Digital Images

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Digital Images

• Rudy Scott

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Graphics File Formats

• If you were a computer how would you store
  pictures?
• What characteristics would be important?
• Image quality? Size?
• Color
• etc...

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Different Types of Images
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Raster vs. Vector

• Raster (or bitmap)
• breaks down image into discrete pieces, pixels
  and describes each picture. It is analogous to
  coordinate geometry with xs and ys.
• Vector
• stores information about objects like points and
  lines and how they are connected. It is more
  analogous to analytic geometry.

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Raster Images
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Vector Images
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Compression

• Lossy
• Data is lost in the compression of the image.
  JPEG is a good example of a lossy compression
  algorithm. Usually lossy techniques result in
  higher levels of compression.
• Loseless
• No data is lost during compression.

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Lossy Compression
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Lossy Compression
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Resolution

• Applies to raster images only
• Describes the number of pixels in an image
• eg. 640x480, 800x600, 1024x768, 1280x1024
• or 1.2 megapixel, 2.1 megapixel, 3.2 megapixel

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Palette vs. Truecolor

• Palette
• a set of colors (usually 256) from which an image
  is composed. Commonly used in formats like .GIF
• Hicolor/Truecolor
• The color of each pixel is described directly
  using 16,24, or 32 bits. This allows the use of
  millions of colors in each image.

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Palettes

• SamplePalette

Truecolor/Hicolor
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Common Gotchas

• Big Slow
• Dont use more resolution than you need.
• Resizing
• When resizing isnt really resizing
• Picking the right file format

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.BMP format

• Microsoft Windows Bitmap format
• Used internally by Windows quite a bit
• Loseless raster format
• Not very good compression (RLE)
• Newer version supports both palette and truecolor
• Commonly supported

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.GIF files

• Graphics Interchange Format
• CompuServe/UniSys
• Raster image format (generally lossless)
• Good for clip-art and animation or other images
  with few colors (not suitable for photos)
• Monday, 23 June 2003 patents in U.S. expired.

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.JPEG files

• pronounced "jay-peg stands for Joint
  Photographic Experts Group
• lossy, raster image format
• Designed to take advantage of the human eye
• Good for photographs or images with many colors
  or shades of gray
• Variable compression quality

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.PNG format

• Portable network graphics
• Lossless raster format (like .GIF)
• Designed to replace .GIF with more advanced
  features eg. alpha-channel transparency
• Also supports high-color images (with lossless
  compression)
• .MNG supports animation

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.WMF files

• Vector based format (not raster)
• Windows Metafile Format (Microsoft)
• Specific set of instructions for vector based
  drawing
• Good for Clipart on Windows machines
• Small file sizes
• Easily scable without loss of quality

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.TIF format

• Tagged Image File Format owned by Aldus (freely
  available)
• Usually lossless format for scans and photographs
• Frequently used by faxing software
• Cross-platform
• Very flexible (can incorporate JPEG and other
  lossy algorithms)

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Questions?

• Which would be better for a photograph you want
  to email to a friend
• 1) a lossy format or
• 2) a loseless format?

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Questions?

• Which would be better for clipart image you need
  to blow-up really big
• 1) a raster format or
• 2) a vector format?

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Questions?

• Would a photograph be better stored as
• 1) raster image or
• 2) a vector image?

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Questions

• Would this image be more efficiently stored in 1)
  raster or 2) vector format?

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Questions

• Would this image be more efficiently stored in as
  a 1) .BMP or as a 2) .WMF file?

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Questions

• Would this image be more efficiently stored in 1)
  raster or 2) vector format?

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Questions?

• Would this image be more efficiently stored as a
  1) .JPG or 2) .GIF file?

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Questions?

• Would this image be more efficiently stored as a
  1) .JPG or 2) .GIF file?

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Questions?

• Your friend is taking pictures with her digital
  camera. When she prints 8x10s of the pictures,
  the images (which looked fine on the screen) look
  blocky. Do you have any suggestions?
• A. Use a vector based format.
• B. Use a higher resolution setting or camera.
• C. Use less lossy compression.
• D. Buy a film based camera.

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Questions?

• Your colleague is making a newsletter in Word.
  Hes added a number of digital images to his
  document but now his computer is responding VERY
  slowly. What would you suggest?
• A. Make the images smaller in Word.
• B. Switch to a loseless compression format.
• C. Resize the images in another paint program.
• D. Buy a faster computer from Rudy.