and compresses them by grouping similar pixels together in large blocks, so poor video compression sometimes results in blocky images that lack detail. Video compression becomes easier when there are limited changes between frames.
About.com gives the following example:
Imagine video of a blue sky and lawn with a dog running across the screen. Uncompressed, the video contains information for every pixel, in every frame. Compressed, the video contains less information because similar pixels are grouped together. So, by recognizing that all pixels in the top half of the frame are blue, and all pixels in the bottom half are green, the compressed video significantly reduces the file size. The only changing pixels are those that show the dog in motion.
There are a number of video compression formats, sometimes referred to as codecs. The International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) sponsor group called the Moving Pictures Experts Group (MPEG) which is responsible for the common compression formats MPEG-1, MPEG-2 and MPEG-4.
The International Telecommunications Union (ITU-T) standardises several popular compression formats including H.261 and H.264. There are other compression formats, such as Intel Indeo and RealVideo produced by large companies without the involvement of international standards groups.
Most video player software, such as Windows Media Player or VLC Media Player will play a range of formats, though not necessarily all of them. Video compression formats should not be confused with media container formats. A media container contains data produced using a video compression format – it is the end product of video compression.
This Wikipedia article gives further information about video compression.
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