Scanning and Digital
Imaging Basics
Digital Initiatives Team
October 18, 2006
Gretchen Gueguen
ggueguen@umd.edu
301.314.2558
Introduction
• What will we cover?
– What do I need to know about digital
images?
– How do I make/edit digital images?
– How do I use digital images?
• Why are we covering it?
– To ensure a level of quality in images
that we scan.
– To ensure a level of quality in images
that we keep for archival purposes.
Digital Images and Pixels
What are Digital Images Made of?
• Digital elements called pixels.
– Pixels are bundles of digital information. They are
organized into a grid to convey the image.
Dimensions of the digital image are given in pixels.
• Image elements called dots.
– Dots are smallest unit of a color. The more dots in an
inch of the picture, the better it looks. This aspect is
called resolution.
• Print size
– Pixels determine the size of the digital image, but the
print size determines what you will see on the
screen or when you print out an image.
Patterns of dots
produce the
effects of
consistent color
The many sizes
of Testudo
Changing the document size alone doesn’t
change the pixel dimensions or the file size
Pixel size
Before:
6.167 x 4.014 inches (444x289 pixels)
72dpi
375.9K
After:
3 x 1.953 inches (444x289 pixels)
148dpi
375.9K
Print size
The many sizes
of Testudo
“Resampling” changes both the pixel dimensions, the
print size, and the file size.
Before:
6.167 x
4.014 inches
Pixel size
444 x
289 pixels)
After:
12 x
7.811
inches
864 x
562 pixels
72dpi
72dpi
375.9K
1.39 M
Print size
Digital Images and Pixels
• The ratio of the number of dots to
the PRINT size is the Resolution.
– Resolution is used to judge the
quality of the image. The higher the
resolution, the sharper the image
details are.
– Resolution should change as print
size changes if the image is not
resampled.
– Resolution is most often measured in
dots per inch or ‘dpi’.
Digital Images and Pixels
What are pixels made of?
• Pixels are groups of digital information
– groups of bits (0 or 1) that give
directions for different colors
– The number of bits in the pixel determines
the color palette:
More bits = more combinations = more
possible colors
This is called ‘bit-depth’ and it is closely
related to color profiles or modes
Color profile and bit-depth
• How does bit-depth affect color?
– 1-bit
– 2-bit
– 4-bit
– 8-bit
= two colors (‘0’ or ‘1’ for each pixel)
= four colors (’00’, ’11’, ’01’, ’10’)
= 16 colors (‘0000’, ‘0001’, etc.)
= 256 colors (‘00000000’, ‘00000001’, etc.)
The more colors you can use,
the more realistic a picture looks
Color profile and bit-depth
• So can I get more than 256 colors?
– 3 channels (R,G,B) @ 8-bit =
8-bit/channel, or 24-bit/ pixel
“millions of colors”
• Can I get even more than that?
– 3 channels (R,G,B) @ 16-bit =
16-bit/channel or 48-bit/pixel =
“millions of colors” x 2
The Big Picture
Which Color Profile to Use?
1-bit:
– Black & White [text with no artifactual value mainly
for large-scale book scanning]
8-bit, 1 channel:
– Indexed color [web graphics and thumbnails]
– Grayscale [text with some artifactual value; rarely
with black and white images]
8-bit, 3 channels:
– RGB (red, blue, green, the primary colors of light…and
your monitor) [manuscript text, photographs, slides, etc.]
8-bit, 4 channels:
– CMYK (cyan, magenta, yellow, black, the primary
colors of ink used in printing) [images destined for
offset printing]
The Big Picture
• Next you need to decide how many dots
to use to capture your image.
Theoretically, the more you cram in the
better the image is, right?
Answer: yes and no
– Image quality relates both to the number of dots of
color and the size at which the image is displayed.
– Once the image is scanned, dots can be stretched or
compressed but should not be added.
The Big Picture
=
Increasing size without increasing
resolution stretches out the dots of color.
