Input devices
Hard copy devices
Display Devices
Scan conversion algorithms
2-dimensional transformations

Display Devices
2-D Transformations
Some Questions

Hardcopy Devices

All the output devices can be categorised into two categories

Ø  Hard Copy Devices

Ø  Soft Copy Devices

Hard copy devices are those that give the output in the tangible form. Printers and Plotters are two common hard copy devices.

Soft copy devices give output in the intangible form or the virtual form, e.g. something displayed on a screen. All the computer monitors are covered under this category.

Printers

All the printers irrespective of the technology used can be categorised as

Ø  Impact Printers

Ø  Non Impact Printers

Impact printers are those printers in which there is a direct contact between the printing head and the paper on which the print is produced.

·         They work by striking a head or a needle against an inked ribbon which leaves a mark on the paper.

·         These printers produce a lot of noise when printing, because of the head striking the paper.

·         Examples are Dot Matrix, Daisy Wheel and Line printers.

In the case of non-impact printers the printing head never comes in direct contact with the paper.

·         These printers work by spraying ink on the paper.

·         Electrostatic or electromagnetic charge is used in these printers.

·         Examples are Ink-Jet and Laser printers.

Dot-Matrix Printers :

Ø  Dot Matrix is an impact printer.

Ø  These printer forms characters from individual dots.

Ø  These printers have a print head which runs back and forth on a paper.

Ø  The print head has a two-dimensional array of pins called dot matrix. There may be 9 to 24 pins in the dot matrix.

Ø  From this array of pins some pins are drawn out (or driven forward) to form the shape of a character.

Ø  The drawn out pins strike an ink soaked cloth ribbon against a paper. This forms that particular character on the paper.

Ø  Thus dot matrix printers can be used to print different fonts of characters.

Ø  Since mechanical force is used, carbon copies of documents can be taken.

Ø  40 to 250 characters can be printed per second.

Daisy Wheel Printers :

Ø  This is an impact printer.

Ø  Only preformed fonts of characters can be printed.

Ø  This printer contains a daisy wheel. Daisy wheel is made of plastic or metal. This holds an entire character set as raised characters moulded on each "petal".

Ø  A motor rotates the daisy wheel to position the required character between the hammer and the ribbon.

Ø  A small hammer then strikes the petal, which in turn strikes the inked ribbon to leave the character mark on the paper.

Ø  The daisy wheel and hammer are mounted on a sliding carriage similar to that used by dot matrix printers.

Ø  Different fonts cannot be printed using this technology.

Line Printers :

Ø  The line printer is a high speed impact printer in which one line is printed at a time.

Ø  600-1200 lines can be printed per minute.

Ø  Drum printer is an example of line printers.

Ø  These printers are very expensive.

Ø  These kind of printers were popular in the early days of computers, but the technology is still in use.

Drum Printers

·         In a drum printer, a fixed font character set is engraved onto a number of print wheels.

·         There are as many print wheels as the number of columns (letters in a line) the printer could print.

·         The print wheels are joined to form a large drum (cylinder),

·         This drum spins at high speed and paper and an inked ribbon is moved past the print position.

·         As the desired character for each column passes the print position, a hammer strikes the paper from the rear and presses the paper against the ribbon and the drum, causing the desired character to be printed on the paper.

Ink-Jet Printers :

Ø  Inkjet printer is a non impact printer, Core of an inkjet printer is the print head.

Ø  The print head contains an ink cartridge which has a series of nozzles that are used to spray tiny drops of ink on to the paper.

Ø  Ink cartridges come in various combinations, such as separate black and colour cartridges, colour and black in a single cartridge or even a cartridge for each ink colour.

Ø  A motor moves the print head back and forth across the paper.

Ø  Different types of inkjet printers form their droplets of ink in different ways. There are two main inkjet technologies currently used by printer manufacturers

o    Thermal bubble - This method is commonly referred to as bubble jet. In a thermal inkjet printer, tiny resistors create heat, and this heat vaporizes ink to create a bubble. As the bubble expands, some of the ink is pushed out of a nozzle onto the paper. When the bubble "pops" (collapses), a vacuum is created. This pulls more ink into the print head from the cartridge. A typical bubble jet print head has 300 or 600 tiny nozzles, and all of them can fire a droplet simultaneously.

o    Piezoelectric - This technology uses piezo crystals. A crystal is located at the back of the ink reservoir of each nozzle. The crystal receives a tiny electric charge that causes it to vibrate. When the crystal vibrates inward, it forces a tiny amount of ink out of the nozzle. When it vibrates out, it pulls some more ink into the reservoir to replace the ink sprayed out.

