Chapter 1Introduction

2Chapter 1

Definition

Computer graphics are graphics created using computers and, more generally, the representation and manipulation of image data by a computer.

The development of computer graphics has made computers easier to interact with, and better for understanding and interpreting many types of data. Developments in computer graphics have had a profound impact on many types of media and have revolutionized animation, movies` and the video game industry.

The term computer graphics has been used in a broad sense to describe "almost everything on computers that is not text or sound” Typically, the term computer graphics refers to several different things:

the representation and manipulation of image data by a computer

the various technologies used to create and manipulate images

the images so produced, and the sub-field of computer science which

studies methods for digitally synthesizing and manipulating visual content

Application Presentation Graphics: presentation

graphics deals with graphical representation of information. Typical

3Chapter 1

examples of presentation graphics are bar charts, line graphs, surface graphs, pie charts and other displays showing relationships between multiple parameters.

Entertainment: computer graphics methods are used in making videos, television shows, games, cartoon movies etc.

Morphing: Morphing is a graphical technique in which one object is transformed into another in such a way that the changes appears to be smooth and not rapid. Morphing has been used in advertisement, movies.

Animation: when motion is provided to an object which is not in existence, it is known as animation (put in action).It is to be noted that the motion should be such that it appears to be real life and not artificial. Cartoon movies are a good example.

Computer aided design: A major use of computer graphics is in design processes, particularly for engineering and architectural systems, but almost all products are now computer designed. For some designed applications, objects are first displayed in a wire frame outline form that shows the overall shape and internal features of the objects. CAD are now routinely used in the design of buildings, automobiles, aircraft, computers, textiles and many other products.

Drawing and art: Drawing requires exceptional self-motivation and

4Chapter 1

discipline. This field requires you to find your own artistic vision.

Communication: with videoconferencing, two people or more than two people can communicate with each other without being physically present at a common place.

Image processing: In computer graphics, a computer is used to create a picture. Image processing, on the other hand applies techniques to modify or interpret existing pictures, such as photographs and TV scans. Image processing starts with the images and then performs a pixel based treatment on them to exhibit certain desired features. For example if there is a white triangle in black background it can be convert to black triangle with white background using image processing.

Some other applications of computer graphics are Computer Vision, Graphical user Interface, Interactive Graphics etc.

Note: 1. Computer graphics is a branch of

computer science that deals with theory and technology of computerize image synthesis.

2. Computer graphics refer to the creation, storage, manipulation of picture and drawing using digital computer.

3. The phrase “Computer Graphics” was coined in 1960 by William

5Chapter 1

Fetter, a graphic designer for Boeing.

4. Popular movies such as Terminator, Matrix present the best examples of Morphing

Some important term related to computer graphics

Object space: In computer graphics terminology what we have envisioned is called the object definition, which defines the object in an abstract space of our choosing. This space is continuous and is called the object space.

Image space: Our action to map our idea on paper, which constitutes a display surface in another space called the image space. This mapping action is further influenced by our choice regarding such factors as the location and orientation of the object. It may be discrete also e.g. computer monitor where image is represented by individual pixels.

Pixel: In digital imaging, a pixel is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled. Each pixel has its own address. The address of a pixel corresponds to its coordinates. Pixels are normally arranged in a two-dimensional grid, and are often represented using dots or squares. The intensity of each pixel is variable. In color image systems, a color is typically represented by three or four component intensities such as red, green, and blue, or cyan, magenta, yellow.

6Chapter 1

Resolution: The display resolution of a display device is the number of distinct pixels in each dimension that can be displayed. The term “display resolution” is usually used to mean pixel dimensions, the number of pixels in each dimension (e.g., 1920×1080), which does not tell anything about the resolution of the display on which the image is actually formed: resolution properly refers to the pixel density, the number of pixels per unit distance or area, not total number of pixels.

There are two distinctly different terms, one is Image Resolution and the other is Screen Resolution. Image resolution refers to the pixel spacing i.e. the distance from one pixel to the next pixel. Image resolution is fixed but screen resolution can vary. If the image resolution is more compared to the inherent resolution of the display device then the displayed image quality gets reduced.

Aspect ratio: The aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. The aspect ratio of an image is the ratio of the width of the image to its height, expressed as two numbers separated by a colon. Aspect ratios are mathematically expressed as x:y (pronounced "x-to-y") and x×y (pronounced "x-by-y"), with the latter particularly used for pixel dimensions, such as 640×480The Aspect ratio of a display is the fractional relation of the width of the display area compared to its height.

