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

Digitization of Visual Resources

Jenn Riley

Metadata Librarian

IU Digital Library Program

October 21, 2004 L597: Humanities Computing

Technical overview

Analog to digital conversion Resolution Bit depth Color representation Reflectivity and polarity Compression


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”


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


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


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”)


Bit depth (2)

1 bit (black & white) 2 bit (4 colors)

4 bit (16 colors) 8 bit (256 colors)


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


Reflectivity and polarity

Positive Negative

Reflective Paper

Photographic prints

Transmissive Slide film Negative film


Compression

Makes files smaller for storage Files must be decompressed for viewing Lossless Lossy

“visually lossless”


Technical questions?

Analog to digital conversion Resolution Bit depth Color representation Reflectivity and polarity Compression


Setting specifications

Standards & best practices Capture once, use many Determine purpose Resolution Bit depth & color File formats Quality control


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


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


Determine purpose

Nature of materials Artifactual For content only

Capture all important information But what’s “important”? Not always “what people can see”


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>


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”)


Resolution comparison (1)


Resolution comparison (2)

600 dpi 300 dpi


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”


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


Why not JPEG?

Lossy-compressed every time they are saved

low compression, high quality high compression, low quality


Delivery file formats

Photographic materials: JPEG Text, line drawings: GIF PNG?


Quality control

Essential part of every digitization project Objective criteria

Can be automated Can check all items

Subjective criteria Require human checks Must sample


Specifications questions?

Standards & best practices Capture once, use many Determine purpose Resolution Bit depth & color File formats Quality control


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

Visual Resources Metadata in Libraries

Jenn Riley

Metadata Librarian

IU Digital Library Program


What is metadata?

“Data about data”


A better definition

Other characteristics Structure Control

Origin Machine-generated Human-generated

In practice, often grows to cover data and meta-metadata


Levels of control

Data structure standards Data content standards


Creating metadata

HTML <meta> tags Spreadsheets Databases XML Digital library content management systems

ContentDM Greenstone


Types of metadata

Descriptive metadata Administrative metadata

Technical metadata Preservation metadata Rights metadata

Structural metadata


Purposes of descriptive metadata Description Discovery


General descriptive metadata standards Dublin Core [example]

Unqualified Qualified

MARC/AACR2 [example]

MARCXML MODS [example]


Visual resources metadata standards CDWA [example]

VRA Core [example]

CCO


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


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


METS

metsHdr dmdSec amdSec

techMD rightsMD sourceMD digiprovMD

fileSec structMap structLink behaviorSec


Some other specialized metadata formats TEI EAD [example]

GILS CSDGM [example]

GEM CIDOC


Vocabularies

TGM I TGM II TGN GeoNet AAT LCSH LCNAF DCMI Type MIME Types


Other considerations

Standard formatting Repeatability of elements Describing original vs. digitized item Relationships between records Interoperability


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


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!


More information

Individual schema documentation Caplan: Metadata Fundamentals for all

Librarians, 2003 NISO Press: Understanding Metadata IFLA Functional Requirements for

Bibliographic Records


Thank you!

[email protected] These presentation slides:

<http://www.dlib.indiana.edu/~jenlrile/presentations/slis/04fall/l597walsh/L597.ppt>

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

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