Ø  The ink droplets are subjected to an electrostatic field created by a charging electrode as they form. Charged droplets are separated by one or more uncharged “guard droplets” to minimize electrostatic repulsion between neighbouring droplets. The charged droplets pass through an electrostatic field and are directed (deflected) by electrostatic deflection plates to print on the Paper.

Laser Printers :

Ø  A laser printer is a non impact printer, which produces a page of text at a time.

Ø  Laser printer uses the principle of Static Electricity to print.

Ø  This printer has revolving cylinder called Drum.

Ø  Drum is given a positive charge.

Ø  A Laser beam is used to draw the image to be printed, on the drum with negative charge. This discharges some portion of the charge on the drum. This creates electrostatic image of the print on the drum with no charge, and the background is left positively charged.

Ø  The drum is then exposed to toner from which positively charged toner particles mixed with carbon black are released. Since positive charge repels positive charge, the toner particles settles on the discharged areas of the drum, this is exactly the image to be printed.

Ø  The paper is then pressed against the drum, this transfers the toner particles on to the paper.

Ø  Paper is then passed through a fuser, which is a set of heated rollers, this melts the carbon black on the paper to form the desired print. 

Plotters :

Another hard copy output device is plotter. Plotter is a printing device which can draw continuous lines. This is useful to print vector graphics rather than raster graphics unlike normal printers. Plotters are widely used in applications like CAD.

Ø  Plotters print by moving one or more pen across the surface of a piece of paper. This means that plotters are restricted to line art, rather than raster graphics as with other printers.

Ø  Pen plotters can draw complex line art, including text, but do so slowly because of the mechanical movement of the pens. They are often incapable of efficiently creating a solid region of colour, but can draw an area by drawing a number of close, regular lines.

Ø  Plotters offered the fastest way to efficiently produce very large drawings or colour high-resolution vector-based artwork when computer memory was very expensive and processor power was very limited.

Ø  There are a number of different types of plotters:

·         A drum plotter draws on paper wrapped around a drum which turns to produce one direction of the plot, while the pens move to provide the other direction.

·         A flatbed plotter draws on paper placed on a flat surface; and an electrostatic plotter draws on negatively charged paper with positively charged toner.

Ø  Pen plotters have essentially become obsolete, and have been replaced by large-format inkjet printers and toner based printers.

Ø   They are most frequently used for CAE (computer-aided engineering) applications, such as CAD (computer-aided design) and CAM (computer-aided manufacturing).

Display Devices

Display Devices

Modern day computers use, many kind of display devices, which are softcopy devices, and are used to display the output in a virtual form. There are many kind of display devices used these days, most common of these are CRT, LCD, Plasma, LED etc.

CRT (Cathode Ray Tube) :

Cathode Ray Tube monitors are the most commonly used display devices, though they are being replaced with other technologies very fast. Following figure depicts major parts of a CRT.

 

Working of CRT is as follows:

Ø  CRTs have a distinctive funnel shape. These are glass vacuum tubes.

Ø  At the very back of a monitor is an electron gun.

Ø  The electron gun fires electrons towards the front through a vacuum which exists in the tube of the monitor. The gun can also be referred to as a cathode - hence the electrons fired forward are called Cathode Rays. These are negatively charged rays.

Ø  The cathode rays then pass through negatively charged electrodes, which accelerate and concentrate the cathode rays.

Ø  At the neck of the funnel-shaped monitor are one or more anodes, which is magnetised or positively charged. As negatively charged electrons pass the anode, they are pulled in one direction or the other. This moves the electrons towards the correct part of the screen.

Ø  The electrons then pass through a mesh, and this mesh defines the individual pixels and resolution on the screen. This mesh is called “Shadow Mask”. Modern CRT displays use aperture grill instead.

Ø  Electrons then hit the phosphor coating which is on the inside of the glass screen. When the negative particles hit the phosphor, they immediately light up - causing the light to shine through the front of the monitor, thus making up the picture on the screen.

Ø  In coloured monitors there are three differently coloured phosphorus (Red, Green, Blue) for each pixel (known as phosphor triads), and depending on which phosphor the electron hits, will give the pixel its colour. Whereas in monochrome (Black/White) monitors the phosphors emit only white light.

