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Multimedia presentation video compression

Jul 15, 2015

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MULTIMEDIA PROJECT

MULTIMEDIA PROJECTVideo compressionLaLit DuBeySachin SharmaSomnathLinikerDeePak bhanushaliAjayProject members Video compression technologies are about reducing and removing redundant video data so that a digitalvideo file can be effectively sent over a network and stored on computer disks

Introduction

Uncompressed video that we record from a video camera(e.g. movies) occupies huge amount of data.

For example,

a video clip recorded with a resolution of 720x576(PAL), with a refresh rate of 25fps and 8-bit color depth takes:720 x 576 x 25 x 8 + 2 x (360 x 576 x 25 x 8) = 1.66 Mb/s (luminance+chrominance)

For HDTV(High definition television) which uses a resolution of 1920x10801920 x 1080 x 60 x 8 + 2 x (960 x 1080 x 60 x 8) = 1.99 Gb/s(Note: In YUV color space, each pixel has one brightness(luminance) value and two color(chrominance) values)

Overview to normal videoDigital video sequences are the most demanding form of data in various fields pass through computer network or portable storage devices like pen drives, CDs etc.Uncompressed video impose of following: 1.Large bandwidth requirements for transmission , 2.Enormous demands on the storage capacity of mediaNeed of compressionDifferent compression technologies, both proprietary and industry standards, are available. Most network video vendors today use standard compression techniques. Standards are important in ensuring compatibility and interoperability. They are particularly relevant to video compression since video may be used for different purposes and, in some video surveillance applications, needs to be viewable many years from the recording date. By deploying standards, end users are able to pick and choose from different vendors, rather than be tied to one supplier when designing a video surveillance system.The process of compression involves applying an algorithm to the source video to create a compressed file that is ready for transmission or storage. To play the compressed file, an inverse algorithm is applied to produce a video that shows virtually the same content as the original source video.VIDEO CODECEncoding

Decoding Working diagram of codecoriginal digital dataCompression algorithmCompressed dataStorage Compressed dataDecompression algorithmoriginal digital data

A pair of algorithms that works together is called a video codec (encoder/decoder). Video codecs of different standards are normally not compatible with each other; that is, video content that is compressed using one standard cannot be decompressed with a different standard. For instance, an MPEG-4 decoder will not work with an H.264 encoder. This is simply because one algorithm cannot correctly decode the output from another algorithm but it is possible to implement many different algorithms in the same software or hardware, which would then enable multiple formats to coexist.About codecVideo compression algorithms such as MPEG-4 and H.264 use interframe prediction to reduce video data between a series of frames. This involves techniques such as difference coding, where one frame is compared with a reference frame and only pixels that have changed with respect to the reference frame are coded. In this way, the number of pixel values that is coded and sent is reduced. When such an encoded sequence is displayed, the images appear as in the original video sequence.WORKING OF ALGORITHM

Diagramatric workingWith inter frame prediction, each frame in a sequence of images is classified as a certain type of frame, such as an I-frame, P-frame or B-frame.More about framesAn I-frame, or intra frame, is a self-contained frame that can be independently decoded without any reference to other images. The first image in a video sequence is always an I-frame. I-frames are needed as starting points for new viewers or resynchronization points if the transmitted bit stream is damaged. I-frames can be used to implement fast-forward, rewind and other random access functions. An encoder will automatically insert I-frames at regular intervals or on demand if new clients are expected to join in viewing a stream. The drawback of I-frames is that they consume much more bits, but on the other hand, they do not generate many artifacts, which are caused by missing data.I FRAMEA P-frame, which stands for predictive inter frame, makes references to parts of earlier I and/or P frame(s) to code the frame. P-frames usually require fewer bits than I-frames, but a drawback is that they are very sensitive to transmission errors because of the complex dependency on earlier P and/or I frames.P FRAMEA B-frame, or bi-predictive inter frame, is a frame that makes references to both an earlier reference frame and a future frame. Using B-frames increases latency.B FRAME

When a video decoder restores a video by decoding the bit stream frame by frame, decoding must always start with an I-frame. P-frames and B-frames, if used, must be decoded together with the reference frame(s). - Occupies less disk space.

- Reading and writing is faster.

- File transferring is faster.

- The order of bytes is independent.ADVANTAGES - Compilation need to be done again for compression. - Errors may occur while transmitting data.

- The byte / pixel relationship is unknown

- Has to decompress the previous data.Disadvantages

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