7Chapter 1

Scan Conversion: The image space in computer graphics is, generally speaking, not continuous. It consists of a set of discrete pixels, i.e. picture elements that are arranged in a row-and-column fashion. Hence a horizontal or vertical line segment becomes a group of adjacent pixels in a row or column, respectively, and a slanted line segment becomes something that resembles a staircase. The area of the computer graphics that is responsible for converting a continuous figure, such as a line segment, into its discrete approximation is called scan conversion.

Aliasing-effect: The distortion introduced by the conversion from continuous space to discrete space is referred to as aliasing effect. Anti-aliasing is the process of minimizing the negative impact of the aliasing effect.

Dot pitch: Dot pitch is a specification for a computer display, computer printer, image scanner, or other pixel based device. It is the distance between adjacent sets (triads) of red, green and blue dots. This is also same as the shortest distance between any two dots of the same color i.e. from green to green, or red to red like that.

Display Device

8Chapter 1

The most prominent part in a personal computer is the display system that is responsible for graphic display. The display system may be attached with a PC to display character, picture and video output. The most common types of display system employ

Raster scan method Random scan method

The display systems are often referred to as Video Display Unit (VDU). However every display system has three basic parts- the display adapter that creates and holds image information, the monitor which displays the image information and the cable that carries the image data between the display adapter and the monitor.

Raster Scan Display

This type of display basically employs a Cathode Ray Tube (CRT). The CRT works just like the picture tube of a television set. In a raster scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turn on and off to create a pattern of illuminated spots. Picture definition is stored in a memory area called a refresh buffer or frame buffer. This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and painted on the screen one row at a time. Each screen point is referred to as a pixel.

9Chapter 1

In CRT the viewing surface is coated with a layer of arrayed phosphor dots. At the back of the CRT is a set of the electron guns which produce controlled stream of electrons. The phosphor material emits light when stuck by these high energy electrons. The frequency and intensity of the light emitted depends on the type of the phosphor materials used and energy of the electrons. In order to maintain a stable image, the electron beam must sweep the entire surface of the screen and then return to redraw it a number of times per second. This process is called refreshing the screen. The refresh rate is the number of times per second the screen is refreshed. Refreshing on raster scan display is carried out at the rate of 60 to 80 frames per second. Sometimes, refresher rates are described in units of cycles per second or Hertz where a cycle corresponds to one frame. If the electron beam takes too long to return and redraw a pixel, the pixel will began to fade; it will return to full brightness only when redrawn. Over the full surface of the screen, this becomes visible as a flicker in the image, which can be distracting and hard on the eyes.

At the end of each scan line, the electron beam returns to the left side of the screen to begin displaying the next scan line. The return to the left of the screen, after refreshing each scan line, is called the horizontal retrace of the electron beam. And at the end of each frame the electron beam returns to the top left corner of the screen to begin the next frame is known as vertical retrace.

10Chapter 1

On some raster-scan system each frame is displayed in two pass using an interlaced refresh procedure. In the first pass the beam sweeps across every other scan line from top to bottom. Then after the vertical retrace the beam sweeps out the remaining scan lines. Interlacing of the scan lines in this way allows us to see the entire screen displayed in one-half the time it would have taken to sweep across all the lines at once from top to bottom. Interlacing is primarily used with slower refreshing rates. In the figure, the odd-numbered lines represent scanning one half of the screen and the even-numbered lines represent scanning of the other half

11Chapter 1

Cathode Ray Tube

A CRT is similar to a big glass bottle. It contains three electron guns that emits beam of electrons, deflection apparatus (magnetic or electrostatic) which deflects these beams both up and down and sidewise, and a phosphor-coated screen over which these beams impinge. The vacuum is necessary to let those electron beams travel across the tube without running into air molecules that could absorb or scatter them.

The primary component in an electron gun is a negatively charged cathode encapsulated by a metal cylinder known as the control grid. A heating element inside the cathode causes the cathode to be heated up as the current is passed and as a result electrons emit from the cathode surface. These electrons are accelerated towards the CRT screen by a high positive voltage applied near the screen or by an accelerating anode. If allowed to continue uninterrupted, the

12Chapter 1

naturally diverging electrons would simply flood the entire screen. The cloud of electrons is forced to converge to a small spot on the CRT screen by a focusing system using an electrostatic or magnetic field.

A pair of deflection coils mounted outside/inside the CRT envelope deflects the concentrated electron beam sidewise. Horizontal deflection is obtained by one pair of coils and vertical deflection is obtained by other pair. The deflection amount is controlled by adjusting the current passing through the coils.