Ø  Some CRT monitors use a single electron gun at the rear of the monitor to produce the electrons that will become the red, green and blue pixel after hitting the correct phosphor. However, higher quality monitors have an individual gun for each colour.

Ø  The entire front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. An image is produced by controlling the intensity of the electron beams.

Ø  The surface of the CRT glows for a fraction of a second before it starts to fade. This means that the picture has to be redrawn many times per second to avoid flicker.

Different Kinds of CRT based on REFRESHING TECHNOLOGY.

Based on how the CRT refreshes the screen once it is fully drawn, different CRT can be categorized as:

·         Raster Scan

·         Random Scan

·         Direct View Storage Tube (DVST)

Raster Scan

Ø  In a raster scan, an image is subdivided into a sequence of (usually horizontal) strips known as "Scan Lines".

Ø  This ordering of pixels by rows is known as raster order, or raster scan order.

Ø  In raster scanning, the electron beam sweeps a row of pixels horizontally left-to-right, then stops and rapidly moves back to the left, where it turns back on and sweeps out the next line of pixels. This is called “Horizontal Retrace”.

Ø  This process continues until the electron beam reaches the end of the bottom line of pixels.

Ø  The electron beam then stops and goes to the first pixel of the first line.

This is called “Vertical Retrace”.

Ø  These systems produce realistic graphics.

Ø  They can produce many different colours.

Ø  Major disadvantage of these systems is that they are

low in resolution.

Ø  These are expensive systems.

Ø  Raster scan system are use shadow mask methods to

produce colours images.

                                                                                    Raster scan system -->

Shadow Mask Method

§  In this, CRT has three phosphor colour dots. One phosphor dot emits a red light, second emits a green light and third emits a blue light.

§  This type of CRT has three electrons guns and a shadow mask grill as shown in figure below

§  Three electrons beams are deflected and focused as a group onto the shadow mask which contains a series of holes. When three beams pass through a hole in shadow mask they activate the dot triangle.

§  Different colours can be produced as combination of red, blue and green.

An example of Raster scans system is the Television. With every vertical retrace the next frame starts. Scanning happens so fast that human eye cannot distinguish between one frame and the other. Every vertical retrace leads to refreshing the screen. Thus refresh rate is also roughly equal to number of vertical retrace per second.

Interlacing is the technique used in standard CRT monitors when refreshing the screen. In this technique, with every horizontal retrace, the electron gun skips one line and moves to the alternate line. So in one frame every odd line of pixels is refreshed and in the next frame every even line of pixels is refreshed.

Progressive Scan in this technique all the lines of pixels are scanned in each frame. This technique is used in better quality TVs these days.

Random Scan

Ø  In this system images are defined in term of Line segments instead of pixels.

Ø  The electron beam is directed to only that part of the screen where the image is to be drawn.

Ø  Random Scan monitor draw one picture at a time therefore they are also known as vector display.

Ø  Refresh rate of random scan system depends on the number of lines to be displayed.

Ø   Picture definition is stored in an area of memory called refresh display file.

Ø 

Random Scan

Random-scan displays are designed to draw all the component lines of a picture 30 to 60 times each second.

Ø  These systems produce smooth lines.

Ø  These systems are high in resolution.

Ø  These systems are less expensive.

Ø  These systems are designed only for drawing applications.

Ø  These cannot produce realistic images.

Ø  These cannot produce all the different colours.

Ø  Random scan monitors use the beam penetration method

for displaying colour picture.

Beam Penetration Method

§  In this, the inside of CRT screen is coated with two layers of phosphor namely red and green.

§  A beam of slow electrons excites only the outer red layer, while a beam of fast electrons penetrates red layer and excites the inner green layer.

§  At intermediate beam speeds, combination of red and green light is emitted to show two additional colours- orange and yellow.

Direct View Storage Tube (DVST).

Ø  DVST is a modification of the Random scan system.

Ø  Much cheaper than Random scan systems.

Ø  Do not require a CPU for drawing image.

Ø  In DVST phosphor is replaced by Potassium Chloride (KCl).

Ø  KCl has a property that when its crystal is struck by an electron beam, it changes its colour to magenta.

Ø  Further it does not lose its brightness, so an image can be retained for a longer period as compared to a phosphor.

Ø  The image thus drawn can be erased by flooding the screen with high intensity infrared light.

