How video compression works?

Digital video compression and decompression algorithms (codecs) are at the heart of many modern video products, from DVD players to multimedia jukeboxes to video-capable cell phones. Video compression can be an art and a science; but most of us don't want to get that deep. Instead of spending hours on video compression trial and error, we want to create videos, and quickly compress them for clear images and smooth playback. If you know your video is internet-bound, there are a few things you can do when you are shooting to make it look better on a computer screen and make video compression a simple process.

First, it helps to understand the basics of video files and video compression. Video compression algorithms is essential for developers of the systems, processors, and tools that target video applications. Video compression software examines the pixels in each frame of video, and compresses them by bunching similar pixels together in large blocks.

Because video clips are made up of sequences of individual images, or "frames," video compression algorithms share many concepts and techniques with still-image compression algorithms. 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. So, the less a video changes frame to frame, the easier video compression becomes.

Video takes up a lot of space. Uncompressed footage from a camcorder takes up about 17MB per second of video. Because it takes up so much space, video must be compressed before it is put on the web. "Compressed" just means that the information is packed into a smaller space. There are two kinds of compression: lossy and lossless.

Lossy compression means that the compressed file has less data in it than the original file. In some cases, this translates to lower quality files, because information has been lost, hence the name. However, you can lose a relatively large amount of data before you start to notice a difference. Lossy compression makes up for the loss in quality by producing comparatively small files. For example, DVDs are compressed using the - MPEG-2 format, which can make files 15 to 30 times smaller, but we still tend to perceive DVDs as having high-quality picture.

Lossless compression is exactly what it sounds like, compression where none of the information is lost. This is not nearly as useful because files often end up being the same size as they were before compression. This may seem pointless, as reducing the file size is the primary goal of compression. However, if file size is not an issue, using lossless compression will result in a perfect-quality picture. For example, a video editor transferring files from one computer to another using a hard drive might choose to use lossless compression to preserve quality while he or she is working.