13Chapter 1

When the electron beam converges on to a point on the phosphor-coated face of the CRT screen, the phosphor dots absorb some of the kinetic energy from the electrons. This causes the electrons in the phosphor atoms to jump to higher energy state. After a short time these excited electrons drop back to their earlier stable state, releasing their extra energy in the form of light energy. As long as these electrons return back to their stable state phosphor continues to glow gradually losing brightness which is known as phosphorescence. The duration of phosphoresce exhibit by phosphor is known as persistence. The brightness of the light emitted by a phosphor depends on the intensity with which the electron beam strikes the phosphor. The intensity of the beam can be regulated by applying measured negative voltage at the control grid. Corresponding to zero value in the frame buffer a high negative voltage is applied in the control grid which in turn will shut off the electron flow by repelling the electrons and stopping them from coming out of the gun and hitting the screen. The corresponding points on the screen will remain black. Similarly a bright white spot can be created at a particular point by

14Chapter 1

minimizing the negative voltage at the control grid of the three electron guns when they are directed to that point by the deflection mechanism.

Color Monitor

In a monochrome CRT there is only one electron gun, whereas in color CRT there are three electron guns each controlling the display of red, green and blue light respectively. Unlike the screen of a monochrome CRT which has a uniform coating of phosphor, the color CRT has three color phosphor-dots (dot triad) – red, green and blue. When struck by electron beam the red dot emits red light, the green dot emits green light and the blue dots emit blue light. Each triad is arranged in a triangular pattern, as are the three electron guns. The beam deflection arrangement allows all the three beams to be deflected at the same time to form a raster scan pattern. There are separate video stream for each RGB color component which drive the electron guns to create different intensities of RGB colors at each point on the screen. To ensure that the electron beam emitted from individual electron guns strikes only the correct phosphor dots, a shadow mask is used just before the phosphor screen. The mask is a fine metal sheet with a regular array of holes punched on it. The mask is so aligned that as the set of three beams sweeps across the shadow mask they converge and intersect at the holes and then hits the correct phosphor dot; the beams are prevented or masked from intersecting other two dots of the triad.

15Chapter 1

Cutaway rendering of a color CRT:

1. Three Electron guns (for red, green, and blue phosphor dots)2. Electron beams3. Focusing coils4. Deflection coils

16Chapter 1

5. Anode connection6. Mask for separating beams for red, green, and blue part of displayed image7. Phosphor layer with red, green and blue zones.8. Close-up of the phosphor-coated inner side of the screen

Color Model

A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three values or color components. The purpose of a color model is to facilitate the specification of colors in some standard generally accepted way. In computer graphics we generally use two color models RGB and CMY.

RGB Color Model

In the RGB model, each color appears as a combination of red, green, and blue. This model is called additive, and the colors are called primary colors. The primary colors can be added to produce the secondary colors. Each primary color can take on an intensity value ranging from 0 to 1. Mixing these three primary colors at different intensity levels produces a variety of colors. The collection of all the colors obtainable by such a linear combination of red, green and blue forms the cube shaped RGB color space. The corner of the RGB color cube that is at the origin of the coordinate system corresponds to black, whereas the corner of the cube that is

17Chapter 1

diagonally opposite to the origin represents white. The diagonal line connecting black and white corresponds to all the gray colors between black and white. It is called gray axis.

Color specification using the RGB model is an additive process. We begin with black and add on the appropriate primary components to yield a desired color. On the other hand there is a complementary color model, called the CMY color model that defines colors using a subtracting process.

RGB and CMY Color Models

The importance of the RGB color model is that it relates very closely to the way that the human eye perceives color. RGB is a basic color model for computer graphics because color displays use red, green, and blue to create the desired color. Therefore, the choice of the RGB color space simplifies the architecture and design of the system. However, RGB is not very efficient

18Chapter 1

when dealing with real-world images. To generate any color within the RGB color cube, all three RGB components need to be of equal pixel depth and display resolution. Also, any modification of the image requires modification of all three planes.

CMY Color Model

The CMY color model is a subset of the RGB model and is primarily used in color print production. CMY is an acronym for cyan, magenta, and yellow. In the CMY model we begin with white and take away the appropriate primary components to yield a desired color. The corner of the CMY color cube that is at the origin corresponds to white, whereas the corner just opposite to origin represents black.

Primary and Secondary Colors for RGB and CMY Models

The following formulae summarize the conversion between the two color models.

19Chapter 1

= -

=

-

Differences between RGB and CMY 

RGB is based on projecting. Red light plus Green light plus blue light all projected together create white. Black is encoded as the absence of any color. 

CMY is based on ink.  Superimpose Cyan ink plus Magenta ink plus Yellow ink, and you get black. White is encoded by the absence of any color.

Prism uses RGB internally. Exporting in RGB will give you results very close to what you see on screen.

Even though it uses one more number to encode a color, the CMY scheme encodes a smaller "color space" than does RGB.