Ø  An image can be retained even after the current to CRT is switched off.

There are some definitions that need to be understood in order to understand display devices. These are:

1.       Pixel: “The smallest displayable point on the screen is called the pixel.”

2.       Resolution: “The maximum number of pixels per inch that can be displayed without overlap is the resolution”, it’s measured as dpi (dots per inch) or ppi (points per inch).

Sometimes resolution is simply measured as the “Physical number of columns and rows of pixels creating the display”. E.g. 1024 X 768

3.       Bit Depth (colour depth): Number of bits used to represent a single pixel. This concept is known as bits per pixel (bpp).  RGB colour system with 24 bits per pixel is known as full colour system or true colour system.

4.       Refresh Rate: Number of times the screen is redrawn by the raster scan in a second.  I.e. how many times the electron guns are told to fire by the video source. The refresh rate is thus also limited by the resolution. This is normally more than 60Hz.

5.       Dot Pitch (Pixel Pitch): “This refers to the  the distance (usually Diagonal) between two phosphors of the same colour”

6.       Persistence: “The time taken by the light emitted from a phosphor to decay to one tenth (1/10^th) of its original intensity”. The phosphor when struck by an electron beam starts to emit light, but this light soon starts to fade. The time taken by this light to reduce to 1/10 is the persistence. It is due to this fading that the electron beam is required to strike the same pixels again and again to avoid flicker.

7.       Aspect Ratio: “Number of pixels in the horizontal direction to the number of pixels in the vertical direction that are required to produce a line of same length”. The pixels on all the display devices are not evenly distributed in horizontal and vertical direction. That is why the screens are rectangular. All latest TVs and monitors have aspect ratio of 16:9, for a better viewing experience. The aspect ratio of human eye is 21:9.

8.       Contrast Ratio: “Difference between the brightest and the darkest part of an image”. Higher the contrast ratio more realistic is the image.

Plasma Panel Displays

The latest technology available in the various display devices is the Plasma panel technology. Plasma panels are used to produce displays which are large in size (more than 32”). These display devices are generally less than 10 cm in depth, as a result they can be easily wall mounted. It is not economical to produce these displays in smaller sizes. Working of these displays is as follows:

Ø  Plasma display is made of two glass plates.

Ø  There are millions of tiny compartments (cells) inside the two glass plates.

Ø  These cells are filled with Neon or Xenon gases, these are inert gases.

Ø  Long tiny electrodes, which extend across the entire screen, are placed on both sides of cells between the glass plates.

Ø  Electrodes placed behind the cells are placed vertically and are called Address Electrodes.

Ø  Electrodes in front of cells, along the front glass plates are placed horizontally and are called display electrodes. These display electrodes are essentially Transparent.

Ø  Every pixel on the screen is made of three tiny cells.

Ø  These three cells have Red, Green or Blue phosphor coating inside.

Ø  Each of these three sub pixel has a different address electrode passing under it.

Ø  Initially a pixel is in a resting (OFF) state.

Ø  When very high voltage (+200V) is applied to an address electrode, the resistance of the inert gas inside the cell is overcome and electric discharge is made across the electrodes.

Ø  Once the discharge is made, the gas inside the cell is ionized into a “Plasma” state. Which means it starts to conduct electricity.

Ø  This emits intense Ultraviolet (UV) light.

Ø  This burst of UV energy exits the coloured phosphor, and they start to glow brightly.

Ø  Now the pixel is in an ON state.

Ø  Once the pixel is ON, a much smaller voltage of electricity (50V), is required to sustain the pixel in this state.

Ø  Once the sustain voltage is removed the pixel goes back to OFF state.

Ø  Varying the voltage of the signals to the three sub cells of a single pixel allows different colours to be displayed, with different brightness.

Advantages

Ø  Superior contrast ratio, latest plasma’s have contrast ratio of 5,000,000 : 1.

Ø  Wide viewing angle, as compared to CRT or LCD.

Ø  Less bulky.

Ø  Can be wall mounted.

Disadvantages

Ø  Small size is not easily available.

Ø  Heavy.

Ø  Electricity consumption is more than LCD, but comparable to CRT.

Ø  Cannot work at high altitude, because of pressure differential between gases inside the glass.

