Computer Graphics Unit 1 Notes

1. Introduction to Computer Graphics

A computer graphics system accepts data from the user, processes it and displays the result as an image. The basic purpose is to communicate information visually because diagrams and pictures are often easier to understand than plain text.

Basic Components of a Graphics System

• Input devices: keyboard, mouse, scanner, joystick, trackball and tablet.
• Processor: performs calculations and controls graphics operations.
• Frame buffer: stores pixel values of the image.
• Display device: presents the generated image.
• Output devices: printers and plotters generate hard copies.

2. Raster Scan Display

The screen is divided into a rectangular matrix of small picture elements called pixels. The colour or intensity value of every pixel is stored in a memory area known as the frame buffer or refresh buffer.

Working

1. The image is converted into a grid of pixels.
2. Pixel values are stored in the frame buffer.
3. The display controller reads the pixel values line by line.
4. The electron beam or display electronics illuminate corresponding points.
5. After reaching the bottom, scanning starts again from the top.

Advantages

• Suitable for realistic images, filled areas and colour pictures.
• Low-cost and widely used.
• Can display complex scenes.
• Compatible with television technology.

Disadvantages

• Requires large memory for high-resolution images.
• Lines may appear jagged because of discrete pixels.
• Needs continuous refreshing.
• Quality depends on resolution and colour depth.

3. Storage Tube Display

The most common example is the Direct View Storage Tube (DVST). It uses a storage mesh and a phosphor-coated screen. Once a picture is written, the stored charge pattern keeps the image visible.

Main Parts

• Primary electron gun
• Flood guns
• Storage mesh
• Collector mesh
• Phosphor-coated screen

Working

1. The primary electron gun writes the required pattern on the storage mesh.
2. The mesh stores an electrical charge pattern corresponding to the image.
3. Flood guns produce low-energy electrons continuously.
4. Stored charge permits electrons to pass only through selected areas.
5. These electrons strike the phosphor screen and create the visible image.

Advantages

• No frequent refresh is required.
• Stable, flicker-free image.
• High resolution for static line drawings.

Disadvantages

• Selective erasing is difficult.
• Animation and dynamic images are not suitable.
• Colour display capability is limited.
• Changing one part generally requires redrawing the entire screen.

Raster Scan vs Storage Tube Display

4. Refreshing

In traditional CRT displays, phosphor emits light only for a short time after being struck by the electron beam. Therefore, the entire image must be redrawn many times per second. The number of complete redraws per second is called the refresh rate.

A refresh rate of 60 Hz means the screen is refreshed 60 times every second. Higher refresh rates generally produce smoother visual output and reduce flicker.

5. Flickering

Causes

• Low refresh rate
• Slow phosphor persistence
• Large bright display area
• Unstable display signal

Effects

• Eye strain
• Headache
• Poor viewing comfort
• Reduced image stability

Methods to Reduce Flicker

• Use a higher refresh rate.
• Use interlaced scanning.
• Use phosphor with suitable persistence.
• Use modern LCD or LED display technology.

6. Interlacing

First, all odd lines are displayed. Then all even lines are displayed. The two fields combine to form one complete frame. This technique reduces the amount of data transmitted in one pass and improves the apparent refresh rate.

Advantages

• Reduces apparent flicker.
• Requires less bandwidth than progressive scanning.
• Historically useful in television systems.

Disadvantages

• Can produce line twitter and motion artifacts.
• Less suitable for modern high-quality digital displays.

7. Colour Monitors

A colour monitor produces images using combinations of primary colours. In most computer displays, the primary colours are Red, Green and Blue (RGB).

Beam Penetration Method

In this method, the screen is coated with multiple layers of phosphor, usually red and green. By controlling the speed of the electron beam, different layers are activated.

• Low-cost method
• Limited number of colours
• Mainly used in random scan systems

Shadow Mask Method

This method uses three electron guns for red, green and blue. A metal sheet called the shadow mask contains tiny holes that guide each beam to the correct phosphor dot.

Advantages of Shadow Mask

• Produces a large range of colours.
• Provides good image quality.
• Suitable for raster scan displays.

Disadvantages

• More complex construction.
• Lower brightness because some electrons are blocked by the mask.

8. Display Processors

Functions

• Converts graphics commands into pixel information.
• Performs line drawing, clipping, filling and transformations.
• Controls the frame buffer and display device.
• Improves speed of graphics rendering.
• Supports animation and interactive applications.

Modern graphics cards contain a GPU (Graphics Processing Unit), which is an advanced form of display processor capable of performing many operations in parallel.

9. Resolution

For example, a resolution of 1920 × 1080 means the screen contains 1920 pixels horizontally and 1080 pixels vertically. Higher resolution allows more detail to be displayed.

Types of Resolution

• Spatial resolution: total number of pixels on the screen.
• Colour resolution: number of colours that each pixel can represent.
• Printer resolution: measured in dots per inch (DPI).

Factors Affecting Image Quality

• Screen size
• Pixel density
• Colour depth
• Refresh rate
• Display technology

10. Printers

Printers produce a permanent hard copy of text and graphics. They are broadly classified as impact printers and non-impact printers.

10.1 Dot Matrix Printer

A dot matrix printer is an impact printer. Its print head contains a group of pins. The pins strike an inked ribbon against paper to form characters and graphics as patterns of dots.

Advantages

• Low printing cost
• Can print multipart forms
• Durable and suitable for continuous stationery

Disadvantages

• Noisy operation
• Low print quality
• Slow compared with modern printers

10.2 Inkjet Printer

An inkjet printer is a non-impact printer. It sprays microscopic droplets of ink through nozzles onto paper. Colour inkjet printers normally use cyan, magenta, yellow and black cartridges.