When a color is converted from RGB to CMY, the appearance may change. Most noticeably, bright colors in RGB will look duller and darker in CMYK

20Chapter 1

Bit Planes, Color Depth

The appearance and color of a pixel of an image is a result of intersection of three primary colors (red, green, blue) at different intensities. When the intensities of all three electron beams are set to the highest level the result is a white pixel; when all are set to zero, the pixel is black. And for many different combinations of intermediate intensity levels, several million color pixels can be generated. For a black and white monitor using a single electron gun, the phosphor material can glow with varies intensities depending on the intensity of the electron beam. As a result a pixel can be black or white or have different shades of gray.

The number of different colors that can be displayed depends upon the intensities of the primary color that can be generated. The number of memory bit required to store color information about a pixel is called color depth or bit depth. A minimum of one memory bit (color depth=1) is required to store intensity value either 0 or 1 for every screen point or pixel. Corresponding to the intensity value 0 or 1, a pixel can be black or white. So if there are n pixels in an image a total of n bits of memory used for storing intensity values will result in a pure black and white image. The block of memory (Frame buffer) which stores intensity values for each pixel of a full screen pure black and white image is called

21Chapter 1

a bit plane or bitmap, however for multiple bits per pixel the frame buffer is referred to a pixmap.

A single bit plane black & white frame buffer

Color or gray levels can be achieved in the display using additional bit planes. First consider a single bit plane – a planer array of bits, with one bit for each screen pixel. This plane is replicated as many times as there are bits per pixel, placing each bit plane behind its predecessor. Hence the result for n-bits per pixel (color depth=n) is a collection of n bit planes that allows specifying any one of 2n colors or gray shades at every pixel.

22Chapter 1

N bit plate gray level frame buffer

The more the number of bits used per pixel, the finer the color detail of the image. However, increased color depths also require significantly more memory for storage and also more data for the video card to process, which reduces the allowable refresh rate.

For true color three bytes of information are used, one for each of the red, blue and green signals that make a pixel. A byte can hold 256 different values and so 256 voltage settings are possible for each electron gun which means that each primary color can have 256 intensities, allowing over 16 million (256 x 256 x 256) color possibilities. This allows for a very realistic representation of the images, without necessitating any color compromise.

23Chapter 1

24-bit plane color frame buffer

In 256 color mode the PC uses only 8 bits; this means something like 2 bits for blue and 3 each for green and red. There are chances that most of the colors of a given picture are not available, and choosing between only 4 or 8 different values for each primary color would result in rather blocky or grainy look of the display image. A palette or look-up table is used here. A palette is a separate memory block created containing 256 different colors. The intensity values stored therein are not constrained within the range 0 to 3 for blue and 0 to 7 each for green and red. Rather each color is defined using the standard 3 – byte color definition that is used for true color. Thus the intensity values for each of the three primary color component can be anything between 0 to 255, in each of the table entries. Upon reading the bit planes, the resulting number instead of directly specifying the pixel color, is used as a pointer to the 3 – byte color value entry in the look-up table. For example, if the number read from the bit planes is 3 for a given pixel, then the intensity to be displayed for that pixel will

24Chapter 1

be found in the 3rd entry of the table. The full range of true color can be accessed in the same way but only 256 of the available 16 million colors can be used at a time.

N bit plane gray level frame buffer with w bit wide look-up table

25Chapter 1

The palette is an excellent compromise at the cost of moderate increase in memory: it allows only 8 bits of the frame buffer to be used to specify each color in an image and allows the creator of the image to decide what the 256 colors in the image should be. Because the palette can be reloaded any time with different combination of 256 colors out of 16 million without changing the frame buffer values.

Raster Scan Systems

Interactive raster graphic systems typically employ several processing units. In addition to the central processing unit, or CPU, a special purpose processor, called the video controller or display controller, is used to control the operation of the display device. In a simple raster scan system the frame buffer can be any where in the system memory and the video controller accesses the frame buffer to refresher the screen.

26Chapter 1

Video Controller

Video Controller is a specialized hardware which actually drives the CRT and produces the display on the screen.In a commonly used organization for raster systems a fixed area of the system memory is reserved for the frame buffer, and the video controller is given direct access to the frame buffer memory.

Raster system with fixed portion of the system memory reserved for the frame buffer

Frame-buffer locations, and the corresponding screen positions, are referenced in Cartesian coordinates. For many graphic monitors, the coordinate origin is defined at lower left corner. The screen surface is then represented as the first quadrant of a two dimensional system, with a positive x values increasing to the right and positive y values increasing from bottom to top (in some computer, the coordinate origin is referenced at the upper left corner of the screen).

