University College Dublin1 Output devices COMP 3003.

University College Dublin 1

Output devices

COMP 3003


Hardware Issues (Display Technology) Different output devices may be used - monitors, printers, plotters

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


Phosphors Once struck by the electron beam most phosphors

relax back to the ground state by emitting a photon of light

This light is called fluorescence, which normally decays in under a millisecond

Some molecules may be further excited, and emit a light call phosphorescence, which decays slower, but still rapidly (15-20 milliseconds)

Therefore, the screen must be refreshed by redrawing the image


Phosphors So phosphors may be characterised by their

persistence– (time to decay of emitted light)

High persistence cheap and good for text, bad for animation (original IBM PC monitor)

Low persistence, good for animation, but needs a high refresh rate or flicker can be observed

50-60 Hz is usually sufficient to avoid flicker


CRT

Interior metallic coating at high positive voltage


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

15- 20,000 V Accelerates e-beam to screen


Vector and Raster Two common techniques are used to draw the

graphic on the screen– Vector and Raster

Vector was developed in the mid-sixties and was in common use until the mid-eighties

Raster was developed in the early seventies and today has mostly replaced vector based systems


Vector (Random) Scan System The electron beam directly draws the picture Refresh rate depends the number of lines drawn 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

To display the picture the system cycles through the set of commands in the display file

Good for line-drawing applications CAD– not good for shading etc

Also good for smooth curved surfaces– e.g. oscilloscope


Vector (Random) Scan System (cont) Advantages are high resolution, easy animation, and

requires little memory (just display program), e.g:

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.


Raster Scan Devices Scans the screen from top to

bottom in a regular pattern (common TV technology)

A Raster is a matrix of pixels (picture elements) covering the screen

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.


Frame Buffer The image information is stored in a special

graphics memory area called a frame buffer (or bit map for b/w)

Each memory location corresponds to a pixel A display processor scans this memory controls the

electron beam at each pixel accordingly 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

For grey scale (single electron gun), 8 bits per pixel gives 256 (28) different intensities of grey


Accessing the Frame Buffer Frame-buffer locations, and the corresponding screen

positions, are referenced in Cartesian co-ordinates Two registers are used to store the co-ordinates of the

screen pixels (x,y) Initially x and y are set to zero The associated value is retrieved and used to set the

intensity of the electron guns Then x is incremented by 1 and process is repeated for the

next pixel until the complete row has been scanned Then set x to zero increment y and start again After cycling through all the pixels start at 0,0 again


Aliasing and Anti-aliasing In raster systems curved primitives such as circles

can only be drawn by approximating them with pixels on a raster grid– jaggies or staircasing

This effect is a manifestation of a sampling error called aliasing

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


Aliasing and Anti-aliasing Sampling Technique ?


Aliasing and Anti-aliasing - Examples


Colour Raster Scan System 3 Electron guns used, for R G and B Each pixel consists of 3 dots of phosphor, arranged

as triangle (triads) Combining different intensities of these phosphors

can generate different colours


High-Quality Raster Graphics High-quality raster systems have three electron

guns which can have a variety of settings 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

These systems are generally referred to as full-colour or true-colour systems


Windows NT Example

Palette Number of pixels Refresh Frequency


Colour Lookup Table In many colour raster systems, the display controller

includes a colour lookup table (LUT) The value of a pixel in the frame buffer is not used to

directly control the beam, but is an index into the LUT

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 (28) entries in the LUT – Each entry in the LUT is 24 bits (8 bits per primary colour) – So 256 (28) colours out of 17 million available colours– The application set up the LUT as required– Provides a fast method of controlling beam intensity


Raster Scan System: conclusion Advantages of Raster Scan systems:

– Low cost (memory has become cheap)– Refresh rate independent of image complexity– Can handle colour and filled areas

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


Direct View Storage tube (DVST) Similar to standard CRT except image is stored as a

distribution of charges on the inner side of the screen.

Advantages:– No Constant Refresh Required

Disadvantages:– To update any part of image must redraw all


Liquid Crystal Display (LCD)

Used in flat panel displays (reduced volume, weight and power requirements

Called non-emissive: they do not covert electrical energy into light but use optical effects to convert light into graphical patterns

Polarised light is passed through a liquid crystal material whose molecules can be aligned to block or transmit the light



Orientation of molecules controls polarization of light

Current forces allignment of molecules Light can’t pass – absorbed – black. Current applied using 2 grids to give

X, Y co-ords



Liquid crystal: material that has crystalline arrangement of molecules but flows like a liquid

It is a (nematic) threadlike material that tends to keep the long axes of the rod-shaped molecules aligned



The light entering through the front layer is polarised vertically

If the molecules are arranged in a spiral fashion, they rotate the direction of the light by 90 degrees. Then the light passes through

If the crystals are in an electrical field they do not change the direction of the light and the light does not pass



Six Layers

Viewing Direction

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Hor

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Pol

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Hor

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Grid

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Ver

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University College Dublin1 Output devices COMP 3003.

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