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Multimedia

Multimedia. Discrete Media: Text, Still Images. Contents. Discrete media types: Text Still images: Vector Graphics Bitmapped Images Continuous media types: Audio Moving images: Animations Video Media/Tools Comparisons. Capture vs. Synthesize. Computer synthesis. Analog video

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Multimedia

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  1. Multimedia Discrete Media: Text, Still Images T.Sharon-A.Frank

  2. Contents • Discrete media types: • Text • Still images: • Vector Graphics • Bitmapped Images • Continuous media types: • Audio • Moving images: • Animations • Video • Media/Tools Comparisons T.Sharon-A.Frank

  3. Capture vs. Synthesize Computer synthesis Analog video camera Capture A/D Animation Video Recognition Vector Graphics Electronic pen Text memo Printed documents Images Audio Sound T.Sharon-A.Frank

  4. Conventional Classification of Media Types Time/space nature Audio Video Animation Continuous (time based) Images Text Vector Graphics Discrete (space based) Origin Captured from real world Synthesized by computers T.Sharon-A.Frank Remark: Conventionally for Origin means usually, not always

  5. Synthesis vs. Captured  T.Sharon-A.Frank

  6. Text • Formats • Plain (ASCII) • Rich text (Word, Latex, HTML) • Issues • Tags • Special characters • Bilingual • Creation • Mostly synthesized (keyboard, OCR) • Presentation T.Sharon-A.Frank

  7. Text in Multimedia • Text is critical to communication even in multimedia. • Hypertext uses text but is fundamentally different than traditional text. • Choice of words is very important and should be done carefully. T.Sharon-A.Frank

  8. Text File Size • 2 bytes per character • Each character 8x8 pixels (picture elements) • Characters per page = = 4,800 • Storage required = 4,800 * 2 = 9,600 bytes Total: ~9.4KB 640 * 480 8* 8 T.Sharon-A.Frank

  9. Computer Graphics • Production and display of still images stored in digital form: • Digitize printed image with a scanner • Capture image from digital camera • Grab frame from video camera • Create in digital form using graphics package • Generate visual representation of data. T.Sharon-A.Frank

  10. Use of Image/Picture • Provide facts • Explain a process • Set a mood • Evoke an idea • Pinpoint location • Illustrate relationships • Tell stories • Identify/Compare entities T.Sharon-A.Frank

  11. Graphics File Formats • Many graphics file formats in existence. • Different ways of encoding image data. • Different amounts/form of supplementary data. • Different compression methods for images: • Lossless: image can be reconstructed exactly fromcompressed version. • Lossy: some information discarded; image can only be reconstructed approximately. T.Sharon-A.Frank

  12. Rendering • Image is displayed on monitor as array of pixels (picture elements) • Rectangular (usually square) dots of color. • Program (e.g., Web browser) sets pixels to anappropriate color to produce desired image • Pixels merge optically to produce effect of continuoustone. • Program must maintain a model of the image • May be stored in a file and read by program. T.Sharon-A.Frank

  13. Vector and Bitmapped Graphics • Vector graphics – Image is modelled as mathematical description of curves and shapes: • e.g., a vector is a line that is described by the location of its two endpoints. • Rendered by computing pixels from description. • Bitmapped graphics – Image is modelled as a matrix/grid of pixelvalues • Rendered by direct mapping of logical pixels to physicalpixels of screen, may need scaling and clipping. T.Sharon-A.Frank

  14. Revisable documents Format can retain structural information Semantic content is preserved in the representation Described as objects Not revisable Format unaware of structural information Semantic content is not preserved Described as bitmaps formed of individual pixels Graphics (Vector) vs. Images (Bitmapped) A T.Sharon-A.Frank

  15. Vector Graphics • Format of Vector Graphics is: • Compact • Scalable • Resolution-independent • Hence it is attractive for networked multimedia. • Widely used on Web, e.g., SWF and SVG. T.Sharon-A.Frank

  16. Sample Vector Graphics Formats • EPS (Encapsulated PostScript) • Primarily print, use declining, superseded by PDF. • SVG (Scalable Vector Graphics) • W3C standard, still needs to be widely used. • SWF (Shockwave Flash) • From Adobe Flash; de facto standard; primarily for vector animation, but can be used for vector graphics. • XAML (Extensible Application Markup Language) • From Microsoft Silverlight; used for marking up vector graphics and animations. T.Sharon-A.Frank

  17. Bitmapped Image • An image is a spatial representation of an object. • An image can be represented as a matrix/grid of pixels. • Records a value for every pixel in the image. • Also known as raster graphics. T.Sharon-A.Frank

  18. Bitmapped Image Representation • An image can be thought of as a function with resulting values of the light intensity at each point over a planar region. • In computers, this function must be sampled at discrete intervals. • Intensity at each pixel is represented by an integer into which the color of the image is encoded. • If there are just 2 intensity values, 0 and 1 represent black and white (1 bit). • If 8-bit integers, color levels range from 0 (black) to 255 (white). T.Sharon-A.Frank

  19. ImageCharacteristics • Image/Device Resolution (ppi/dpi) • Color Depth (1 bit and halftone, grayscale, color, true color) – bits per pixel • Color Model (RGB, CMY, YUV) T.Sharon-A.Frank

