Computer Graphics 4j1c5y

Chapter

1

B ASICS OF C OMPUTER G RAPHICS [20 M ARKS ] Chapter Details 1.1

Introduction

1.2

Basic of Computer Graphics

1.3

Raster Scan Display and Random Scan Display

1.4

Frame Buffer

1.5

Display Devices

1.6

Primitive Operation

1.7

Display File Interpreter

1.8

Graphics file format

1.9

Applications of Computer Graphics

1.10

Various Input Devices

1.11

Display Processor

1.12

Co-ordinate System

1.13

Graphics Modes

1.14

Examples initialize

of

graphics

mode

1-2

Computer Graphics

Graphical and Statistical representation of questions asked from this chapter in previous years MSBTE Question Papers

Statistical Analysis

MSBTE paper S-2010 W-2010 S-2011

Total marks questions asked in this chapter 24 16 32

Basic of Computer Graphics

1.1

INTRODUCTION

Computer Graphics is a field which deals with picture and images. In literature, one can find many definitions of computer Graphics, of which a few are given below. “Computer Graphics is a process associated with producing images by digital rendering of picture model”. “Computer graphics is a process of generation of image of virtual scenes using computer hardware”. “Computer Graphics is human-oriented system that uses the capabilities of a computer to create, transform, and display pictorial symbolic data”.

1-3 Definition “Computer graphics is a process of generation of image of virtual scenes using computer hardware.”

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English Word

Interactive

d: ‘kd.kkjs

Computer graphic is the use of computer hardware and software to create, manipulate, and present picture and images”. Computer graphics is pictorial representation and manipulation of data by a computer. Looking at the preceding definitions, it becomes clear that computer graphics deals with the creation and manipulation of images and pictures. During the presentation of picture and images, s would like to communicate through graphics packages. Communicating with the graphics packages during creation and manipulation of images and pictures is called interactive computer graphics.

1.2

BASICS OF COMPUTER GRAPHICS

The Origins of Computer Graphics In 1950 the first computer driven display, attached to MIT's computer, was used to generate simple pictures. This display used a Cathode-Ray tube (CRT). Interactive computer graphics made progress and the term computer graphics was first used in 1960. Interactive Computer Graphics It involves two way communications between computer and . The computer up on receiving signals from the input device, can modify the displayed picture appropriately.

1-4 English Word

Computer Graphics

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The main reason for the effectiveness of interactive computer graphics in many applications is the speed with which the of the computer can assimilate the display information.

Assimilate = vkRelkr

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DP

Refresh buffer

Advantages: Realistic shading No flickering

1.3

RASTER SCAN DISPLAY AND RANDOM SCAN DISPLAY

1.3.1

Raster Scan Display

VC Disadvantages: Aliasing Scan conversion

Raster scan system

Raster scan CRT much works like television set. Light occurs when an electron beam stimulates a phosphor.

BTE [S.2010] - 4M Write short note on raster scan display.

A fixed area of system memory reserved for frame buffer U

Frame System memory buffer

Video controller

Monitor

System bus

I/O devices Figure 1.1

Principle of Raster Scan System In Raster scan, the electron beam from electron gun is swept horizontally across the phosphor. After each horizontal sweep the beam is moved. After the bottom line is swept, the beam returns to the top and the sweep process begins again.

Basic of Computer Graphics

1-5 Horizontal retracing

0 Scanline

1 2 3

It is used to refer to the beam moving back to the left at the end of each line.

Horizontal retrace

4

Vertical retracing

5

Refer to the movement from bottom to top.

6 7 Vertical retrace

8 9 10 Figure 1.2

The lines numbered as 0, 1, 2, 3, 4, 5, …, 10 represent lines that the electrons beam is “painting” on the screen from left to right. When the beam reaches the right side of the bottom line, it has to move back to the upper left corner of the screen. i.

Horizontal retracing: 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.

ii.

Vertical retracing: At the end of each frame, the electron beam returns to the top left corner of the screen to begin the next frame is called the vertical retrace.

iii.

Interlacing: It is the method of incrementally displaying a visual on a CRT. On some raster scan systems, each frame is displayed in two es using an interlaced refresh procedure. In the first , 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. The screen is painted 60 times per second but only half of the lines are painted per frame. For example: Even numbered lines are painted first and then odd numbered lines are painted on each . So the entire screen in two es is painted 30 times every second.

