note by anna: I would really like this chapter to include specific licence information where possible. x
By the end of this module you will
Tools you will need for this module:
Video Codecs: When you talk about “watching a video,” you’re probably talking about a combination of one video stream and one audio stream. But you don’t have two different files; you just have “the video.” Maybe it’s an AVI file, or an MP4 file. These are just container formats, like a ZIP file that contains multiple kinds of files within it. The container format defines how to store the video and audio streams in a single file.
When you “watch a video,” your video player is doing at least three things at once:
A video codec is an algorithm by which a video stream is encoded, i.e. it specifies how to do #2 above. (The word “codec” is a portmanteau, a combination of the words “coder” and “decoder.”) Your video player decodes the video stream according to the video codec, then displays a series of images, or “frames,” on the screen. Most modern video codecs use all sorts of tricks to minimize the amount of information required to display one frame after the next. For example, instead of storing each individual frame (like a screenshot), they will only store the differences between frames. Most videos don’t actually change all that much from one frame to the next, so this allows for high compression rates, which results in smaller file sizes.
There are lossy and lossless video codecs. Lossless video is much too big to be useful on the web, so I’ll concentrate on lossy codecs. A lossy video codec means that information is being irretrievably lost during encoding. Like copying an audio cassette tape, you’re losing information about the source video, and degrading the quality, every time you encode. Instead of the “hiss” of an audio cassette, a re-re-re-encoded video may look blocky, especially during scenes with a lot of motion. (Actually, this can happen even if you encode straight from the original source, if you choose a poor video codec or pass it the wrong set of parameters.) On the bright side, lossy video codecs can offer amazing compression rates by smoothing over blockiness during playback, to make the loss less noticeable to the human eye.
There are tons of video codecs. The three most relevant codecs are H.264, Theora, and VP8.
Open Video codecs: Free formats like Ogg Theora or WebM don't have licensing restrictions. Where proprietary codecs charge licensing fees for use of their codecs, open video codecs allow anyone to freely create tools and distribute content as they like. An open specification should allow anyone to implement their own player, encoder or other tools to encode or decode videos in a free codec. It is important that this can be done without requiring a special contract or patent agreement.
While H.264 is a big step forward compared to proprietary codecs owned by a single vendor like Real or Microsoft, it still requires anyone implementing an encoder or decoder and even anyone distributing a video in H.264 offline to pay license fees for patents covering algorithms used in the format to do so. For videos distributed on the web for free this has been removed but if you sell or distribute videos on disks or broadcast you need to pay up.
For more info you can read this article about The H.264 Licensing Labyrinth
Video Containers: You may think of video files as “AVI files” or “MP4 files.” In reality, “AVI” and “MP4” are just container formats. Just like a ZIP file can contain any sort of file within it, video container formats only define how to store things within them, not what kinds of data are stored. (It’s a little more complicated than that, because not all video streams are compatible with all container formats, but never mind that for now.)
A video file usually contains multiple tracks — a video track (without audio), one or more audio tracks (without video) and some containers even contains one or more subtitletracks or picture tracks. Tracks are usually interrelated. An audio track contains markers within it to help synchronize the audio with the video. Individual tracks can have metadata, such as the aspect ratio of a video track, or the language of an audio track. Containers can also have metadata, such as the title of the video itself, cover art for the video, episode numbers (for television shows), and so on.
There are lots of video container formats. Some of the most popular include
.mp4 or .m4v extension. The MPEG 4 container is based on Apple’s older QuickTime container (.mov). Movie trailers on Apple’s website still use the older QuickTime container, but movies that you rent from iTunes are delivered in an MPEG 4 container..flv extension. Flash Video is, unsurprisingly, used by Adobe Flash. Prior to Flash 9.0.60.184 (a.k.a. Flash Player 9 Update 3), this was the only container format that Flash supported. More recent versions of Flash also support the MPEG 4 container..ogv extension. Ogg is an open standard, open source–friendly, and unencumbered by any known patents. Firefox 3.5, Chrome 4, and Opera 10.5 support — natively, without platform-specific plugins — the Ogg container format, Ogg video (called “Theora”), and Ogg audio (called “Vorbis”). On the desktop, Ogg is supported out-of-the-box by all major Linux distributions, and you can use it on Mac and Windows by installing the QuickTime components or DirectShow filters, respectively. It is also playable with the excellent VLC on all platforms..avi extension. The AVI container format was invented by Microsoft in a simpler time, when the fact that computers could play video at all was considered pretty amazing. It does not officially support features of more recent container formats like embedded metadata. It does not even officially support most of the modern video and audio codecs in use today. Over time, companies have tried to extend it in generally incompatible ways to support this or that, and it is still the default container format for popular encoders such as MEncoder.There has been error in communication with Booktype server. Not sure right now where is the problem.
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