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Humanities Computing: Digitzation of Visual Resources Technical Overview

Learn about analog-to-digital conversion, resolution, bit depth, color representation, reflectivity, and compression in visual resource digitization. Get insights on best practices, standards, and specifications for optimal outcomes.

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Humanities Computing: Digitzation of Visual Resources Technical Overview

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  1. Digitization of Visual Resources Jenn Riley Metadata Librarian IU Digital Library Program

  2. Technical overview • Analog to digital conversion • Resolution • Bit depth • Color representation • Reflectivity and polarity • Compression L597: Humanities Computing

  3. Analog to digital conversion • Image is converted to a series of pixels laid out in a grid • Each pixel has a specific color, represented by a sequence of 1s and 0s • Pixel-based images are called “raster” images or “bitmaps” L597: Humanities Computing

  4. Resolution (1) • Often referred to as “dpi” or “ppi” • RATIO of number of pixels captured per inch of original photo size • 8x10 print scanned at 300ppi = 2400 x 3000 pixels • 35mm slide (24x36mm!) scanned at 300ppi ≈ 212 x 318 pixels L597: Humanities Computing

  5. Resolution (2) • “Spatial resolution” refers to pixel dimensions of image, e.g., 3000 x 2400 pixels • Flatbed and film scanners have a fixed focus, so they know how big the original is; digital cameras don’t L597: Humanities Computing

  6. Bit depth (1) • Refers to number of bits (binary digits, places for zeroes and ones) devoted to storing color information about each pixel • 1 bit (1) = 21 = 2 shades (“bitonal”) • 2 bit (01) = 22 = 4 shades • 4 bit (0010) = 24 = 16 shades • 8 bit (11010001) = 28 = 256 shades (“grayscale”) L597: Humanities Computing

  7. Bit depth (2) 1 bit (black & white) 2 bit (4 colors) 4 bit (16 colors) 8 bit (256 colors) L597: Humanities Computing

  8. Color representation • RGB • Scanners generally have sensors for Red, Green, and Blue • Each of these “channels” is stored separately in the digital file • 8 bits for each of 3 channels = 24 bit color • CMYK (Cyan, Magenta, Yellow and Black) is used for high-end “pre-press” printing purposes L597: Humanities Computing

  9. Reflectivity and polarity L597: Humanities Computing

  10. Compression • Makes files smaller for storage • Files must be decompressed for viewing • Lossless • Lossy • “visually lossless” L597: Humanities Computing

  11. Technical questions? • Analog to digital conversion • Resolution • Bit depth • Color representation • Reflectivity and polarity • Compression L597: Humanities Computing

  12. Setting specifications • Standards & best practices • Capture once, use many • Determine purpose • Resolution • Bit depth & color • File formats • Quality control L597: Humanities Computing

  13. Standards & best practices • NARA Technical Guidelines for Digitizing Archival Materials for Electronic Access • Moving Theory into Practice • Book • Online tutorial • California Digital Library Digital Image Format Standards • Western States Digital Imaging Best Practices L597: Humanities Computing

  14. Capture once, use many • Create master image when scanning • Capture all “important” information • Meets all foreseeable needs • For long-term storage and later use • Create derivatives for specific uses later • Web delivery • Printing • Publication L597: Humanities Computing

  15. Determine purpose • Nature of materials • Artifactual • For content only • Capture all important information • But what’s “important”? • Not always “what people can see” L597: Humanities Computing

  16. Resolution (1) • Higher is not always better • Scan at highest resolution necessary to achieve your stated purpose, no higher chart from Cornell’s online digital imaging tutorial: <http://www.library.cornell.edu/preservation/tutorial/conversion/conversion-03.html> L597: Humanities Computing

  17. Resolution (2) • Color photographic materials • 3000-6000 pixels on the long side • 24-bit RGB • B/W photographic materials • 3000-6000 pixels on the long side • 8-bit grayscale (unless sepia tone is “important”) L597: Humanities Computing

  18. Resolution comparison (1) L597: Humanities Computing

  19. Resolution comparison (2) 600 dpi 300 dpi L597: Humanities Computing

  20. Bit depth & color • Match current photo or match original scene • Final master images should be 8 bits per channel (8-bit grayscale, 24-bit RGB); some specialized projects using higher bit depths • Any color adjustments & other processing should be done in scanning software before final scan is done • Use almost the full tonal range; avoid “clipping” L597: Humanities Computing

