from analog to digital peter hirtle hbcu-cul digital imaging workshop ii july 31, 2007
TRANSCRIPT
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From Analog to Digital
Peter HirtleHBCU-CUL Digital Imaging
Workshop IIJuly 31, 2007
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Session Goal
Workshop participants will understand the issues involved in converting analog materials into digital formats for preservation
and access.
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Session Objectives Overview the application of the
scanning process in the digitization chain
In-depth presentation on the factors affecting image quality
Discuss the issues involved in presentation
Participate in “reality checks” on lessons learned
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Learner Outcomes Be able to Discuss issues involved
in converting analog materials to digital images
Compare and Contrast three types of image scanning
Understand factors affecting image presentation
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Analog to Digital
We’re scanning 2D images – (textual and graphical) & 3D images (photographic)
Other 3D images are tape-based, such as: Video Audio
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What are Digital Images? “electronic photographs” created through
scanning or digital photography Sampled 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
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Digital Image Also known as a raster or bitmap
image
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Three types of scanning BITONAL GRAYSCALE
COLOR
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Bitonal Scanning
Information is presented as either black or white
Gray shades simulated by clustering black dots
Suitable for printed materials
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Grayscale Scanning Shades of gray
represented; normally 8 bits per pixel Suitable for
manuscripts, photographs, halftones, and stained documents
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Bitonal vs. Grayscale Capture of Stained Manuscript
Bitonal scan Grayscale scan
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Color Scanning Full color range; normally
24 bits per pixel Suitable for documents
with significant color information, such as photographs, works of art
Helpful in determining a document’s age, physical condition, or previous use
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Digital Image Quality Affected By
Document attributes Resolution and threshold Bit depth and dynamic range Image enhancement Compression/format Equipment and performance over
time Operator judgment and care
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Important Document Attributes Physical type, size, and
presentation Physical condition Document type The Big Three
Detail, Tone, Color
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Scanned from OriginalOn H-P ScanJet 3c
Scanned from 2N FilmOn Sunrise SRI 50
Halftone: 1-bit image at 400 dpi
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600 dpi 1-bit2.9 Mb file
400dpi 8-bit10.3 Mb file
400 dpi 24-bit30.9 Mb file
Matching Informational Content to Scanning Approach: Quality vs. File Size and Cost
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Resolution Determined by number of pixels used to
represent the image expressed in dots per square inch (dpi)
zoom in
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Resolution
100 dpi = 1002 (100 x 100) or 10,000 dots
200 dpi = 2002 or 40,000 dots
400 dpi = 4002 or 160,000 dotsincreasing resolution increases detail
captured and geometrically increases the file size
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Effects of Resolution
600 dpi600 dpi
300 dpi300 dpi
200 dpi200 dpi
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Threshold Setting in Bitonal Scanning
defines the point on a scale from 0 (black) to 255 (white) at which gray values will be interpreted either as black or white
0 255black dark
gray lightgray
white
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Effects of Threshold
threshold = 100
threshold = 60
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Defining Detail in Text Fixed metric: smallest lower case letter Variables: quality, resolution, feature
size Bitonal QI formula for text
dpi = 3QI/.039h QI values: 8(excellent), 5(good),
3.6(marginal), 3(barely legible) Grayscale/Color QI formula for text
dpi = 2QI/.039h
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Example Text page with smallest character
measuring 1mm, which must be fully captured (QI=8, h= 1)
Bitonal Scanning: dpi = 3QI/.039h dpi = 3(8)/[.039(1)] = 615 dpi
Grayscale: dpi = 2QI/.039h dpi = 2(8)/[.039(1)] = 410 dpi
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http://images.library.uiuc.edu/projects/calculator/
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Bit Depth Determined by the number of
binary digits (bits) used to represent each pixel
1-bit 8-bit 24-bit
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Bitonal scanning has a bit-depth of 1
each pixel represented by one bit, with a tonal range of 2:
0 = black 1 = white
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Binary Calculations
21 = 222 = 423 = 824 = 1628 = 256210 = 1024212 = 4096224 = 16.7 million
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Bit Depth
Grayscale is typically 8 bits or more, representing 256 (28) levels
Color is 24 bits or more , representing 16.7 million (224) levels
example: 8-bit grayscale pixel 00000000 = black 11111111 = white
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Bit Depth