27Chapter 1

Scan lines are then labeled from ymax at the top of the screen to 0 at the bottom. Along each scan line, screen pixel positions are labeled from 0 to xmax.

The basic refresh operations of the video controller can be described using two register by storing the coordinates of the screen pixel. As the origin is consider at the left-bottom corner the x register is set to 0 and y register is set to ymax .The value of the pixel position is taken from the frame buffer and used to set the intensity of the CRT beam. Then through out the same scan line x value is incremented for the next pixel but the y value remain same. After the last pixel of the top scan line the x value is again set to 0 and the y value is decremented by 1. Pixels along the scan line are then processed and the procedure is then repeated for each scan line. After reaching the bottom scan line (y=0) the video controller resets the register for the pixel position at the top-left corner and the refresher process is over.The video controller in the output circuitry generates the horizontal and vertical drive signals so that the monitor can sweep its beam across the screen during raster scan. Memory reference addresses are generated in a synchronous way with the raster scan, and the contents of the memory are used to control the CRT beam intensity or color. The (x,y) address is translated into memory address of frame buffer where the color value for this pixel position is stored. The controller retrieves the color value (binary number) from the frame buffer and supplies it to digital-to-analog converter. The analog voltages in turn controls the intensity of the electron beam that are

28Chapter 1

focused at the (x,y) screen position by the horizontal and vertical drive signal.

Logical operation of the video controller

Since the screen must be refreshed at the rate of 60 frames per second and the cycle time is too slow it is necessary to speed up the pixel processing time. Video controllers are allowed to retrieve multiple pixel values from the frame buffer on each pass. Separate registers are used for storing multiple pixel intensities and used to control the beam intensity for adjacent pixels. When the groups of pixels are processed the next block of pixel values is retrieved from the frame buffer. In high quality system sometimes two frame buffers are used so that one buffer can be used for refreshing while the other is filled with intensity values. Then the buffers alternate their roles. Sometimes a separate display processor is used to free the CPU from the graphics cores. In that case a separate display memory area is also provided. Display processor helps the process of scan conversion, generating various line styles,

29Chapter 1

displaying color areas, performing certain transformations and manipulation of displayed objects. Display processors are typically designed to interface with interactive input devices, such as a mouse. To reduce the memory requirements in raster system frame buffer is organized a linked list. One way to do this is to store each scan line as asset of integer pairs. One number for each pair indicates an intensity value and the second number specifies the number of adjacent pixels on the scan line having the same intensity. This technique is known as run-length encoding. A picture having mostly single-color pixels can be saved with minimum storage space.

Random Scan Display

In the random scan technique the electron beam is directed straight way to those particular points of the screen where the image is to be drawn. It generates the image by drawing a set of random straight lines much in the same way one might move a pencil over a piece of paper to draw an image – drawing strokes from one point to another, one line at a time. For this reason this type of display is known as vector display or calligraphic display. The component lines of a picture can be drawn and refreshed by a random scan system in any specified order. Refresh rate on a random-scan system depends on the number of lines to be displayed. Picture definition is stored as a set of line drawing commands in area of memory referred to as refresh display file. To display a specified picture, the system cycles though the set of commands in the display file, drawing each component line in turn. After all

30Chapter 1

line-drawing commands have been processed, the system cycles back to the first line command in the list.

In a simple random scan system is an application

program is taken as input and stored in the system memory along with a graphics package. Graphics commands in the application program are translated by the graphics package into a display file stored in the system memory. This display file is then accessed by the display processor to refresh the screen. The display processor cycles through each command in the display file program once during every refresh cycle. Graphics patterns are drawn on the screen by directing the electron beam along the component lines of the picture. Lines are defined by the endpoints coordinate and these input coordinates are converted to x and y deflection voltages. A scene is then drawn one line at a time by positioning the beam to fill the line between the specified endpoints.

31Chapter 1

Difference between Raster Scan and Random Scan

The main difference between the two is the technique with which the image is generated on the screen. In raster scan method the electron beam is allowed to sweep the entire screen in the same way one write a full page text in a notebook, word by word, character by character, from left to right and from top to bottom, whereas in random scan the electron beam directed only to the parts of the screen where a picture is to be drawn.

32Chapter 1

Random scan systems are designed for line drawing application and cannot display realistic shaded scenes. Since picture definition is stored as a set of line-drawing instructions and not as a set of intensity values for all screen points.

Random scan displays can work at higher resolution than the raster displays.

The images are sharp in random scan displays and have smooth edges unlike the jagged edges and lines in raster displays.

Vector Scan Display Raster Scan DisplayIn vector scan display the beam is moved between the end points of the graphics primitives.

In raster scan display the beam is moved all over the screen one scan line at a time, from top to bottom and then back to top.

