Still Images

1
Still Images
raster graphics
file formats
2
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

3
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

4
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

5
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

6
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)

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Raster Graphics Bit Depth
RGB colour 24 bits 8 bit R, 8 bit G, 8 bit B
256 256256 16,722,216
8
Graphics File Formats
R raster V vector C compression
numbers bit depth - applicable only to raster
graphics files
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Graphics File Formats
R raster V vector C compression
numbers bit depth - applicable only to raster
graphics files
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GIF

• 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 gt 16 million colours
• each image limited to 256 colour palette
• original specification still images only
• animated GIF now supported

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GIF

• 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

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GIF vs JPEG
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GIF vs JPEG
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GIF

• 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

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JPEG

• 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)

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JPEG

• 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

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PNG

• 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

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PNG

• 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)

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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

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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)
• TARGA (TGA)
• most common in the video industry
• also used by high-end paint and ray-tracing
  programs
• has many variations
• supports several types of compression.

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Other File Formats

• PCX
• 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
• BMP
• 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

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Other File Formats

• AutoCAD DXF
• 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

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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)

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Dithering

• http//en.wikipedia.org/wiki/Dithering

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References

• 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