University College Dublin1 Output devices COMP 3003

University College Dublin2 Hardware Issues (Display Technology) u Different output devices may be used - monitors, printers, plotters u Most common is the Cathode Ray Tube (CRT) monitor – Horizontal and vertical deflectors focus an electron beam emitted by an electron gun on any spot on a phosphor coated screen – The maximum number of points, or pixels that can be displayed without overlap is called the resolution, e.g. 1024x768, 800x600 etc. – Colour systems have groups of 3 different phosphors, for red, green and blue (the primary colours) – The CRT uses a combination of these phosphors to emit different coloured light

University College Dublin3 Phosphors u Once struck by the electron beam most phosphors relax back to the ground state by emitting a photon of light u This light is called fluorescence, which normally decays in under a millisecond u Some molecules may be further excited, and emit a light call phosphorescence, which decays slower, but still rapidly (15-20 milliseconds) u Therefore, the screen must be refreshed by redrawing the image

University College Dublin4 Phosphors u So phosphors may be characterised by their persistence –(time to decay of emitted light) u High persistence cheap and good for text, bad for animation (original IBM PC monitor) u Low persistence, good for animation, but needs a high refresh rate or flicker can be observed u Hz is usually sufficient to avoid flicker

University College Dublin5 CRT Interior metallic coating at high positive voltage

University College Dublin6 CRT E GUN Produces constant stream of electrons Control Grid Sets intensity of spot on screen (the more negative the control grid voltage the fewer electrons pass through) Focusing System Forces e-beam into narrow stream (otherwise repel) Deflection Coils Indicates target phosphor spot High positive V coating ,000 V Accelerates e-beam to screen

University College Dublin7 Vector and Raster u Two common techniques are used to draw the graphic on the screen – Vector and Raster u Vector was developed in the mid-sixties and was in common use until the mid-eighties u Raster was developed in the early seventies and today has mostly replaced vector based systems

University College Dublin8 Vector (Random) Scan System u The electron beam directly draws the picture u Refresh rate depends the number of lines drawn u The picture definition is stored as a set of line-drawing commands in the memory called the refresh display file – also known as display list, display program or refresh buffer u To display the picture the system cycles through the set of commands in the display file u Good for line-drawing applications CAD – not good for shading etc u Also good for smooth curved surfaces – e.g. oscilloscope

University College Dublin9 Vector (Random) Scan System (cont) u Advantages are high resolution, easy animation, and requires little memory (just display program), e.g: u Disadvantages are limited colour capability and flicker occurs as complexity of image increases. b a Turn e beam off, move to a. Turn e beam on and draw to b. Repeat move draw sequence.

University College Dublin10 Raster Scan Devices u Scans the screen from top to bottom in a regular pattern (common TV technology) u A Raster is a matrix of pixels (picture elements) covering the screen u The electron beam is turned on/off so the image is a collection of dots painted on screen one row (scan line) at a time.

University College Dublin11 Frame Buffer u The image information is stored in a special graphics memory area called a frame buffer (or bit map for b/w) u Each memory location corresponds to a pixel u A display processor scans this memory controls the electron beam at each pixel accordingly u For a monochrome system, each pixel is either on or off, so only one bit per pixel is required, and the electron beam is either on or off u For grey scale (single electron gun), 8 bits per pixel gives 256 (2 8 ) different intensities of grey

University College Dublin12 Accessing the Frame Buffer u Frame-buffer locations, and the corresponding screen positions, are referenced in Cartesian co-ordinates u Two registers are used to store the co-ordinates of the screen pixels (x,y) u Initially x and y are set to zero u The associated value is retrieved and used to set the intensity of the electron guns u Then x is incremented by 1 and process is repeated for the next pixel until the complete row has been scanned u Then set x to zero increment y and start again u After cycling through all the pixels start at 0,0 again

University College Dublin13 Aliasing and Anti-aliasing u In raster systems curved primitives such as circles can only be drawn by approximating them with pixels on a raster grid – jaggies or staircasing u This effect is a manifestation of a sampling error called aliasing u Anti-aliasing is a technique by which neighbouring pixels at edges of primitives are set to graduating levels of intensity – I.e. not set to maximum or zero

University College Dublin14 Aliasing and Anti-aliasing u Sampling Technique ?

University College Dublin15 Aliasing and Anti-aliasing - Examples

University College Dublin16 Colour Raster Scan System u 3 Electron guns used, for R G and B u Each pixel consists of 3 dots of phosphor, arranged as triangle (triads) u Combining different intensities of these phosphors can generate different colours

University College Dublin17 High-Quality Raster Graphics u High-quality raster systems have three electron guns which can have a variety of settings u For example 8 possible settings per gun – allowing 256 voltage settings for each colour electron gun – in total 16,777,216 (256 * 256 * 256) or 17 million approx. possible colours for each pixel – 24 bits used for each pixel u These systems are generally referred to as full- colour or true-colour systems

University College Dublin18 Windows NT Example u Palette u Number of pixels u Refresh Frequency

University College Dublin19 Colour Lookup Table u In many colour raster systems, the display controller includes a colour lookup table (LUT) u The value of a pixel in the frame buffer is not used to directly control the beam, but is an index into the LUT u The entry in the LUT is used to directly control the colour of the pixel e.g. – 1 byte (8 bits) per pixel in frame buffer – This gives address for 256 (2 8 ) entries in the LUT – Each entry in the LUT is 24 bits (8 bits per primary colour) – So 256 (2 8 ) colours out of 17 million available colours – The application set up the LUT as required – Provides a fast method of controlling beam intensity

University College Dublin20 Raster Scan System: conclusion u Advantages of Raster Scan systems: – Low cost (memory has become cheap) – Refresh rate independent of image complexity – Can handle colour and filled areas u Disadvantages – Uses more memory » Over come by some degree by using an LUT – Mathematical models of objects must be scan converted by the host processor – aliasing

University College Dublin21 Direct View Storage tube (DVST) u Similar to standard CRT except image is stored as a distribution of charges on the inner side of the screen. u Advantages: –No Constant Refresh Required u Disadvantages: –To update any part of image must redraw all

University College Dublin22 Liquid Crystal Display (LCD) u Used in flat panel displays (reduced volume, weight and power requirements u Called non-emissive: they do not covert electrical energy into light but use optical effects to convert light into graphical patterns u Polarised light is passed through a liquid crystal material whose molecules can be aligned to block or transmit the light

University College Dublin23 Liquid Crystal Display (LCD) u Orientation of molecules controls polarization of light u Current forces allignment of molecules u Light can’t pass – absorbed – black. u Current applied using 2 grids to give X, Y co-ords

University College Dublin24 Liquid Crystal Display (LCD) u Liquid crystal: material that has crystalline arrangement of molecules but flows like a liquid u It is a (nematic) threadlike material that tends to keep the long axes of the rod-shaped molecules aligned

University College Dublin25 Liquid Crystal Display (LCD) u The light entering through the front layer is polarised vertically u If the molecules are arranged in a spiral fashion, they rotate the direction of the light by 90 degrees. Then the light passes through u If the crystals are in an electrical field they do not change the direction of the light and the light does not pass

University College Dublin26 Liquid Crystal Display (LCD) u Six Layers Viewing Direction Reflective Layer Horizontal Polarizer Horizontal Grid Wires Vertical Grid Wires Vertical Polarizer Liquid Crystal Layer