1-6

Computer Graphics A

Odd field

Even fi eld

C

B

D

Figure 1.3

iv.

The picture definition is stored in memory area called refresh buffer or frame buffer. Buffer stands for temporary memory area. The main storage in refresh buffer is the intensity values for all screen points. a. b. c. d.

In raster scan display black and white systems are used for intensity indications in of 0 and 1. In case of colour displays the different intensities are stored in frame buffer. Raster scan display is well suited for realistic images showing colour patterns and shading. The refreshing rate of raster scan display is 60 to 80 frames per second.

Advantages of Raster Scan display i. ii. iii.

It uses interlacing techniques to paint the image on screen. It gives realistic effect to images. Cost is low.

Disadvantages of Raster Scan Display Not suitable for line drawing applications.

1-7

Basic of Computer Graphics

1.3.2

Random Scan Display

In a random scan system, also called vector, stroke writing or calligraphic the electron beam directly draws the picture. B

C

Display file Advantages: Light pen pick Built-in lines

DP Disadvantages: Flickering No realistic shading Programming more difficult

Random scan display D

A F

E

BTE [S.2010] - 4M Write short note on random scan display.

Figure 1. 4

Working Graphics commands are translated into a display file stored in the system memory. U

System memory

Display processor

Monitor

System bus

I/O devices Figure 1.5

Here the electron gun of the CRT illuminates points and / or straight lines in any order. The display processor repeatedly reads a variable ‘display file defining a sequence of X, Y co-ordinates the electron gun the whole display area is updated many times a second from image data held in raster memory.

English Word

Calligraphic

=

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vlysyk Illuminates = ped.ks

1-8

Computer Graphics

Random scan monitor draw pictures line by line hence known as vector displays. The component lines of pictures can be drawn and refreshed in particular order. e.g.: Pen plotter. Picture definition is stored as set of line drawing commands in an area of memory referred as a refresh display file. For displaying picture line commands in display file executed in sequence or cycle. Refresh rate of random scan display is 30 to 60 times per second. Random scan display is used in vector based application and its resolution is high as compare to raster scan display. Advantages of Random Scan i.

Very high resolution, limited only by monitor.

ii.

Easy animation, just draw at different position.

iii.

Requires little memory.

Disadvantages of Random Scan i.

Requires “intelligent“ electron beam, i.e. processor controlled.

ii.

Limited screen density before have flicker, can’t draw a complex image.

iii.

Limited colour capability.

iv.

It is very expensive.

Difference between Random Scan Display and Raster Scan Display i.

ii.

Random Scan Display In random scan display the beam is moved between the end points of the graphics primitives.

iii.

Random display flickers when the number of primitives in the buffer becomes too large. Scan conversion is not required.

iv.

Scan conversion hardware is not required.

v.

Random display draws a continuous and smooth lines.

vi. vii.

Cost is more Random display only draws lines and characters.

Raster Scan Display In raster display the beam is moved all over the screen one scan line at a time, from top to bottom and then back to top. In raster display, the refresh process is independent of the complexity of the image. Graphics primitives are specified in of their endpoints and must be scan converted into their corresponding pixels in the frame buffer. Because each primitive must be scan-converted, real time dynamics is far more computational and requires separate scan conversion hardware. Raster display can display mathematical smooth lines, polygons and boundaries curved primitives only by approximately them with pixels on the ratster grid. Cost is low. Raster display has ability to display a filled with solid colours of patterns.

1-9

Basic of Computer Graphics

Applications of Raster Scan Graphics i.

ii.

iii.

iv.

v.

vi.

Flight simulation: The use of the computer base flight simulators saves air line companies huge sums of money. Animation: The raster scan display can also be used for painting surface on which individual frames are prepared for animated sequences. Design: Realistic views of solid objects, machine part, glass bottles, buildings can be generated on raster scan display using store geometric description of the objects. Technical illustration: The surface of the raster scan display can be used as interactive drafting table for the preparation of technical illustration. Printing and plotting: Raster scan printers have been design for generating hard copy output of the text and images. Image processing: New techniques are constantly developed for dealing with sampled images of real world scenes.

Applications of raster scan graphics: i. ii. iii. iv. v. vi.

Flight simulation Animation Design Technical illustration Printing and plotting Image processing

Definition

1.4

FRAME BUFFER

A frame buffer is a video output device that drives a video display from a memory buffer containing a complete frame of data.

