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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 l.jpg
Still Images

raster graphics

file formats


Raster graphics bitmaps l.jpg
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 l.jpg
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 l.jpg
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


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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


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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 l.jpg
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 l.jpg
Graphics File Formats

R raster V vector C compression

numbers: bit depth - applicable only to raster graphics files


Graphics file formats9 l.jpg
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 > 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




Slide14 l.jpg
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


Slide17 l.jpg
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)


Other file formats l.jpg
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 l.jpg
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.


Other file formats21 l.jpg
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


Colour depths and compression formats l.jpg
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)


Dithering l.jpg
Dithering

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


References l.jpg
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


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