  20. Device Resolution • Printers, scanners: specified as dots per unit length, often dots per inch (dpi): • Desktop printer 600dpi, typesetter 1270dpi, scanner 300–3600dpi, … • Video, monitors: specified as pixel dimensions: • PAL TV 768x576px, 17" CRT monitor 1024x768px, … • dpi depends on physical size of screen. T.Sharon-A.Frank

  21. Image Resolution • Bitmaps are resolution dependent. • Array of pixels has pixel dimensions, but no physical dimensions. • By default, displayed size depends on resolution (dpi) of output device • physical dimension = pixel dimension/resolution. • Can store image resolution (ppi) in image file to maintain image's original size • Scale by device resolution/image resolution. T.Sharon-A.Frank

  22. Commonly used Color Models • RGB: Red, Green, Blue – additive primary colors (color monitors). • CMY: Cyan, Magenta, Yellow – subtractive primary colors plus black (color printers). • YIQ (YUV): luminance, chrominance are decoupled (TV). • HSV: Hue, Saturation and Value (color imagemanipulation). T.Sharon-A.Frank

  23. Image Format • A stored image is an array of values: • Each value represents data associated with a pixel in the image. • For color, this value may be: • 3 numbers – intensities of red, green and blue components of color at that pixel • 3 numbers – indices to tables of RGB intensities • 1 number – index to table of color triples • 1 number – index to other color models, such as CMY, YIQ, HSV, etc. T.Sharon-A.Frank

  24. Sample Image File Formats • BMP (Bit Map Picture) • TIFF (Tagged Image File Format) • GIF (Graphics Interchange Format) • Lossless, 256 colors (indexed), transparency • PNG (Portable Network Graphics) • Lossless, variable number of colors, W3C standard • JPEG (Joint Photographic Experts Group) • Lossy (variable quality), millions of colors T.Sharon-A.Frank

  25. GIF (Graphic Interchange Format) • Most popular image format on Web. • Good for condensing rows of pixels of identical color: logos, icons, line art, cartoons, etc. • Compression is lossless since uses LZW algorithm which takes advantage of repetition in data streams. • 8-bit indexed color – indexed means each pixel contains index to cell in color table. T.Sharon-A.Frank

  26. More about GIF • Interlacing: • normal GIFs are displayed either 1 row of pixels at a time or all at once when entire image is downloaded. • Interlaced GIFs displayed in 4 passes – every 4th line in each pass. • GIF87a vs. GIF89a • GIF89a allows animation and transparency. T.Sharon-A.Frank

  27. JPEG (Joint Photographic Experts Group) • Compression is lossy: • based on spatial frequency. • low frequency areas compressed more efficiently than high frequency areas. • Good for photographic images (better than GIF compression anyway): • how much compression depends on photo. • e.g., detail-rich photo takes 85K as GIF but requires only 35K as JPEG. T.Sharon-A.Frank

  28. More about JPEG • Allows variable compression: • Q setting between 1 and 100 (usually) • varies among tools that create JPEGs • lower number = lower image quality but better compression rates (smaller files) • Q setting has no mathematical significance • actual compression ratio depends on individual image. • Lossy compression makes flat colors blotchy and pixilated. T.Sharon-A.Frank

  29. PNG (Portable Network Format) • Developed in early 1995 as non-proprietary alternative to GIF. • Taking a while to catch on. • Uses lossless compression: • horizontal compression similar to GIF • vertical compression using series of filters • typically compresses 5-25% better than GIF. T.Sharon-A.Frank

  30. More about PNG • A good GIF substitute. • Not a good JPEG substitute (lossless compression results in bigger files than JPEGs lossy compression). • Supports 24 bit indexed color (and higher so could be a good TIFF substitute for detailed photos). • Supports interlacing, transparency, and lots of other unique features. T.Sharon-A.Frank

  31. Image Acquisition • Capturing (as bitmaps) • scanner • digital camera • Videotape/Analog camera + video card • Synthesizing • Drawing programs (Adobe Illustrator, Macromedia Freehand, Paint Shop Pro, MS Office, etc) • Image-editing software (MS Paint, Adobe Photoshop, Photoshop Elements, ImageReady, GIMP, etc) T.Sharon-A.Frank

  32. Image Presentation • Computer Display • PDA screens • Printers T.Sharon-A.Frank

  33. Memory Requirements • Bitmapped – any picture of w x h pixels, using cbytes per pixel, occupies w x h x c bytes. • Vector – space required depends on complexity of picture (how many shapes, segments of path, etc). • Usually vector graphics memory requirements are smaller than those of bitmapped – when few vectors, more efficient than bitmaps. T.Sharon-A.Frank

  34. Vector Graphics Size • Typical screen – 500 elements. • Say each element has: • 1 attribute/color field – 8 bit • 2 points containing: • Bits per element: 2*(9+10)+8 = 46 bits • Storage required per screen = 500*46/8 = 2,875 bytes Total: ~2.8KB • x position – 10 bit (log2640) • y position – 9 bit (log2480) T.Sharon-A.Frank

  35. Color Raster Picture Size • One screen: 640 * 480 = 307,200 pixels • 256 colors – 1 byte per pixel = 307,200 bytes Total: ~300KB • True color (16M colors) – 3 bytes per pixel = 307,200*3 = 921,600 bytes Total: ~900KB T.Sharon-A.Frank

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