Transcript A Digital Imaging Primer

A Digital
Imaging Primer
Nick Dvoracek
Instructional Resources Center
University of Wisconsin Oshkosh
Often not an end in itself
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Files created for use
On the World Wide Web
In presentation programs
In desktop publishing
For scientific analysis
In order to get
The best image quality
For the least amount of disk space
You must consider
Image type
Intended use (output device)
Size in final application
Digital Images are made from
a pattern of dots
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Called pixels
Short for picture element
2 x 3 pixels
less that 1K
25 x 17 pixels
2K
125 x 87 pixels
32K
375 x 260
286K
1500 x 1040
4.47MB
Image types
Line Art
Black and
White only
Continuous
tone
greyscale
“Graphics”
Flat areas of
few colors
Continuous
tone color
Color Depth
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The amount of data for
each pixel determines the
number of possible colors
Line art (1 bit per pixel)
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Use when you only
have black and
white in an image
For print
applications
Indexed color (8 bits)
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256 (28) colors
Used to reduce
file size
Other methods
accomplish this
better
Transparent &
interlaced GIF
Color Look Up Table
Indexed color (8 bits)
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Best format for
“Graphic”
images
That don’t have
256 colors to
start with
System Palette
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Not very
accurate, but
universal
No longer
necessary
Adaptive pallette
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Best
approximates
true colors
Dithering
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Better color
approximation
But lower
resolution
Grey scale
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8 bits per pixel
256 shades of
grey
About as many
as can be
perceived
Full Color (RGB)
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Millions of colors
24 bits per pixel
8 bits for Red
8 bits for Blue
8 bits for Green
Full Color (RGB)
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Millions of colors
24 bits per pixel
8 bits for Red
8 bits for Blue
8 bits for Green
Full Color (CMYK)
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Use when
printing critical
color
Define RGB
to CMYK
specifically for
your monitor
and printer
Resolution equals
number of pixels
divided by size
125 x 87 pixels
3.5
”
5”
125 x 87 pixels 25 dpi
3.5”
5”
125 x 87 pixels
1.1”
1.6”
125 x 87 pixels 75 dpi
1.1”
1.6”
The right resolution
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Depends on
Image type
What is output device capable of?
There is no “universal” scan.
Too high saves immense amounts of
data without improving image
Too low results in poor image quality
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The right resolution - Print
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Line copy
Use the full resolution of the output
device
Printers range from 300 dpi to 1440
Anything over 400 is hard to see the
difference
See Page 7
The right resolution - Print
Color or Greyscale
Printers simulate colors and shades of
grey by “halftoning”
Halftoning
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“printer dots”
clustered to create
“halftone dots”
Halftoning
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Color printers
work this way also
Except with four
layers of color
The right resolution - Print
Color or Grey
One and a half to two times the
printer’s “halftone frequency”
125 to 200 dots per inch
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See Page 9
The right resolution - Print
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Be sure to specify final size of the
image.
Resolution (dots per inch) changes if
the number of inches changes.
i.e. if you resize in the final application
The right resolution - The screen
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Monitors and projectors can
display any color for each pixel
The size of monitors and projectors is
commonly expressed in numbers of pixels
rather than physical size
640 x 480, 800 x 600, 1024 x 768
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The right resolution - The screen
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Treat line copy, “graphic” images, grey
scale and color the same.
Use indexed color or grey scale instead
of 1-bit for line copy images.
1-bit
greyscale
The right resolution - The screen
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Powerpoint
Assume you will be using a 1024 x 768
projector.
Powerpoint thinks its slides are 10 x 7.5 inches
Acquiring 10 x 7.5 inches at 100 dpi gives you
the right number of pixels
if your image is intended to fill the screen
The right resolution - The screen
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The web
Web browsers ignore any information
that refers to physical size.
Only dimensions in pixels are taken
into account
The right resolution - The screen
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The web
Different users may have different
sized monitors at different resolution.
Assume a browser window 750 pixels
wide.
It won’t always be right, but it’s the
most common.
The right resolution - The screen
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The web
You can resize images with HTML
commands
But it’s a really bad idea.
Scanning control
La Cie
Silverscanner
Digital Camera resolution
6 Megapixels – 2832 x 2128
3 Megapixels – 2048 x 1536
1 Megapixel – 1280 x 960
VGA – 640 x 480
File formats - TIFF
Supports any size, resolution or color
depth.
Best format for “desktop publishing”
Modest file compression (LZW)
File formats - JPEG
Use for “natural” pictures:
photographs, drawings
For print applications use highest quality
For internet, medium quality is OK
“Lossy” compression - don’t use if you’re
going to manipulate image
File formats - GIF
Use for “graphic” pictures:
charts, diagrams
Requires indexed color
Only format that features transparency and
interlace.
Transparent
Not
Transparent
Embedded files
When you place a file in PowerPoint…
your file size increases by the size of the
placed files.
Linked files
When you place a file in Pagemaker…
your file size increases only slightly
but, you have keep the image file in the same
place for Pagemaker to use when it needs to
display or print the file
Put everything in a folder
If you move it, move the whole folder.
Optical Character Recognition
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Requires scanner and OCR program
Results are pretty good but not perfect
Images and text saved as separate files
Make sure original is placed in scanner
straight.
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