The Big Picture
=
2.7x4.5 @ 72dpi;
18.5kb
10x16 @ 19 dpi
18.5kb
The Big Picture
≠
However, keeping resolution constant results in
‘interpolation’. The computer adds in
new dots and guesses their color.
The Big Picture
=
2.7x4.5 @ 72dpi;
18.5kb
10x16 @ 72dpi
4.6Mb
The Big Picture
The Big Picture
• So the only reliable way to
increase the size of an image (or
zoom into a detail) is to scan at a
higher resolution than what you
would need at the regular size
• How do you know what resolution
you need?
– Print limitations: 280 to 300 dpi
– Screen limitation: 72 dpi
–Institutional Guidelines
The Big Picture
=
Resolution SHOULD decrease when you enlarge,
but if you scan at a high resolution, the loss
won’t be as severe.
The Big Picture
=
2x1.5 @ 300 dpi
95kb
9x6.5 @ 72 dpi
95kb
The Big Picture
If we want to create a smaller file size, we can change
the print size and resolution through resampling as long
as the resulting image decreases these values and still
meets the needs of viewing medium.
Saving and Sharing
• Now that we know what settings
to use to make a good image, how
do we save it?
– Need a copy to keep
(hi-res, original size, big
file)
– Need a copy to print
(med- to hi-res,
determined size, big to medium file)
– A copy to view
(lo-res, determined size, small
file)
– A copy for comparison
small file)
(lo-res, small size,
Saving and Sharing
• Other factors that affect file size:
– Compression:
• Lossy
– Similar information is lumped together
• Lossless
– Compresses data but keeps the directions to
reconstruct it. Doesn’t compress as much as
lossy compression.
Saving and Sharing
– Format:
• TIFF files are usually the largest because they can
be uncompressed or compressed using a lossless
method.
• JPEG files use a lossy compression and are much
smaller than TIFFs
• GIF files also use lossy compression in
combination with an index color scheme making
them very small.
For archival purposes a TIFF file is usually kept as a
‘master’ file. JPEGs are created as high-quality
derivatives and GIFs may be created for use as
thumbnails.
Making Decisions
• Now that you understand some of
the variables, resources exist to
help you make decisions
– Best Practices for Digital projects
• http://www.lib.umd.edu/dcr/publications
– Workshop handouts
– Digital Collections and Research
• http://www.lib.umd.edu/dcr
• 301-314-2558
• ggueguen@umd.edu
Scan Interfaces
• What variables can I control when
scanning?
– Color profile/bit depth
– Resolution
– Size
– Histogram balance / Tone Curve
Scan Interfaces
Epson Scan in ‘Professional Mode’
Color settings lists bitdepth but not “RGB”
Both resolution and
print (document) size
are changeable
The histogram tool is
opened from here. It
will adjust the
exposure settings
Epson Interface
Scan Interfaces
Epson Scan in ‘Professional Mode’
Histogram input
sliders and levels
Histogram output
sliders and levels
Scan Interfaces
Microtek Interface
Color
settings
Resolution
and print
size settings
Open Color
Correction
tool
Open Tone
Curve tool
Overview,
Prescan, and
Scan buttons
Open
correction
tools
Preview
Window
Scan Interfaces
Microtek Interface
Color
wheel
Info
window
Scan Interfaces
Microtek Interface
Interface with a
correction tool
open
Scan Interfaces
Nikon SuperCoolScan 5000ED
(for slides)
Option to see the
image with or
without corrections
Color options
are:
Calibrated
RGB or
Grayscale
Preview and scan buttons
“Keep this File Size” and
“Resolution” controls
Scan Interfaces
Nikon SuperCoolScan 5000ED
(for slides)
Curves palette does the
same thing as the histogram
adjustment in the Epson
scanner.
Examples of Noise, Distortion,
Channel Problems
Fixed pattern
noise
Random
noise
Moiré patterns
Banding
Channel misalignment