A Digital Imaging Primer

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A DigitalImaging Primer

• Nick Dvoracek
• Instructional Resources Center
• University of Wisconsin Oshkosh

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Often not an end in itself

• Files created for use
• On the World Wide Web
• In presentation programs
• In desktop publishing
• For scientific analysis

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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
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Digital Images are made from a pattern of dots

• Called pixels
• Short for picture element

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2 x 3 pixels

• less that 1K

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25 x 17 pixels

• 2K

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125 x 87 pixels

• 32K

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375 x 260

• 286K

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1500 x 1040

• 4.47MB

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Image types
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Color Depth

• The amount of data for each pixel determines the
  number of possible colors

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Line art (1 bit per pixel)

• Use when you only have black and white in an
  image
• For print applications

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Indexed color (8 bits)

• 256 (28) colors
• Used to reduce file size
• Other methods accomplish this better
• Transparent interlaced GIF

Color Look Up Table
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Indexed color (8 bits)

• Best format for Graphic images
• That dont have 256 colors to start with

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System Palette

• Not very accurate, but universal
• No longer necessary

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Adaptive pallette

• Best approximates true colors

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Dithering

• Better color approximation
• But lower resolution

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Grey scale

• 8 bits per pixel
• 256 shades of grey
• About as many as can be perceived

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Full Color (RGB)

• Millions of colors
• 24 bits per pixel
• 8 bits for Red
• 8 bits for Blue
• 8 bits for Green

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Full Color (RGB)

• Millions of colors
• 24 bits per pixel
• 8 bits for Red
• 8 bits for Blue
• 8 bits for Green

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Full Color (CMYK)

• Use when printing critical color
• Define RGB to CMYK specifically for your monitor
  and printer

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Resolution equals

• number of pixels
• divided by size

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125 x 87 pixels
3.5
5
25
125 x 87 pixels 25 dpi
3.5
5
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125 x 87 pixels
1.1
1.6
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125 x 87 pixels 75 dpi
1.1
1.6
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The right resolution

• 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

• 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
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The right resolution - Print

• Color or Greyscale
• Printers simulate colors and shades of grey by
  halftoning

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Halftoning

• clustered to create

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Halftoning

• Color printers work this way also
• Except with four layers of color

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The right resolution - Print

• Color or Grey
• One and a half to two times the printers
  halftone frequency
• 125 to 200 dots per inch

See Page 9
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The right resolution - Print

• 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

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The right resolution - The screen

• 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

• 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.

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The right resolution - The screen

• 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

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The right resolution - The screen

• The web
• Web browsers ignore any information that refers
  to physical size.
• Only dimensions in pixels are taken into account

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The right resolution - The screen

• The web

• Different users may have different sized monitors
  at different resolution.
• Assume a browser window 750 pixels wide.
• It wont always be right, but its the most
  common.

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The right resolution - The screen

• The web

• You can resize images with HTML commands
• But its a really bad idea.

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Scanning control
La Cie Silverscanner
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Digital Camera resolution
6 Megapixels 2832 x 2128
3 Megapixels 2048 x 1536
1 Megapixel 1280 x 960
VGA 640 x 480
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File formats - TIFF

• Supports any size, resolution or color depth.
• Best format for desktop publishing
• Modest file compression (LZW)

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File formats - JPEG

• Use for natural pictures
• photographs, drawings
• For print applications use highest quality
• For internet, medium quality is OK
• Lossy compression - dont use if youre going
  to manipulate image

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File formats - GIF

• Use for graphic pictures
• charts, diagrams
• Requires indexed color
• Only format that features transparency and
  interlace.

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Embedded files

• When you place a file in PowerPoint
• your file size increases by the size of the
  placed files.

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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.

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Optical Character Recognition

• 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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dvoracek_at_uwosh.edu