Advantages

• Good colour printing
• Quiet operation
• Affordable initial cost

Disadvantages

• Ink cartridges may be expensive.
• Ink can dry or clog the print head.
• Print speed is moderate.

10.3 Laser Printer

A laser printer is a non-impact page printer that uses a laser beam, photoconductive drum, toner and heat to produce high-quality output.

Working of Laser Printer

1. The drum receives a uniform electric charge.
2. A laser beam creates an electrostatic image on the drum.
3. Toner particles stick to the charged image.
4. The toner image is transferred to paper.
5. Heat and pressure permanently fuse toner to the paper.

11. Keyboard

A keyboard is a common input device used to enter text, numbers, commands and shortcuts. When a key is pressed, the keyboard controller identifies its scan code and sends it to the computer.

Types of Keys

• Alphanumeric keys
• Function keys
• Control keys
• Navigation keys
• Numeric keypad

In graphics applications, the keyboard is used for command entry, object parameters, coordinate values, menu shortcuts and text labels.

12. Mouse

Working

A mechanical mouse detects movement using a ball and rollers, while an optical mouse uses a light source and image sensor. The detected movement is converted into cursor movement.

Operations

• Pointing
• Clicking
• Double-clicking
• Dragging
• Scrolling

In computer graphics, a mouse is used to draw, select, move, resize and edit graphical objects.

13. Scanner

Working

1. A light source illuminates the document.
2. Reflected light is captured by sensors.
3. The sensor converts light intensity into electrical signals.
4. An analog-to-digital converter creates digital pixel values.
5. The digital image is stored in the computer.

Types

• Flatbed scanner
• Handheld scanner
• Sheet-fed scanner
• Drum scanner

14. Digitizing Camera

A digitizing camera captures a real-world scene and converts it into a digital image. It uses image sensors such as CCD or CMOS to measure light.

Steps

1. The lens focuses light onto the image sensor.
2. The sensor converts light into electrical signals.
3. The signals are sampled and quantized.
4. The final image is stored as digital data.

Applications

• Photography
• Video conferencing
• Medical imaging
• Surveillance
• Computer vision

15. Trackball

Unlike a mouse, the entire device does not move. Rotation of the exposed ball is detected and translated into cursor motion.

Advantages

• Requires less desk space.
• Provides accurate cursor control.
• Useful in industrial control and CAD systems.

16. Graphics Tablet

The tablet detects the position of the stylus and sends corresponding coordinates to the computer. Pressure-sensitive tablets can also measure pen pressure and tilt.

Applications

• CAD and engineering design
• Digital painting
• Signature capture
• Animation
• Map digitization

17. Joystick

Movement of the stick is converted into electrical signals. It may provide two-dimensional or three-dimensional control.

Applications

• Computer games
• Flight simulators
• Robotics
• Industrial machines
• Interactive graphics

18. Graphical Input Techniques

Graphical input techniques define how users interact with objects displayed on the screen. They make graphics software interactive and user-friendly.

18.1 Positioning Technique

Positioning is used to specify a coordinate or location on the screen. A mouse, tablet, light pen or cursor keys may be used to select the point.

18.2 Pointing and Selection

The user moves the cursor to a graphical object and selects it by clicking. Selection is commonly used for editing, deleting, moving and formatting objects.

18.3 Rubber-Band Technique

It is widely used to draw lines, rectangles, circles and selection boxes. The temporary shape behaves like a stretched rubber band.

18.4 Dragging Technique

Steps

1. Move the pointer over an object.
2. Press and hold the mouse button.
3. Move the mouse.
4. Release the button at the desired location.

18.5 Constraints

Constraints limit input movement. For example, holding a key may restrict a line to horizontal, vertical or 45-degree directions.

18.6 Grid and Snap

Grid provides reference points on the screen. Snap automatically places the cursor or object at the nearest grid point, improving alignment and accuracy.

Complete Unit Summary

• Raster scan systems display pixel-based images stored in a frame buffer.
• Storage tube displays retain static images without continuous refreshing.
• Refreshing redraws the screen; a low refresh rate may cause flickering.
• Interlacing divides a frame into odd and even fields.
• Colour monitors commonly use RGB and the shadow mask method.
• Display processors handle graphics operations and reduce CPU workload.
• Resolution represents the number of pixels or printable dots.
• Dot matrix, inkjet and laser printers produce hard copies using different methods.
• Keyboard, mouse, scanner, camera, trackball, tablet and joystick are important input devices.
• Positioning, selection, rubber-banding and dragging are common graphical interaction techniques.

Important RGPV Exam Questions

Long Answer Questions

1. Explain the working of raster scan display with a neat diagram. State its advantages and disadvantages.
2. Explain Direct View Storage Tube display and compare it with raster scan display.
3. What are refreshing, flickering and interlacing? Explain with suitable diagrams.
4. Explain beam penetration and shadow mask methods of colour display.
5. Explain the working principles of dot matrix, inkjet and laser printers.
6. Describe different graphical input devices used in computer graphics.
7. Explain positioning, rubber-band and dragging techniques with examples.
8. Explain the role and functions of a display processor.

Short Answer Questions

1. Define pixel and frame buffer.
2. What is refresh rate?
3. What is display resolution?
4. Write two advantages of a trackball.
5. What is a graphics tablet?
6. Differentiate between impact and non-impact printers.
7. What is rubber-banding?
8. What is dragging?

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