Lec6 compression

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Information about Lec6 compression

Published on March 15, 2014

Author: dommike33

Source: slideshare.net


DATA COMPRESSION • In computer science and information theory, data compression is the process of encoding information using fewer bits than an un-encoded representation would use, through use of specific encoding schemes. • Compression is useful because it helps reduce the consumption of expensive resources, such as hard disk space or transmission bandwidth. On the downside, compressed data must be decompressed to be used, and this extra processing may be detrimental to some applications.

NEED FOR COMPRESSION • The large data objects size represents few issues as 1. Storage 2. Transmission 3. Access Time

COMPRESSION STANDARDS • Standardization has been an essential requirement for any technology that is supported by a large number of manufacturers. Few of the major compression issuing bodies are: • CCITT (INTERNATIONAL CONSULTING COMMITTEE IN TELEPHONY & TELEGRAPHY) • CCIR (INTERNATIONAL CONSULTING COMMITTEE FOR RADIO) • ISO (INTERNATIONAL STANDARDS ORGANISATION)

COMPRESSION TYPES • Lossless and Lossy compression are terms that describe whether or not, in the compression of a file, all original data can be recovered when the file is uncompressed. • With lossless compression, every single bit of data that was originally in the file remains after the file is uncompressed. All of the information is completely restored. This is generally the technique of choice for text or spreadsheet files, where losing words or financial data could pose a problem. The Graphics Interchange File (GIF) is an image format used on the Web that provides lossless compression.

LOSSLESS COMPRESSION • Lossless compression algorithms usually exploit statistical redundancy in such a way as to represent the sender's data more concisely without error. • Lossless compression standards are designed to retain all information in the original multimedia object. Used for text documents, databases. • Group 3 1-D • Group 3 2-D • Group 4 COMPRESSION TYPES

LOSSY COMPRESSION • There are times when a picture may have more detail than the eye can distinguish or an audio file has immensely high quality that might not be noticed by the audience it is intended for; this is where Lossy Compression comes into play. • This technique results in some loss of information & is used where data accuracy is not essential. Usually for audio, image and video. • JPEG • MPEG • DVI (Digital Visual Interface is a video interface standard designed to provide very high visual quality on digital display devices)

COMPRESSION GROUPS • Group 2 Very early (1980s) compression scheme developed for facsimile machines featuring resolutions as high as 100 dpi. It did not provide a very high level of compression and is generally not in use anymore. • Group 3 1D Also known as run-length encoding (RLE), its based on the assumption that a typical scan line has long runs of pixels of the same color (black or white). Primary applications has been facsimile and very early document imaging systems.

• Group 3 2D Also known as modified run-length encoding. It is more commonly used for software-based document imaging systems. While it provides fairly good compression, it is easier to decompress as well. The compression ratio for this scheme averages between 10 and 25. It utilizes a modified READ (Relative Element Address Designated) algorithm. • Group 4 Two dimensional coding scheme. In this method, the first reference line is an imaginary all-white line above the top of the image. It was designed to address high-resolution images in black-n-white. • Group 5 Designed to address the need for an efficient content-based encoding methodology that also addresses the color and shade information.

CODING TYPES • Entropy Coding Used for different media regardless of the medium specific characteristics. It is lossless. Here semantics are ignored. • Source Coding It takes semantics of the information to be encoded. It depends on medium. Its lossy. • Hybrid Coding It is the combination of both.

CODING TYPES Coding Type Basis Technique Entropy Coding Run-Length Coding Huffman Coding Arithmetic Coding Source Coding Prediction DPCM DM Transformation FFT DCT Hybrid Coding JPEG MPEG H.263 & Many proprietary systems

MAJOR STEPS IN DATA COMPRESSION • The preparation step generates an appropriate digital representation of the information in the medium being compressed. • The processing step makes use of the various compression algorithms. • Quantization takes place after the mathematically exact picture processing step. Values determined in the previous step cannot be processed with full exactness. • Entropy coding starts with a sequential data stream of individual bits and bytes and lossless compression is performed.

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