Quality Levels of Reproduction Adolf Knoll National Library of the Czech Republic

Archival imageAccess image Facsimile printing or display Preprocessed set of various quality level images Dynamically postprocessed Image on demand Transition from analogue to digital = first loss of information Acceptable technical parameters (resolution, colour depth, lossy compression) Acceptable colour fidelity (ICC profiles, calibration, metadata for still images) Original Digital world Analogue world continuous discrete

What is necessary and what not? Visual representation of documents To deliver only the necessary quantity of information from the whole source Where is the limit between necessary and unnecessary? It is given by the purpose of communication: facsimile printing, understanding of text, study of illuminations, general idea what the document is about, structure and character of objects on the digitized page, …?

Typical documents Illuminated manuscripts Textual manuscripts and old printed books Old periodicals Typewritten documents Modern journals (with colour photographs) Maps Administrative documents and forms Computer generated documents...

Access images Acceptable illusion of reality Colour depth Resolution Compression Colour fidelity

300 dpi 15,173 KB Colour depth

256 shades of gray 5,062 KB 256 colours same size Colour depth

16 shades of gray 2,531 KB Colour depth

Black-and-white = 2 colours 637 KB Colour depth

80 dpi = 3.75 times smaller file 300 dpi Resolution

Possible solutions Decreased colour depth  16.7 mio colours  256 colours or shades of gray (1/3 of volume)  16 colours/shades of gray (1/6)  2 colours (black-and-white images, 1/24) Decreased resolution (down to the limit when necessary details are still seen) Combined colour depth with resolution Loss of information (colour or detail)

Lossless  Colour LZW = 11,441 KB PNG = 8,849 KB JP2 = 6,647 KB  BW Fax Group 3 Fax Group 4 = 272 KB Fax Group 4 = 166 KB (JBIG, JBIG2)  JB2 (in DjVu) Lossy  Colour DCT JPEG = 318 KB Wavelet = better, but mixed photos with text do not give too much space for improvement  BW JBIG2-based  JB2 = 98 KB  JB2 = 32 KB BEST LOSSY METHODS COMBINED = Mixed Raster Content = DjVu = 55 KB55 KB Compression We have lost already a lot going from analogue to digital

Wavelet JP2 DCT JPEG Identical Compression Ratio Artefacts problem

MRC = Mixed Raster Content

How about other originals? Typewritten text (93 dpi) 2,326 KB BW  GIF = 23 KB  Fax Gr. 3 = 20 KB  PNG = 17 KB  Fax Gr. 4 = 14 KB  JB2 DjVu = 8 KB JB2 DjVu JPEG = 546 KB JPEG DjVu = 10 KB DjVu Illuminated manuscript 225 dpi; 54,654 KB TIFF/LZW = 36,355 PNG = 28,404 JP2 lossless = 22,595 Wavelet = 1,322 (JP2, IW44 in DjVu)

1,322 KB for the entire image IW44 in DjVuDjVu JP2 DCT in JPEGJPEG

Colour fidelity ICC profile included Calibration table:  Printed on permanent paper with permanent colours  Scanned with original  Stored with the digital copy Metadata description of the image stored in a special SGML/XML file

Solutions for archiving Rely on well established ISO standards :  uncompressed TIFF  TIFF/Fax Gr. 4 for black-and-white images  JPEG Can rely on other well established formats:  GIF (simple graphics and bw)  PNG (everything for efficient lossless compression)  PDF for computer generated documents

Solutions for delivery The archival formats and pre-processed sets of imagespre-processed sets or More efficient modern solutions:  DjVu (for almost everything)  MrSID (maps) MrSID Viewing comfort at the user’s side must be solved (integration of plug-ins or ActiveX components into web browsers) IE has problems with larger JPEG, but DjVu and MrSID very good viewing tools Netscape Communicator – everything possible, but obsolete in handling Netscape 6.xx – good plug-in and file association properties, but latest viewing components only as ActiveX (DjVu)

Critical points in modern solutions Slow encoding and high requirements for computation power at larger images Many of them depend on expensive software that must be purchased Viewing problems and integration in web browsers Good solutions have free browser plug-ins available and free encoding software for home use (smaller images or less functionality); they enable flexible display of very large files (DjVu, MrSID)

Quality Levels of Reproduction Usage-driven Creators:  Must have tools for easy and good-quality production  Must provide tools for users to enable comfortable access (digital library solutions)  Must preserve what they have created we are responsible for what we do