Vector display flickers when the number of primitives in the buffer becomes too large.

In raster display, the refresh process is independent of the complexity of the image.

Scan conversion is not required.

Graphics primitives are specified in terms of their endpoints and must be scan converted into their corresponding pixels in the frame buffer.

Scan conversion hardware is not required.

As each primitive must be scan converted, real time dynamics is far more computational and requires separate scan conversion hardware.

33Chapter 1

Vector display draws continuous and smooth lines.

Raster display can display mathematically smooth lines, polygons, and boundaries of curved primitives only by approximating them with pixels on the raster grid.

Cost is more. Cost is low.

Vector display only draws line characters.

Raster display has ability to display areas filled with solid colors or patterns.

Direct View Storage Device

Direct View Storage Tube is different from the usual refresh type display. In DVST there is no refresh buffer, the image is created by drawing

34Chapter 1

vectors or line segments with a relatively slow-moving electron beam. The beam is designed not to draw directly on phosphor but on a fine wire mesh coated with dielectric and mounted just before the screen. A pattern of positive charge is deposited on the grid, and this pattern is transferred to the phosphor coated screen by a continuous flood of electrons emanating from a separate flood gun. Just behind the storage mesh is a second grid, the collector, whose purpose is to smooth out the flow of flood electrons. These electrons pass through the collector at low velocity and are attracted to the positively charged portions of the storage mesh but repelled by the rest. Electrons not repelled by the storage mesh pass right through it and strike the phosphor. To increase the energy of these slow electrons and thus create a bright picture, the screen is maintained at a high positive potential.

Advantage: No refreshing is necessary and

the image is absolutely flicker free

Supports a very high resolution which is good for displaying complex images.

Disadvantage: Inability to erase parts of an

image from the screen. To erase an image from the displayed image, one has to first erase the complete image and then redraw it by omitting the line segment.

No color facility is there.

35Chapter 1

Liquid crystal display

This is a non emmissive type display which uses optical effects to convert light from some other source into graphics patterns. LCD basically consists of a layer of liquid crystal, sandwiched between two polarizing plates. The polarizers are aligned perpendicular to each other (one vertical and the other horizontal), so that the light incident on the first polarizer will be blocked by the second. Because a polarizer plate only passes photons with their electric fields aligned parallel to the polarizing direction of that plate. The LCD displays are addressed in a matrix fashion. Rows of matrix are defined by a thin layer of horizontal transparent conductors, while columns are defined by another thin layer of vertical transparent conductors; the layers are placed between the LCD layer and the respective polarizer plate. The intersection of the two conductors defines a pixel position. This means that an individual LCD element is required for each display pixel.

36Chapter 1

Polarizing filter – control light input/outputGlass substrateTransparent electrodes – drives the LCDAlignment layer – aligns the LC in fixed directionsLiquid crystalsSpacer – controls the cell gapColor filter – Color is expressed through the use of R, G and B filters.Backlighting – The display is lit from behind to make the screen brighter

37Chapter 1

The two polarizing filters are arranged along perpendicular polarizing axes

The liquid crystal materials are made up of long rod-shaped crystalline molecules containing cyanobiphenyl units. The individual polar molecules in a nematic LC layer are normally

When voltage is not applied, light entering from the above is re-directed 90 degrees along the helix arrangement of the liquid crystal molecules so that it passes through the lower filter.

When voltage is applied, the liquid crystal Molecules straighten out of their helix pattern and stop redirecting the angle of the light, thereby preventing light from passing through the lower filter.

38Chapter 1

arranged in a spiral fashion such that the direction of polarization of polarized light passing through it is rotated by 90 degrees. Light from an internal source enters the first polarizer and is polarized accordingly. As the light passes through the LC layer it is twisted 90 degrees so that it is allowed to pass through the rear polarizer and then reflect from the reflector behind the rear polarizer. The reflect light when reach the viewers eye traveling in the reverse direction, the LCD appears bright.

When an electric current is passed through the LCD layer, the crystalline molecules align themselves parallel to the direction of light and thus have no polarizing effect. The light entering through the front polarizer is not allowed to pass through the rear polarizer due to mismatch of polarization direction. The result is zero reflection of light and the LCD appears black.

39Chapter 1

Fig1 Fig2

Fig 1: The light also "twists" as it passes through the twisted liquid crystals and bends 90 degrees

Fig 2: When voltage is applied to the liquid crystal structure, the twisted light passes straight through.

40Chapter 1

In a color LCD there are layers of three liquid crystal panels one on top of another. Each one is filled with a colored liquid crystal. Each one has its own set of horizontal and vertical conductors. Each layer absorbs an adjustable portion of just one color of light passing through it. This is similar to how color images are printed. The principal advantage of the design is that it helps create as many screen pixels as intersections, thus making high-resolution LCD panels.

