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Graphics Devices Principles and Image Structures

Graphics Devices Principles and Image Structures. Agenda. Basic CRT Principles Vector Graphic Displays Raster Graphics Displays Aliasing Image Transformations. Cathode Ray Tube (CRT). Basic Principle of the CRT.

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Graphics Devices Principles and Image Structures

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  1. Graphics Devices Principlesand Image Structures

  2. Agenda • Basic CRT Principles • Vector Graphic Displays • Raster Graphics Displays • Aliasing • Image Transformations

  3. Cathode Ray Tube (CRT)

  4. Basic Principle of the CRT Phosphor coating emits light from energy provided by electron beam but only lasts for short period and must be ‘refreshed’ to remove flicker of screen image. Typical refresh rates are 60Hz - 80 Hz the higher the more stable is the resultant display. Cheaper CRTs would not always support higher refresh rates. x axis deflection plate Phosphor coating Electrons emitted here Focused here y axis deflection plate

  5. Display Devices • Vector Displays (Random Scan) • Raster Displays (Raster Scan)

  6. Vector Displays

  7. Vector Displays

  8. Vector Displays • Originally designed for architectural and engineering layouts • High resolutions with smooth point-to-point line drawing functions • Not able to display photo-realistic images

  9. Vector Graphics

  10. Vector Graphics • Vector graphic files contain mathematical descriptions of one or more image elements, which are used by the rendering application to construct a final image. • Vector files are thus said to be made up of descriptions of image elements or objects, rather than pixel values. • Objects are composed of shapes, lines and fills (which can be solid, textured, graduated or transparent)

  11. Example Formats • Microsoft Windows metafile .WMF • Corel Computer Graphics Metafile .CGM • Autodesk Data Exchange .DXF • Shockwave Flash .SWF • Fireworks and Photoshop have tools for creating and editing vector graphics

  12. Vector Graphics - Pros • Vector files are useful for storing images composed of line-based elements such as lines and polygons, or those that can be decomposed into simple geometrical objects, such as text. • More sophisticated formats can also store 3D objects such as wire-frame models. • Vector data can be easily scaled and manipulated to accommodate the resolution of a spectrum of output devices.

  13. Vector Graphics - Pros • Many vector files containing only ASCII-format data can be modified with simple text editing tools. • Individual elements may be added, removed, or changed without affecting other objects in the image. • It is usually easy to render vector data and save it to a bitmap format file, or, alternately, to convert the data to another vector format, with good results.

  14. Vector Graphics - Cons • Vector files cannot easily be used to store extremely complex images, such as some photographs, where color information is paramount and may vary on a pixel-by-pixel basis. • The appearance of vector images can vary considerably depending upon the application interpreting the image. Factors include the rendering application's compatibility with the creator application and the sophistication of its toolkit of geometric primitives and drawing operations

  15. Vector Graphics - Cons • Vector data also displays best on vectored output devices such as plotters and random scan displays. High-resolution raster displays are needed to display vector graphics as effectively. • Reconstruction of vector data may take considerably longer than that contained in a bitmap file of equivalent complexity, because each image element must be drawn individually and in sequence.

  16. Scaleable Vector Graphics • SVG is a language for describing two-dimensional graphics and graphical applications in XML • Scaleable means extendable • Interaction - mouse events • Animation - animated components

  17. SVG still in Development • Development by WC3 - SVG Web Consortium Working Group • Group membership made up of interested parties from computer graphics community et al. • Representatives from companies including: • Adobe Systems • Microsoft Corporation • Sun Microsystems • Autodesk • Corel • Nokia

  18. Raster Scan Displays

  19. Vector v Raster

  20. Raster Scan Displays • Based on TV Technology • Electron beam sweeps scan lines row by row down the phosphor screen based on information stored in the Frame Buffer ‘painting’ one line at a time • Screen area comprised of picture elements (pixels) • Often characterised by their resolution, aspect ratio, and colour depth.

  21. Raster Scan

  22. Raster Scan

  23. Raster Scan Properties • Resolution = number of pixels available on display, 640 x 480 ( VGA ), 800 x 600, 1024 x 768 ( SVGA ) • Aspect ratio = number of horizontal pixels / vertical pixels so 800 / 600 = 1.3. or 4 / 3 = 1024 / 768 etc • 4/3 artistically (visually) pleasing? v 16 x 9 (wide screen • Bit depth determined by Frame Buffer - colours available for each pixel - determined by number of bits stored for each pixel - hence bits per pixel. • 1 bit per pixel = monochrome (black and white) • Typically 24 bits per pixel gives ‘true colour’

  24. x Raster Scan Coordinates Origin (0,0) at top left-hand corner y values increase down the screen y

  25. Raster Scan Aliasing

  26. Raster Scan

  27. Raster Scan

  28. Raster Scan

  29. Raster Scan

  30. Aliasing • Bitmap images will exhibit aliasing of lines due to the underlying method of their storage and display. • Algorithms applied to final image to smooth jagged edges and give more gradation between edge colours to provide antialising

  31. Aliasing

  32. Antialiasing Lines Aliased Antialiased

  33. Aliased

  34. Antialiased

  35. Vector -> raster conversion? Indeed! Look at: http://www.fileformat.info/convert/image/svg2raster.htm

  36. Raster -> vector conversion? • Most advanced packages have tracing facilities • Good for simple line drawings, bad for complex images

  37. Plasma displays

  38. LCD displays http://solutions.3m.com/wps/portal/3M/en_US/Vikuiti1/BrandProducts/secondary/optics101/?slideIndex=1

  39. Bitmap Image Editing

  40. Bitmapped Graphics • Allow for high-definition photorealistic images. • Image encoded on a pixel by pixel basis. • Image resolution degraded when scaled • Supports lossless compressions algorithms • File Formats - .BMP, .GIF, .JPG, .TIF .TGA • Software • Adobe Photoshop (with ImageReady) • Fireworks • PaintShop Pro (Shareware)

  41. True Colour • One bit per pixel = 21 = 2 colours (monochrome) • Human eye can distinguish between 224 colours = 16 777 216 colours • Thus any system giving >= 224 colours can be described as ‘Truecolour’ or Truecolor • 24 bits per pixel yields true colour • Earlier technology used 16 bits per pixel often referred to as ‘High Colour’

  42. Image Transformations • Scaling • Rotations • Translations • Reflections (flip) • Freeform

  43. Translation

  44. Scaling

  45. Rotation

  46. Reflection (flip)

  47. Freeform (distort)

  48. Compositing Images

  49. Compositing Images

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