1 CS 502: Computing Methods for Digital Libraries Lecture 9 Conversion to Digital Formats Anne Kenney, Cornell University Library

2 What are Digital Images? Electronic snapshots taken of a scene or scanned from documents samples and mapped as a grid of dots or picture elements (pixels) pixel assigned a tonal value (black, white, grays, colors), represented in binary code code stored or reduced (compressed) read and interpreted to create analog version

Four Scanning Methods BitonalGrayscale Color Special Treatment

4 Digital Image Quality is Governed By: resolution and threshold bit depth image enhancement color management compression system performance operator judgment and care

5 Resolution determined by number of pixels used to represent the image expressed in dots per inch (dpi)-- actually dots/sq. inch increasing resolution increases level of detail captured and geometrically increases file size

Effects of Resolution 600 dpi 300 dpi 200 dpi

7 Threshold Setting in Bitonal Scanning defines the point on a scale from 0 to 255 at which gray values will be interpreted either as black or white

8 Effects of Threshold threshold = 100 threshold = 60

9 Bit Depth number of bits used to represent each pixel, typically 8 bits or more per channel representing 256 (2 8 ) levels for grayscale and 16.7 million (2 24 ) levels for color example: 8-bit grayscale pixel = black = white

10 Bit Depth increasing bit depth increases the level of gray or color information that can be represented and arithmetically increases file size affects resolution requirements

11 Effects of Grayscale on Image Quality 3-bit gray8-bit gray

12 Image Enhancement can be used to improve image capture use raises concerns about fidelity and authenticity

13 Effects of Filters no filters used no filters used maximum enhancement maximum enhancement

14 Image Editing

15 Compression reduces file size for processing, storage, transmission, and display image quality may be affected by the compression techniques used and the level of compression applied

16 Compression Variables lossless versus lossy compression proprietary vs. open schemes level of industry support bitonal vs. gray/color

17 Common Compression Schemes bitonal –ITU Group 4: lossless –JBIG (ISO 11544): lossless –CPC: Lossy –DigiPaper grayscale/color –LZW, lossless –JPEG: lossy –Kodak Image Pac, “visually lossless” –Fractal and Wavelet compression

18 Effects of JPEG Compression 300 dpi, 8-bit grayscale uncompressed TIFF JPEG 18.5:1 compression

19 Compression Observations the richer the file, the more efficient and sustainable the compression the more complex the image, the poorer the compression

20 Equipment used and its performance over time scanners offer wide range of capabilities to capture detail, dynamic range, and color scanners with same stated functionality can produce different results calibration, age of equipment, and environment affect quality

21 Equipment used and its performance over time attributes and capabilities of monitor and/or printer are also factors assess quality visually and computationally –use targets –control QC environment –increasing availability of software to assess resolution, tone, color, artifacts

22 Image Capture: Create digital objects rich enough to be useful over time in the most cost- effective manner.

23 How to determine what’s good enough? Connoisseurship of document attributes Objective characterizations Translation between analog and digital –measurement to scanning requirement to corresponding image metrics –e.g., detail size  resolution  MTF –tonal range  bit depth  signal-to-noise ratio

24 Case Study Brittle Books--printed text, use of metal type, commercial publishers, objective measurement, use of Quality Index from micrographics 600 dpi 1-bit capture adequately preserves informational content of text-based materials

25 Ensuring Full Informational Capture: “No More, No Less” cost image quality and utility desired point of capture

26 Create One Scan To Serve Multiple Uses Derive alternative formats/approaches to meet current and future information needs Base “derivative” requirements on document attributes, technical infrastructure, user requirements, and cost Understand technical links affecting presentation and utility of derivatives

27 User Requirements completeness legibility speed of delivery “cooked” files

28 Derivatives from a Digital Master the richer the image, the better the derivative –a derivative from a rich file is superior in quality to one from a poorer scan –the richer the image, the better the image processing

monitor: 800 x 600 pixels document: 8” x 10”, 200 dpi (1,600 x 2,000 pixels) 2,000 pixels 1,600 pixels document at 60 dpi 480 pixels x 600 pixels document at 100 dpi 800 pixels x 1,000 pixels

TIFF Uncompressed GGIF Compressed 6:1 (NARA) JPEG Compressed 20:1 ( LC) Compressed 20:1 (LC) Compression/File Format Comparison for Derivative Files

33 Alternatives for Displaying Oversize Images File formats and compression schemes that support multi-resolution image delivery, e.g., wavelet compression, GridPix, Flashpix User tools for representing scale (Blake Project ImageSizer, java applet), and improving image quality

34 Recommendations Coalescing Intent of conversion drives decisions –issues of access considered at conversion –notion of long-term utility and cross- institutional resources gaining ground Access images will change with: –changing user needs and capabilities –changes in technologies: file formats, technical infrastructure,compression, web browsers, processing programs, scaling routines