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Still Images raster graphics file formats Raster Graphics: Bitmaps bit - binary on or off 1 or 0 map two-dimensional matrix fig 6-1 (Vaughan 2004) below memory (bits) needed to display various levels of colour 2 to 16,722,216 possible colours Raster Graphics: Bitmaps

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still images
Still Images

raster graphics

file formats

raster graphics bitmaps
Raster Graphics: Bitmaps
  • bit - binary
    • on or off
    • 1 or 0
  • map
    • two-dimensional matrix
  • fig 6-1 (Vaughan 2004) below
    • memory (bits) needed to display various levels of colour
    • 2 to 16,722,216 possible colours
raster graphics bitmaps3
Raster Graphics: Bitmaps
  • sometimes known as pixel maps because
    • “bitmap” … two-colour or

one-colour + transparency image

  • fixed number (perhaps thousand of millions)

of dots (pixels)

    • each pixel a single colour
    • typical size one pixel
  • we perceive an image rather than separate dots
    • our eyes blend pixels together
  • raster graphics superior to vector
      • detail so great vectors cannot effectively represent image
raster graphics
Raster Graphics
  • editing
    • modify pixels (not lines or curves)
  • resolution dependent
    • enlarge : redistribute the pixels
      • affects quality
    • if output device has lower resolution than image
      • may degrade image quality
  • uses
raster graphics map
Raster Graphics - Map
  • represents rows and columns of dots
  • contain value (colour) of each dot
    • (background colour if not filled in)
  • density of the dots – resolution
    • determines how sharply image is represented
    • expressed in dpi or number of rows and columns (eg.640 * 480)
  • to display on monitor
    • translate map into pixels
  • to print on printer
    • translate map into ink dots
  • optical scanners, fax machines
    • transform text or pictures into maps
raster graphics files
Raster Graphics Files
  • always saved as raster graphics files
    • scanned photos
  • disadvantages
    • takes up lots of room
    • may suffer from the jaggies
    • enlargements – unnatural blocky appearance
  • applications creating raster based image files
    • hundreds
    • Photoshop dominates
    • Macromedia Fireworks (also handles vector)
raster graphics bit depth
Raster Graphics: Bit Depth

RGB colour: 24 bits … 8 bit R, 8 bit G, 8 bit B = 256 *256*256 = 16,722,216

graphics file formats
Graphics File Formats

R raster V vector C compression

numbers: bit depth - applicable only to raster graphics files

graphics file formats9
Graphics File Formats

R raster V vector C compression

numbers: bit depth - applicable only to raster graphics files

  • pronunciation?
  • Graphics Interchange Format
    • UNISYS Corporation and Compuserve
    • to transmit graphical images over phone lines via modems
    • hold patent – so new standard (PNG) developed
      • covers only compression, not decompression
      • so UNIX gunzip can decompress LZW-compressed files
  • LZW lossless compression
    • Lempel-Ziv-Welch
  • limited to 8 bit (256) colour images
    • has access to > 16 million colours
    • each image limited to 256 colour palette
  • original specification still images only
    • animated GIF now supported

Features making GIF well-suited for Web

  • file compression
  • transparency – not true alpha-transparency
  • interlacing
  • storage multiple images in single file
    • allows for a primitive form of animation
  • GIFs still very popular (despite patent)
    • well-supported
  • best suited for
    • images with large areas of flat and limited colour
    • drawings: logos, text, line art

Limitations useful

  • rule:

do not use GIF to display photographs online

  • BUT limitations used for creating special effects

Turn off dithering when you use GIF

Because of the way GIF compression works

    • images with horizontal stripes have smaller file size than one with vertical stripes
  • Joint Photographic Experts Group
  • most important current standard for image compression
  • created by working group of ISO
    • International Organisation for Standardisation
  • lossy compression
    • takes advantage of limitations in human vision system to achieve high rates of compression
      • cannot see extremely fine detail

(more pronounced for colour)

