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Video compression
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Page 1: Multimedia presentation video compression

Video compression

Page 2: Multimedia presentation video compression

LaLit DuBey

Sachin Sharma

Somnath

Liniker

DeePak bhanushali

Ajay

Page 3: Multimedia presentation video compression

“ Video compression technologies are about

reducing

and removing redundant video data so that a digital

video file can be effectively sent over a network and

stored on computer disks ”

Page 4: Multimedia presentation video compression

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 1920x1080

1920 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)

Page 5: Multimedia presentation video compression

Digital video sequences are the most demanding form

of data in various fields pass through computer network

or portable storage devices like pen drives, CD’s etc.

Uncompressed video impose of following:

1.Large bandwidth requirements for transmission ,

2.Enormous demands on the storage capacity of media

Page 6: Multimedia presentation video compression

Different 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.

Page 7: Multimedia presentation video compression

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.

Page 8: Multimedia presentation video compression

Encoding

Decoding

original digital data

Compression algorithm

Compressed data

Storage

Compressed data

Decompression algorithm

original digital data

Page 9: Multimedia presentation video compression

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.

Page 10: Multimedia presentation video compression

Video 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.

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With 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.

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An 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.

Page 14: Multimedia presentation video compression

A 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.

Page 15: Multimedia presentation video compression

A 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.

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- Occupies less disk space.

- Reading and writing is faster.

- File transferring is faster.

- The order of bytes is independent.

Page 18: Multimedia presentation video compression

- 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.

Page 19: Multimedia presentation video compression