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Discrete Media: Text, Still Images

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

2 Contents Discrete media types: Continuous 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 Animation
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 

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

7 Text is critical to communication even in multimedia.
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 Production and display of still images stored in digital form:
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 from compressed 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 an appropriate color to produce desired image Pixels merge optically to produce effect of continuous tone. 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 pixel values Rendered by direct mapping of logical pixels to physical pixels of screen, may need scaling and clipping. T.Sharon-A.Frank

14 Graphics (Vector) vs. Images (Bitmapped)
Not revisable Format unaware of structural information Semantic content is not preserved Described as bitmaps formed of individual pixels Revisable documents Format can retain structural information Semantic content is preserved in the representation Described as objects 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 Image Characteristics
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 image manipulation). 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) Synthesizing 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

33 Memory Requirements Bitmapped – any picture of w x h pixels, using c bytes 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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