Still Images - PowerPoint PPT Presentation

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  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)

  7. Raster Graphics: Bit Depth RGB colour: 24 bits … 8 bit R, 8 bit G, 8 bit B = 256 *256*256 = 16,722,216

  8. Graphics File Formats R raster V vector C compression numbers: bit depth - applicable only to raster graphics files

  9. Graphics File Formats R raster V vector C compression numbers: bit depth - applicable only to raster graphics files

  10. 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

  11. 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

  12. GIF vs JPEG

  13. GIF vs JPEG

  14. 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

  15. 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)

  16. 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

  17. 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

  18. 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)

  19. 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

  20. 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.

  21. 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

  22. 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

  23. 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)

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

  25. 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