LCD (Liquid Crystal Display) :

LCD is fast becoming the most popular display device. These are being used in all the devices like laptops, monitors, microwaves, calculators, CD Players etc. These LCDs are passive displays that mean they do not emit any light, instead use external source of light, which may or may not be inbuilt. LCDs manipulate the light to display images. This technology uses liquid crystals, which are neither liquid nor solid. LCD work on the principle that

·         Light can be polarized and

·         Liquid crystals are capable of transmitting and changing polarized light.

By polarized light we mean that light is always vibrating and this vibration can be aligned in one direction. The working of the LCD is as follows:

Ø  An LCD primarily consists of two polarizing filters.

Ø  The axes of transmission of these two filters are perpendicular to each other such that when the light passes from one filter it will be blocked by the other filter.

Ø  These polarizing filters are attached on to two glass surfaces.

Ø  Inside these polarized glass surfaces are the Negative and Positive electrodes. These are transparent conductors made of Indium Tin Oxide (ITO).

Ø  Between the electrodes is a layer of Liquid Crystals.

Ø  Most LCDs today use a type of liquid crystal called Twisted Nematic (TN). These liquid crystals are rod shaped arranged in a twisted or helical shape.

 

Ø  At the back of this whole arrangement is an external light source called Backlighting.

Ø  When no current is applied to the electrodes the light from the back source passes from the vertical polarized filter at the back, this gives the light vertical polarization.

Ø  This polarized light then passes through the liquid crystals. These twisted crystals twist the polarized light along with themselves. Now the light is twisted by 90 degrees. (More the twist more is the contrast and better viewing, STN-SuperTN can twist up to 270 degree.)

Ø  The light now passes through the horizontally polarized filter in the front. This filter allows the light to pass without any problem as the light by now is also horizontally polarized.

Ø  So when the LCD is switched OFF we see the light as light gray.

Ø  When switched ON the liquid crystal molecules will rotate in the direction of the electric field.

Ø  The polarized light now passes from the liquid crystals unaffected and is completely blocked by the front polarizing filters.

Ø  This blockage appears as black pixels against the light silver gray background.

Ø  In coloured LCD screens a layer of RGB filters is also added in front of the front transparent electrodes.

Ø  When the light passes from these filters it appears as a combination of RGB colour.

Ø  The polarization of the light can be adjusted to adjust the brightness of the colour.

Types of LCD

Based on Display technology

·         Passive Display

·         Active Display (TFT)

Based on Light Source

·         Backlit

·         LED

·         Reflective

Passive Display LCD

                In passive matrix LCD the transparent electrodes are arranged as horizontal and vertical matrix. All the active electronics are outside the display screen. The integrated circuits control when the charge is sent down a particular row and column. This sends down the voltage to untwist the liquid crystal at that particular pixel. The major Disadvantage of this technology is the Slow response time and fuzzy images.

Active Display (TFT)

                Also called Thin Film Transistors (TFT). These are basically a grid of tiny transistors. Each sub pixel on the screen is controlled by a transistor. For example, a 1024x768 colour screen requires 2,359,296 transistors, one for each red, green and blue sub pixel. To address a particular pixel, the proper row is switched on, and then a charge is sent down the correct column. The amount of current can be easily controlled to allow less or more untwisting of the liquid crystal, which in turn allows more or less light to get through. This allows very crisp images, fast refresh rates and high contrast.

Since LCDs do not emit any light of their own, they have to use some external source of light. Based on the source of light these LCDs can be categorized as Backlit or reflective.

Backlit LCDs

Backlit LCD screens use their own source of light. This source of light is fluorescent tube lights used at the back or side of the LCD screen. They also use a diffuser to evenly spread the light across the screen. These LCDs work well in dim light also. These are widely used in Laptops and mobiles.

LED

LED Screens are essentially LCD screens that use LEDs as a source of light instead of fluorescent tubes to send light to the liquid crystals. A major advantage of LED is that they are very thin as compared to tubes and consume much less electricity.

Reflective

These LCDs do not use their own light source but instead use the ambient light, i.e. the light available from the environment. These devices reflect the light falling on them from external light sources. These are used in watches and microwave timers. These devices work well only in bright light, and are poor in dim lights.

Advantages of LCDs

·         Compact & Light.

·         Very Low Power Consumption.

·         High resolution.

·         Many shapes and sizes.

Disadvantages

·         Limited Viewing Angel.

·         Slow response time.

·         Low Bit Depth.

·         Cannot be used with light pens.

·         Cannot tolerate Huge variations in temperature.