1. MULTIMEDIA TECHNOLOGY SMM 3001 DATA COMPRESSION

2. In this chapter The basic principles for compressing data The basic principles for compressing data Data compression methods for text & numeric data Data compression methods for text & numeric data How digital images & graphics are compressed for storage & transmission How digital images & graphics are compressed for storage & transmission Compression methods for digital video & audio Compression methods for digital video & audio

3. Data compression concepts Compression techniques are used to replace a file with another that is smaller Compression techniques are used to replace a file with another that is smaller Decompression techniques expands the compressed file to recover the original data -- either exactly or in facsimile Decompression techniques expands the compressed file to recover the original data -- either exactly or in facsimile A pair of compression/decompression techniques that work together is called a codec for short A pair of compression/decompression techniques that work together is called a codec for short

4. Data compression concepts

5. Types of codecs Codecs that upon decompression always reproduce the original file exactly are called lossless codecs Codecs that upon decompression always reproduce the original file exactly are called lossless codecs Codecs that reproduce only an approximation of the original file upon decompression are called lossy codecs Codecs that reproduce only an approximation of the original file upon decompression are called lossy codecs Codecs that take approximately the same amount of time to compress and decompress a file are referred to as symmetric codecs Codecs that take approximately the same amount of time to compress and decompress a file are referred to as symmetric codecs By contrast, codecs that feature simple fast decompression but significantly slower compression are called asymmetric codecs By contrast, codecs that feature simple fast decompression but significantly slower compression are called asymmetric codecs

6. Compression encoding Compression is an encoding process that filters the original file in several successive stages Compression is an encoding process that filters the original file in several successive stages

7. Codec Methods Syntactic encoding (also called entropy encoding) methods attempt to reduce the redundancy of symbolic patterns in a file without any regard to the type of information represented Syntactic encoding (also called entropy encoding) methods attempt to reduce the redundancy of symbolic patterns in a file without any regard to the type of information represented Semantic methods consider special properties of the type of information represented to reduce nonessential information in a file Semantic methods consider special properties of the type of information represented to reduce nonessential information in a file Hybrid methods combine both syntactic and semantic methods Hybrid methods combine both syntactic and semantic methods

8. Compressing Text and Numerical Data Lossless Syntactic Methods Run-Length Encoding (RLE) Run-Length Encoding (RLE) looks for repeated sequences of symbols looks for repeated sequences of symbols widely used for fax (facsimile) transmissions widely used for fax (facsimile) transmissions Huffman Codes Huffman Codes exploits frequency distribution of symbols in a source exploits frequency distribution of symbols in a source adaptive Huffman coding builds its own frequency tables rather than use predefined statistics adaptive Huffman coding builds its own frequency tables rather than use predefined statistics Lempel-Ziv-Welch (LZW) compression Lempel-Ziv-Welch (LZW) compression based on recognizing patterns of strings in original file based on recognizing patterns of strings in original file fast and yields good results (50% typical reduction) fast and yields good results (50% typical reduction)

9. Compressing Images GIF (Graphic Interchange Format) Codec GIF (Graphic Interchange Format) Codec employs LZW method for lossless compression employs LZW method for lossless compression TIFF (Tagged Image File Format) Codec TIFF (Tagged Image File Format) Codec lossless syntactic method lossless syntactic method JPEG (Joint Photographics Experts Group) Codec JPEG (Joint Photographics Experts Group) Codec umbrella term covering several lossy and lossless methods umbrella term covering several lossy and lossless methods baseline method is most commonly used one -- lossy method based on a hybrid method baseline method is most commonly used one -- lossy method based on a hybrid method

10. Compressing Video Video compression employs both spatial and temporal compression Video compression employs both spatial and temporal compression spatial techniques compress individual frames spatial techniques compress individual frames temporal methods compress data in frames over time temporal methods compress data in frames over time QuickTime and AVI (Audio Video Interleaved) are two popular (and incompatible with each other) formats QuickTime and AVI (Audio Video Interleaved) are two popular (and incompatible with each other) formats

11. Compressing Video Some additional methods DVI (Digital Video Interactive) DVI (Digital Video Interactive) Motion-JPEG Motion-JPEG MPEG (Motion Pictures Experts Group) MPEG (Motion Pictures Experts Group) The px64 Standard The px64 Standard

12. Temporal Compression in Video Lossy strategies for eliminating redundancy of information between frames employ temporal compression -- referred to as interframe compression Lossy strategies for eliminating redundancy of information between frames employ temporal compression -- referred to as interframe compression Sequence of frames are considered together Sequence of frames are considered together key frames key frames difference frames difference frames Used in QuickTime and DVI Used in QuickTime and DVI

13. Temporal Compression in Video (cont’d) MPEG and related codecs employ a more complex frame-referencing method MPEG and related codecs employ a more complex frame-referencing method intrapictures (I pictures) intrapictures (I pictures) predicted pictures (P pictures) predicted pictures (P pictures) bidirectional pictures (B pictures) bidirectional pictures (B pictures)

14. Compressing Audio Widely used method is ADPCM (Adaptive Differential Pulse Code Modulation) Widely used method is ADPCM (Adaptive Differential Pulse Code Modulation) ADPCM ADPCM lossy method lossy method employs a differencing technique related to those used in video compression employs a differencing technique related to those used in video compression used in DVI used in DVI

15. Summary Compressing data means reducing the effective size of a data file for storage or transmission Compressing data means reducing the effective size of a data file for storage or transmission Particular paired compression/decompression methods are called codecs Particular paired compression/decompression methods are called codecs Codecs that cannot reproduce the original file exactly are called lossy methods; those that reproduce the original exactly are called lossless methods Codecs that cannot reproduce the original file exactly are called lossy methods; those that reproduce the original exactly are called lossless methods Text and numbers usually require lossless methods Text and numbers usually require lossless methods Images, video, and sound codecs are usually lossy Images, video, and sound codecs are usually lossy

16. Summary (cont’d) Syntactic methods attempt to reduce the redundancy of symbolic patterns in a file without any regard to the type of information represented Syntactic methods attempt to reduce the redundancy of symbolic patterns in a file without any regard to the type of information represented Semantic methods exploit characteristics inherent in the type of information being represented Semantic methods exploit characteristics inherent in the type of information being represented The use of codecs is not an exact science - - the effectiveness and suitability of any method will depend on the exact nature of the original file and the intended use for the compressed file The use of codecs is not an exact science - - the effectiveness and suitability of any method will depend on the exact nature of the original file and the intended use for the compressed file