The image painting operation in LCD panels is a different from that of CRT though both are of raster scan type. In a simple LCD panel an entire line of screen pixels are illuminated at one time. Then the next line and so on till the entire screen image is completed. Once set, the screen pixels stay at fixed brightness until they are reset. The time required to set the brightness of a pixel is high compared to that of the CRT.

Advantage: Very compact and light. Low power consumption. No geometric distortion. Little or no flicker depending on

backlight technology. Not affected by screen burn-in. No high voltage or other hazards

present during repair/service. Can be made in almost any size or

shape. No theoretical resolution limit.

Disadvantage:

41Chapter 1

Limited viewing angle, causing color, saturation, contrast and brightness to vary, even within the intended viewing angle, by variations in posture.

Bleeding and uneven backlighting in some monitors, causing brightness distortion, especially toward the edges.

Smearing and ghosting artifacts caused by slow response times (2-8 ms) and "sample and hold" operation.

Only one native resolution. Displaying resolutions either requires a video scaler, lowering perceptual quality, or display at 1:1 pixel mapping, in which images will be physically too large or won't fill the whole screen.

Fixed bit depth, many cheaper LCDs are only able to display 262,000 colors. 8-bit S-IPS panels can display 16 million colors and have significantly better black level, but are expensive and have slower response time.

Input lag Dead pixels may occur either during

manufacturing or through use. In a constant on situation,

thermalization may occur, which is when only part of the screen has overheated and therefore looks discolored compared to the rest of the screen.

Not all LCD displays are designed to allow easy replacement of the backlight.

Cannot be used with light guns/pens.

42Chapter 1

Plasma Display

This is a emissive type display that convert electrical energy into light. Here a layer of gas (usually neon) is sandwiched between two glass plates. Thin vertical strips of conductor run across one plate, while horizontal conductors run up and down the other plate. By applying high voltage to a pair of horizontal and vertical conductors, a small section of the gas at the intersection of the conductors breaks down into glowing plasma of electrons and ions. Thus in the array of gas bulbs, each one can be set to ‘on’ state or ‘off’ state by adjusting voltages in the appropriate pair of conductors. Once set ‘on’ the bulbs remain in that state until explicitly made ‘off’ by momentarily reducing the voltage applied to the pair of conductors. Here no refreshing is necessary.

Advantages Picture quality

o Capable of producing deeper blacks allowing for superior contrast

43Chapter 1

ratio o Wider viewing angles than

those of LCD; images do not suffer from degradation at high angles like LCDs.

o Less visible motion blur, thanks in large part to very high refresh rates and a faster response time, contributing to superior performance when displaying content with significant amounts of rapid motion.

Physical o Slim profile o Can be wall mounted o Less bulky than rear-projection

televisions

Disadvantages Picture quality

o Earlier generation displays were more susceptible to screen burn-in and image retention, although most recent models have a pixel orbiter that moves the entire picture slower than is noticeable to the human eye, which

reduces the effect of burn-in but does not prevent it.

o Earlier generation displays (2006 and prior) had phosphors that lost luminosity over time, resulting in gradual decline of absolute image brightness

44Chapter 1

o Earlier generation (circa 2001 and earlier) models were susceptible to "large area flicker"

o Heavier screen-door effect when compared to LCD or OLED based TVs

Physical o Generally do not come in

smaller sizes than 37 inches o Heavier than LCD due to the

requirement of a glass screen to hold the gases

Other o Use more electricity, on

average, than an LCD TV o Do not work as well at high

altitudes due to pressure differential between the gases inside the screen and the air pressure at altitude. It may cause a buzzing noise. Manufacturers rate their screens to indicate the altitude parameters.

o For those who wish to listen to AM radio, or are Amateur Radio operators (Hams) or Shortwave Listeners (SWL), the Radio Frequency Interference (RFI) from these devices can be irritating or disabling.

o Due to the strong infrared emissions inherent with the technology,

standard IR repeater systems cannot be used in the viewing room. A more expensive "plasma compatible" sensor must be used.