  21. Master file formats • TIFF (uncompressed) • Virtually unanimously recommended by digital imaging best practices • “De facto” standard • JPEG2000 • ISO/IEC IS 15444-1 | ITU-T T.800 • Not patent-free • Up-and-coming but not quite there yet • Supports embedded metadata • Uses wavelet-based compression L597: Humanities Computing

  22. Why not JPEG? • Lossy-compressed every time they are saved low compression, high quality high compression, low quality L597: Humanities Computing

  23. Delivery file formats • Photographic materials: JPEG • Text, line drawings: GIF • PNG? L597: Humanities Computing

  24. Quality control • Essential part of every digitization project • Objective criteria • Can be automated • Can check all items • Subjective criteria • Require human checks • Must sample L597: Humanities Computing

  25. Specifications questions? • Standards & best practices • Capture once, use many • Determine purpose • Resolution • Bit depth & color • File formats • Quality control L597: Humanities Computing

  26. More information • NARA Technical Guidelines for Digitizing Archival Materials for Electronic Access • Moving Theory into Practice • Book • Online tutorial • IU DLP Use of Digital Imaging Standards & Best Practices L597: Humanities Computing

  27. Visual Resources Metadata in Libraries Jenn Riley Metadata Librarian IU Digital Library Program

  28. What is metadata? “Data about data” L597: Humanities Computing

  29. A better definition • Other characteristics • Structure • Control • Origin • Machine-generated • Human-generated • In practice, often grows to cover data and meta-metadata L597: Humanities Computing

  30. Levels of control • Data structure standards • Data content standards L597: Humanities Computing

  31. Creating metadata • HTML <meta> tags • Spreadsheets • Databases • XML • Digital library content management systems • ContentDM • Greenstone L597: Humanities Computing

  32. Types of metadata • Descriptive metadata • Administrative metadata • Technical metadata • Preservation metadata • Rights metadata • Structural metadata L597: Humanities Computing

  33. Purposes of descriptive metadata • Description • Discovery L597: Humanities Computing

  34. General descriptive metadata standards • Dublin Core[example] • Unqualified • Qualified • MARC/AACR2[example] • MARCXML • MODS[example] L597: Humanities Computing

  35. Visual resources metadata standards • CDWA[example] • VRA Core[example] • CCO L597: Humanities Computing

  36. Technical metadata • For recording technical aspects of digital objects • Of use for long-term maintenance of data • Standards for still images • NISO Z39.87: Data Dictionary – Technical Metadata for Digital Still Images • MIX L597: Humanities Computing

  37. Structural metadata • For creating a logical structure between digital objects • Multiple copies of same bibliographic item • Multiple pages within item • Grouping of pages into sections • Multiple sizes of each page • METS is the current primary schema L597: Humanities Computing

  38. METS • metsHdr • dmdSec • amdSec • techMD • rightsMD • sourceMD • digiprovMD • fileSec • structMap • structLink • behaviorSec L597: Humanities Computing

  39. Some other specialized metadata formats • TEI • EAD[example] • GILS • CSDGM[example] • GEM • CIDOC L597: Humanities Computing

  40. Vocabularies • TGM I • TGM II • TGN • GeoNet • AAT • LCSH • LCNAF • DCMI Type • MIME Types L597: Humanities Computing

  41. Other considerations • Standard formatting • Repeatability of elements • Describing original vs. digitized item • Relationships between records • Interoperability L597: Humanities Computing

  42. Crosswalks • For transforming between metadata formats • Usually refers to transforming between content standards rather than structure standards, but not always • Good practice to create and store most robust metadata format possible, then create other views for specific needs L597: Humanities Computing

  43. The bottom line • Many concepts from a century and a half of library cataloging inform good metadata practices • But some re-examination needed for new environment • Need for automation • Need for smart people contributing! L597: Humanities Computing

  44. More information • Individual schema documentation • Caplan: Metadata Fundamentals for all Librarians, 2003 • NISO Press: Understanding Metadata • IFLA Functional Requirements for Bibliographic Records L597: Humanities Computing

  45. Thank you! • jenlrile@indiana.edu • These presentation slides: <http://www.dlib.indiana.edu/~jenlrile/presentations/slis/04fall/l597walsh/L597.ppt> L597: Humanities Computing

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