increasing bit depth increases the levels of gray or color that can be represented and arithmetically increases file size
affects resolution requirements
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Effects of Grayscale on Image Quality
3-bit gray 8-bit gray
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Tonal Reproduction Dynamic range: the range of tonal
difference between lightest light and darkest dark
The higher the range, the greater the number of potential shades
Tone distribution as important as dynamic range
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Comparing Key Types
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Mapping Tones Correctly: Use of Histograms
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Color Appearance Difficult to evaluate Hue, saturation, brightness Translating between analog and
digital, between color spaces, between reflected and transmitted light
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Representing Color Appearance
Balanced Color
Color Shift Towards Red
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Color Cast
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Color Imaging and Tone Distribution
Limited tones evident in highlights and shadows
Balanced tonal distribution
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Image Processing and Enhancement
Image editing to modify or improve an image filters (brightness, contrast, sharpness,
blur) file size reduction (scaling) tone and color correction
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Effects of FiltersEffects of Filters
no filters usedno filters used
maximum maximum enhancementenhancement
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Compression/format
image quality may be affected by the compression technique used to reduce file size
Image quality affected by format support for: Bit depth Compression techniques Color management
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Compression /File Format Comparison
GIF (lossless)File Size: 60 KB
JPEG (lossy)File Size: 49 KB
images courtesy of Edison Papers
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Equipment used and its performance over time
scanners offer wide range of capabilities to capture detail, tone, dynamic range, and color
scanners with same stated functionality can produce different results
calibration, age of equipment, and environment are also factors
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300 dpi, scanner A 300 dpi, scanner B
Variations in Image Quality due to Scanner Performance
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Image Capture:
Creating digital files rich enough to be useful over time in the most cost- effective manner.
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Why Create High Quality Images?
preservation original may only withstand one scan
cost one scan may be all that is affordable
access one scan can be used to derive
multiple images
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Calculating File Size
F file size = height x width x bit depth x dpi2
8
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File Size Naming Conventions
Represented in bytes
1 byte = 8 bits 1Kb ~ 1,000 bytes 1Mb ~ 1,000Kb 1Gb ~ 1,000Mb
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Example: grayscale image
file file size = height x width x bit depth x dpi2
8
7”
10”
300 dpi, 8-bit
file size = 10 x 7 x 8 x 3002
8
file size = 6,300,000 bytes = 6.3 megabytes
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800
200 dpi , 1 bit
10”
8”
Example: Bitonal Imagefile size = height x width x bit depth x dpi2
8
file size = 10 x 8 x 1x 2002
8
file size = 3,200,000 bytes = 3.2 megabytes
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Reality Check 1
Determine the file size of a letter size page captured bitonally at 200 dpi.
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Answer
(8.5 x 11 x 2002)/8 = 468Kb or .47Mb
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Reality Check 2
Determine the file size of an 8” x 10” photograph scanned in color at 200 dpi.
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Answer
(8 x 10 x 24 x 2002) = 9.6 million
bytes or 89.6Mb
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Estimating File Size for Compressed Images Compressed file size = file size
level of compression
Example: Color photo compressed 20:1
9.6Mb =.48Mb 20
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Aligning Document Attributes with Digital Requirements Identify key document attributes
Detail, tone, color Characterize them, if possible through
objective measurements Determine quality requirements and
tolerance levels Translate between analog and digital
and between scanning requirements and scanning performance
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Aligning Document Attributes with Digital Requirements Calibrate scanner, calibrate the
rest of the system Control lighting and environment Objective and Subjective
Evaluation: targets and software; evaluating images against the originals
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Aligning Document Attributes with Digital Requirements Minimize post-processing in the
master image Save in TIFF; use RGB for color;
and avoid lossy compression Maintain scanning metadata
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One Size Does Not Fit All!
different document types will require different scanning processes
the more complex the document, the higher the conversion/access requirements, the larger the file, and the greater the expense
scan the original whenever possible Follow project recommendations