45Chapter 1

ExerciseMultiple Choice Questions

1. What will be the resolution (in ppf.) of 3X3 inch image that has 768X768 pixels? (a) 256 (b) 254 (c) 255 (d) None of these2. What will be the CMY co-ordinates of a color at (0-2, 1, 0-5) in the RGB space? (a) (0-4, 0-2, 0-3) (b) (0-8, 0, 0-3) (c) (0-8, 0, 0-5) (d) (0-4, 0, 0-5)3. If 3-byte pixel values are used in a 24-bit look up table representation, how many

bytes does the table occupy? Ans.224X24/8 bytes4. What is the name of the path the electron beam takes when returning to the left side of

the CRT screen? (a) Vertical retrace (b) Vertical entrance (c) Horizontal retrace (d) Horizontal comeback5. Refreshing on raster scan displays is carried out at the rate of (a) 60 to 80 frames per sec (b) 40 to 60 frames per sec (c) 30 to 60 frames per sec (d) None of these.6. The maximum number of points that can be displayed without overlap on a CRT is

referred to as (a) Resolution (b) Attenuation

46Chapter 1

(c) Persistence (d) None of these

Short Answer Type

1. What is persistence? Why is the electron beam allowed to over scan?

Ans. When the electron beam strike the phosphor dots they emit light. The incident is known as phosphorescence. The duration of phosphorescence exhibited by a phosphor is known as persistence.

When a dot of phosphor material is struck by the electron beam, it glows for a fraction of second and then fades. As the brightness of the dots begins to reduce, the screen image become unstable and gradually fades out. In order to maintain a stable image, the electron beam must sweep the entire screen or over scan and the rerun to redraw it a number of times per second.

2. Differentiate between Raster scan and

Random scan displays.

3. What do you mean by raster scan and random scan system?

4. What is aspect ratio? Give examples

5. Explain briefly the RGB and CMY color model.

6. Give examples of some Graphics devices.

47Chapter 1

Ans. 1. Display system where the graphics are rendered in the screen of the computer e.g. CRT (raster/random type), flat-panel displays, plasma panels etc. 2. Hardware components with which the user interacts to generate necessary instructions for the creation of the required graphics e.g. printer, plotter, keyboard, mouse, joysticks, scanner etc.

7. What do you mean by resolution of an image?

8. Compute the size of a 640x480 image at 240 pixels per inch.

9. Find the CMY coordinates of a color at (0.2, 1 , 0.5) in the RGB space.

10. Explain shadow mask methods for color

monitor.Ans. In case of color monitor there are three electron guns for each primary color RED, GREEN and BLUE. The phosphor coating inside the display screen also consists of the primary colors. These phosphor dots are capable of red, green and blue light respectively. To ensure that the electron beam strike the correct phosphor dot a shadow mask is used just before the screen. Shadow mask is thin metal sheet with a regular array of holes. The tiny holes on the shadow mask constrain each electron beam to hit its corresponding phosphor dots.

48Chapter 1

11. What Is Flicker? How can this be reduced?Ans. When the refreshing rate falls below certain limit then our visible system is unable to integrate the light impulses from the phosphor dots into a steady picture, the fact is known as flicker.In order to avoid flicker, the screen image must be redrawn sufficiently quickly that the eye cannot tell that refresh is going on. If the refresh rate is sufficiently high then it is possible to reduce flicker. Some monitor use the technique called interlacing to double the refresh rate.

Write short notes on:

Shadow masking. Interlacing Cathode Ray Tube (CRT). Resolution Aspect Ratio Refreshing Rate Bit Plane

Raster graphic display is implemented using frame buffer. A frame buffer is a large contiguous piece of computer memory. Each pixel is in the raster is represented by at least o bit. This amount of memory of one bit per pixel is called a bit plane or bitmap. The picture is built up in the frame buffer one bit at a time. A single bit plane yields a black-and-white or monochrome display, because a memory bit has only two states, namely, 0 or 1.

49Chapter 1

However for systems with multiple bits per pixel, the frame buffer is often referred to as a pixmap.

Descriptive Question

1. State some of the major applications of graphics in real life. Name a few graphics

software’s.2. Briefly describe the main functional components and its functions of a CRT terminal

with a proper diagram.3. Consider the three different raster systems, systems with resolution of 640X480,

1280x1024 and 2560x2048. What size of the frame buffers is needed for each of these systems to store 12-bits per pixel? How much storage is required for each system if 24-bits per pixel are to be stored?Ans. The size of the frame buffers for each of the above raster system to store 12 bits per pixel is 640x480x12, 1280x1024x12, 2560x2084x12 respectively and 40x480x24, 1280x1024x24, 2560x2084x24 respectively if 24-bits per pixel are to be stored.

4. Suppose a raster system with resolution 640 by 480. How many pixels could be accessed per second in this system if the display controller refresh rate is 60 Hz? What is the access time per pixel in this system?

Ans: refresh rate = 60 Hz=60 frames/sec. Resolution of the raster = 640x480.

50Chapter 1

No. of pixel that could be accessed per second is 60x640x480. Access time per pixel =1/(60x640x480) sec=0.00000005 sec.