    • as compression increases information removed
    • quality level should always be 40 or above
      • as high as possible (trade off with download times)
  • JFIF is a new TIFF subformat
    • embeds a JPEG image into a TIFF file
    • not popular because
      • more complex without offering more capability
  • can be saved as progressive jpegs
    • image downloads in multiple passes
  • standard resolution for images for web
    • 72 dpi
    • scan them in at a higher resolution
  • used for images with
  • supports millions of colours
    • no transparency

PNG – Portable Network Graphics

  • pronounced “ping”
  • PNG-8 form
    • works like a patent-free replacement for GIF
      • supports indexed colour
  • PNG-24 form
    • supports greyscale and millions of colours
      • up to 48 bit
    • challenges JPEG
  • supports alpha-transparency
    • true alpha-transparency (unlike GIF)
    • so excellent alternative to JPEG

PNG – Portable Network Graphics

  • high compression rate
    • unsurpassed among “lossless” formats
  • W3C recommendation - BUT
  • poor support from some browser manufacturers
    • IE still does not support PNGs alpha transparency
      • shows up solid white
    • promised for IE 7
  • extensible file format
    • Fireworks PNG stores proprietary data
      • think of it as a PSD (Photoshop)
other file formats
Other File Formats

TIFF – Tagged Image File Format

  • versatile, works on both Mac and PC
  • supports almost any picture depth
  • disadvantages
    • many version and types of compression – no current system can decode them all
    • never sure how it will behave
      • use no compression or LZW lossless
    • not good for Web
      • large file sizes and lack of browser support

PICT - picture

  • meta format
    • can be used for both vector and raster images
  • developed by Apple computers 1984
    • encoded in QuickDraw commands
  • Apple replacing it with PDF
    • Adobe Portable Document Format – hybrid R + V
other file formats20
Other File Formats

PSD – PhotoShop Data - proprietary

  • native Photoshop file format created by Adobe
  • primarily raster but also provides support for
    • layering and multiple alpha channels
    • vector type and multiple paths
  • converting PSD to another raster format
    • flattens it (disables features)


  • most common in the video industry
  • also used by high-end paint and ray-tracing programs
  • has many variations
  • supports several types of compression.
other file formats21
Other File Formats


  • a straightforward raster file originally available only on the PC
  • PCX is migrating to the Mac as programs become cross-platform
  • suppported by
    • Adobe Photoshop
    • QuarkXPress for the Mac


  • available in almost all Windows-based graphics applications
  • primarily used in Windows application development

Windows Metafile (WMF)

  • a list of calls to the Microsoft Windows graphics drawing library
  • small and flexible
  • difficult to display properly without program used to create them

Computer Graphics Metafile (CGM)

  • very flexible vector format
  • can also save raster information
  • so flexible that very few applications can use all the types available
other file formats22
Other File Formats


  • a standard for exchanging CAD drawings
  • vector information is ASCII encoded
    • so files can become very large

Hewlett-Packard GL/2

  • HP plotter language often used as an exchange format for graphics

Encapsulated PostScript (EPS)

  • vector file
  • relies on the PostScript page description language to draw image
  • can also contain raster information

(even though it's not a raster format)

  • generally contains a raster graphic as a screen preview
  • Mac EPS files use a PICT and PC EPS files use a TIFF graphic
  • only format to support transparent white in bitmap mode
colour depths and compression formats
Colour Depths and Compression Formats
  • 24 bits
  • dithered to 8 bits
    • adaptive palette (best 256 colours to represent image)
  • dithered to 8 bits
    • Macintosh system palette (optimised standard mix of 256 colours)
  • dithered to 4 bits
    • any 16 colours
  • dithered to 8 bit grey-scale
    • 256 shades of grey
  • dithered to 4 bit grey-scale
    • 16 shades of grey
  • dithered to 1 bit
    • 2 colours (black and white here)

fig 6-2 Vaughan (2004)



Vaughan, T. (2004) Multimedia: Making It Work 6th edition New York: McGraw Hill Technology Education

Ze-Nian Li and Drew, Mark S. (2004) Fundamentals of Multimedia Upper Saddle River, NJ: Pearson Prentice Hall