Frame Buffer “A frame buffer is a video output device that drives a video display from a memory buffer containing a computer frame of data”.

Or A buffer that stores the contents of an image pixel by pixel. Many different kinds of memory have been used in frame buffers: Drums, disks, integrated circuit, shift s and core stores. Frame buffers

Types of Frame Buffers i. ii. iii. iv.

Rotating memory frame buffer Shift frame buffer Random access frame buffer Multiple plane frame buffer

Rotating memory

Shift

Random access

Multiple plane

1-10 1.4.1

Computer Graphics

Rotating Memory Frame buffer

The earliest frame buffers made use of disks and drums for image storage. The rotational frequency of these devices can be made to coincide with the frequency generally chosen for TV refresh. It is therefore possible to read intensity values from drum or disk, convert them into analog voltage values, and thus construct a video signal. Each track of the memory device provides single stream of bits; therefore one bit of memory represent each pixel. For more intensity precision several tracks must be used in parallel.

Video signal

D/A converter CRT

Drum Figure 1.6: Rotating memory frame buffer

Above diagram shows the use of four tracks in parallel to provide frame buffer with 16 different intensity levels. Frame buffers of this type used to drive clusters of displays, in this way the relatively high cost of rotating memory could be spared among the number of terminals.

1.4.2

Shift Frame Buffer

Shift frame buffer: To generate a video signal from digital memory we must read the contents of the memory at constant high frequency. A fresh intensity value is required each time the CRT’s electron beam moves to a new pixel on a standard TV monitor this occurs every 90 nano seconds and on high resolution display time drops to 25 nano seconds or less. Integrated circuits are better ever to achieve high memory bandwidths. A shift is particularly appropriate circuit for use in a frame buffer every time a pulse is applied to the shift ,its content are shifted by one place ,displacing one bit from one end of and allowing one bit to be added at other end. As each bit emerges from shift it can be used as an intensity value and then insert into the other end of the to keep the content circulating. Several shift in parallel can be used where more than one bit of intensity per pixel is required. We construct the

Basic of Computer Graphics

1-11

frame buffer out of number of shift s, each one representing one column of pixel on TV screen. For example, if screen image has 256 scan lines each of 340 pixels, we use 340 shift s of 256 bits each .Each shift is shifted once per horizontal scan and contribute one bit to scan line. The s are shifted in carefully staggered sequence so that they produce bits of data at the exact movement they are needed to contribute to the video signal. The use of circulating memories, such as disks, drums, and shift s poses serious latency problem.

1.4.3

Random- Access Frame Buffer

The modern frame buffer uses random access integrated memory circuits. Each pixels intensity is represented by 1,2,4,8,or more bits of memory; one bit is sufficient for text and simple graphics and leads to relatively inexpensive display; two and four bits are useful in applications that require to display solid areas of gray or the color and eight or more bits are needed for quality shaded pictures. In a simplest method color components of each pixel are defined. The bits representing the pixel can be divided into three groups of bits, each indicating the intensity of one of the three primary color components. In an 8 bit byte, 3 bits are normally allocated to red, 3 to green and 2 to blue and these three components are then fed to three guns of the color TV monitor. From frame buffer

Red

Green Blue

8-bit pixel

Blue signal Green signal Red signal D/A converters Figure 1.7: Decoding color values stored in a frame buffer

1.4.4

Multiple Plane Frame Buffer

The provision of multiple bits per pixels is not only useful in representing intensity and the colour but allows the frame buffer to be treated as several planes, each containing separate image .Division into planes can be made in several different ways and 8 bits per pixel frame buffer can represent single image to the 8 bits of intensity precision ,two

1-12

Computer Graphics

images to 4 bit of precision and ,4 images to the 2 bits of precision , or 8 separate black and white images. By dividing the frame buffer into the plane we can apply the variety of different kinds of video mixing. One plane can be used to show a static picture and another to show a symbol or a picture part that wish to drag around the screen. In animation system, several moving objects can be displayed as separate planes. Multiple plane buffers should provide following capabilities: i.

Each plane should be provided with pair of s to indicate co-ordinate position of the plane relative to the screen co-ordinate system. It should be possible to define the plane of less than full screen size in order to economize on money use. A full range of logical functions should provide for concatenating the contents of the plane. So it is possible to inclusive-OR two planes together to combine their images.

ii. iii.

Strengths and Weaknesses of Frame Buffer i.

