Quality Levels of Reproduction

Adolf Knoll

National Library of the Czech Republic

Archival image Access image

Facsimile printing or displayPreprocessed set of various

quality level images

Dynamically postprocessedImage 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

Dig

ital w

orld

Ana

logu

e w

orld

continuousdiscrete

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 illusionof reality

Colour depth

Resolution

Compression

Colour fidelity

300 dpi15,173 KB

Colour depth

256 shadesof gray

5,062 KB

256 colourssame size

Colour depth

16 shadesof gray

2,531 KB

Colour depth

Black-and-white = 2 colours637 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 KB

Compression

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

JPEG = 546 KB DjVu = 10 KB

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 DjVu JP2

DCT in JPEG

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 imagesor

More efficient modern solutions: DjVu (for almost everything) MrSID (maps)

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

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