1 / 25

Still Images

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

Download Presentation

Still Images

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related