The frame buffer is one of the most versatile display device. English Word

intensity = Versatile =

iii. iv. v.

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ii.

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For the application that involves shading, solid areas of colour, high quality text, or any type of image processing the frame buffer offers satisfactory form of display.

It does not offer the compact way of representing an image. The large amount of memory it uses make it expensive. The time taken to fill the memory or change its content makes interactive response slow at time.

1.5

DISPLAY DEVICES

In computer system commonly used display device is video monitor, the operation of most video monitors based on it. The following display devices is used in computer graphics:

BTE [S.2010] - 4M List any four display device.

i. ii. iii. iv.

CRT Color CRT monitor DVST Flat display

Basic of Computer Graphics

1.5.1

1-13

Cathode Ray Tube (CRT)

A Cathode Ray Tube (CRT) is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface. Most desktop computer displays make use of CRTs. The CRT in a computer display is similar to the "picture tube" in a television receiver. All CRT's have three main elements: an electron gun, a deflection system, and a screen. The electron gun provides an electron beam, which is a highly concentrated stream of electrons. The deflection system positions the electron beam on the screen, and the screen displays a small spot of light at the point where the electron beam strikes it. •

Working of CRT



BTE [S.2011] - 4M Draw a labeled diagram of CRT.

The basic operation of CRT is shown in fig. below: Horizontal Focusing deflection system plate Healing filament Electron beam Cathode Control Accelerating grid system

Vertical deflection plate

Phosphor coated screen

Figure 1.8: Basic operation of CRT

•

Working Principle of CRT

The electrons are emitted by electron gun. These electrons through focusing and deflection system. The beam of electrons strike at specified points on the phosphor coated screen. When it falls on screen small spot of light is generated at each position where beams strikes. The light of phosphor fades after sometime. So electron beam is repeated quickly to redraw picture in the screen on the same point. The phosphor in screen is of different persistence. The term persistence is used to represent time interval during which phosphor emit after beam is removed. The phosphor may have low or high persistence. If persistence is low then it will be suitable for animation. If persistence is high it will be used for display highly complex pictures in CRT. We use cathode to heat the electrons. Cathode is heated by filament. The electrons are accelerated towards phosphor coating by high positive voltage.

1-14 Electron Focusing gun system

Computer Graphics

Interior metallic coating at high positive voltage

Heating filament

i.

Cathode Control grid Deflection coils

Screen Phosphor coating

List of main components of cathode tube i. ii. iii. iv.

Main Components Cathode Ray Tube

Electron gun Focusing system Deflection coils Phosphor coated screen

English Word

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Electron Gun: The primary components of an electron gun in a CRT are the heated metal cathode and a control grid. The cathode is heated by an electric current ed through a coil of wire called the filament. This causes electrons to be boiled off the hot cathode surface. In the vacuum inside the CRT envelope, negatively charged electrons are then accelerated towards the phosphor coating by a high positive voltage. The accelerating voltage can be generated with a positively charged metal coating on the inside of the CRT envelope near the phosphor screen, or an accelerating anode can be used. Sometimes the electron gun is built to contain the accelerating anode and focusing system within the same unit.

Control Grid is used to surround the cathode. Grid is cylindrical in shape. It is made up of metal. Grid has hole at one end, through which electrons get escaped. The control grid is kept at lower potential as compared to cathode, so that an electrostatic field can be created. It will direct that electrons through point source, so process of focusing will be simplified. The brightness of picture can be controlled by adjusting flow of electrons and current in beam. ii.

Focusing system: Focusing system is done by special focusing structure made up of two or more metal plates. The plates have different potential. An accelerating structure is also used in combination with focusing structure. Accelerating structure has two metal plates, the plates are perpendicular to each other beam can through these plates because holes are provided at center of plates. The plates are kept at high potential so that beam can be accelerated at desired velocity.

iii.

Deflection System: It is also called as yoke. It is located at neck of CRT tube. It is responsible for addressing of CRT. Deflection system consists of two pairs of coils. One coil controls horizontal deflection, whereas other controls vertical deflection. The deflection coils deflect the information rapidly so that it can be displayed clearly on screen without any flickering.

iv.

Phosphors: A variety of phosphor is available. Phosphor has different colors. Color generally used is white. White color is suitable for dark background. It should have small size. It should have high efficiency in of electric energy converted into light. It should have distance to burning under prolonged excitation.