Data delivery Adolf Knoll National Library of the Czech Republic
Data delivery The goal is to make the data transfer easier under slow speed Internet connection In practice Decrease of resolution Decrease of image depth Compression of data Delivery of necessary chunks of data of still acceptable quality
Slow speed Internet connection At home: 28.8 kbps – 56 kbps At work: 155 Mbps What does it mean? 56 kbps = 7 KB of data transferred per each second 155 Mbps = MB transferred per each second 400 simultaneous users = 48.5 KB transferred per each second in average per user !!!
Solutions… Higher speed Research networks (Czech backbone 2.5 Gbps) Used for sharing computational resources and storage (grid computing) Thinner files To deliver only shat is necessary Decrease of resolution Decrease of image depth Compression of data Delivery of necessary chunks of data of still acceptable quality
75 dpi 300 dpi Decrease of resolution WHAT HAPPENS? Computer screen has a fix resolution…
Decrease of image depth 24 bit 8 bit4 bit1 bit 2 colours16 colours256 coloursCa colours
Decrease of image depth 24-bit1-bit4-bit8-bit 1/3 1/241/61
16 million colours 16 colours 256 colours
1 1/24 1/3 1/6 Size reduction through decrease of the colour depth
Methods for decrease of image depth onto 1-bit (dithering) If compressed by CCITT Fax Group B 4364 B 3734 B 3682 B Nearest colour Floyd-Steinberg BurkesStucki
Delivery of necessary chunks Pre-processing and a set of images of various quality levels Thumbnail Preview Internet User Archival Image servers Dynamic on-the-fly conversion Delivery from multiresolutional source Controlled by user
Compression in general lossless lossy
Typical situations Colour Image: PNG vs. JPEG Black-and-White Image: TIFF/G4 vs. JP2 Sound: CDA vs. MP3 Video: ….. Motion pictures are lossy by their nature, as they create illusion of motion through successive display of discrete frames !!! Compression will be explained in more detail in concrete chapters concerning characteristic types of digital data.
Compression – working groups Image JBIG (Joint Bi-level Image Group) JPEG (Joint Photographic Expert Group) Audiovisual MPEG Video (Motion Pictures Expert Group) sound