Unit 1: Introduction to computer graphics
Computer Graphics notes · PTU syllabus (BSIT603/BSBC602)
On this page
- Unit summary
- Active and passive graphics
- Applications of computer graphics
- Input devices
- Video display devices: the CRT
- Raster scan and random scan displays
- Colour CRT monitors
- Direct view storage tube (DVST)
- Flat-panel displays
- Colour models: RGB and CMY
- Lookup tables
- Key terms
- Quick revision
- Important questions
Unit summary
Computer graphics creates and manipulates pictures with a computer — from charts and CAD drawings to games and films. This unit covers active and passive graphics, applications, input devices, video display devices — CRT, raster and random scan, colour CRT, DVST and flat-panel displays — the RGB and CMY colour models, and lookup tables.
After this unit you can
- Distinguish active and passive graphics and list applications
- Describe graphics input devices
- Explain the working of display devices
- Explain colour models and lookup tables
PTU syllabus topics
- Active and passive graphics
- applications
- input devices (light pens, graphic tablets, joysticks, trackball, digitizers, scanners)
- video display devices (CRT, raster/random scan, color CRT, DVST, flat-panel displays)
- color models (RGB, CMY)
- lookup tables
Draws by
Scanning every pixel row by row
Drawing lines directly
Picture stored as
Frame buffer of pixels
Display list of commands
Best for
Realistic images, shading
Line drawings
Example
Modern monitors
Early CAD displays
Topic 1
Active and passive graphics
User control
None — the user only watches
User changes the picture through input devices
Communication
One-way
Two-way between user and computer
Examples
TV broadcast, printed charts, screen savers
Video games, CAD, flight simulators, drawing apps
Topic 2
Applications of computer graphics
Computer-aided design
Buildings, cars, circuits
Presentation graphics
Charts and graphs for reports
Entertainment
Films, animation, video games
Education and training
Simulators, scientific visualisation
Image processing
Medical scans, satellite images
Graphical user interfaces
Windows, icons, menus
Virtual and augmented reality
Immersive environments
Topic 3
Input devices
- Light pen
- Pen-shaped device detecting light from a CRT screen to select or draw
- Graphics tablet (digitiser)
- Flat surface and stylus giving precise coordinates — design and signatures
- Joystick
- Lever controlling direction and speed — games, simulators
- Trackball
- Ball rotated by hand to move the cursor — space-saving
- Digitiser
- Converts drawings or maps into digital coordinates
- Scanner
- Converts printed images and text into digital images (with OCR for text)
- Others
- Mouse, touch screen, data glove, voice input
Topic 4
Video display devices: the CRT
- 1
Heated cathode (electron gun) emits electrons
- 2
Control grid sets beam intensity
- 3
Focusing system narrows the beam
- 4
Deflection plates or coils steer the beam
- 5
Beam strikes phosphor-coated screen
- 6
Phosphor glows to form a spot (pixel)
- Persistence: how long phosphor glows after the beam moves; refresh rate: how often the picture is redrawn (60 Hz or more avoids flicker); resolution: maximum number of points displayed without overlap.
Topic 5
Raster scan and random scan displays
Drawing
Beam sweeps every row from top to bottom
Beam goes only where lines are drawn
Picture stored as
Intensity of every pixel in a frame buffer
Line-drawing commands in a display file
Resolution
Lower; jagged lines (aliasing)
High; smooth lines
Realism
Shaded, filled, realistic scenes
Line drawings only
Cost
Cheaper
Costlier
Example
TVs, monitors
Early CAD systems, plotters
Memory
Horizontal pixels × vertical pixels × bits per pixel ÷ 8 bytes
Example
1024 × 768 × 24 ÷ 8 = 2,359,296 bytes ≈ 2.25 MB
- Interlacing: odd lines drawn in one pass and even lines in the next, reducing flicker at low refresh rates.
Topic 6
Colour CRT monitors
Beam penetration
Two phosphor layers (red and green); beam speed decides penetration and colour
Random scan; only four colours
Shadow mask
Three electron guns (R, G, B) and a metal mask with holes; dot triads glow
Raster displays; millions of colours
- Delta–delta and in-line arrangements of the three guns are used in shadow-mask tubes.
Topic 7
Direct view storage tube (DVST)
- A DVST stores the picture as a charge distribution on a storage grid behind the screen, so it needs no refresh.
- Advantages: no refreshing, flicker-free, complex pictures at high resolution. Disadvantages: no colour, selective erasing impossible — the whole screen must be erased and redrawn; slow erasure.
Topic 8
Flat-panel displays
Plasma panel
Emissive
Gas between glass plates glows when cells are energised
LED and OLED
Emissive
Diodes (or organic layers) emit light at each pixel; OLED needs no backlight
LCD
Non-emissive
Liquid crystals twist polarised light from a backlight; active-matrix (TFT) has a transistor per pixel
- Flat panels are thinner, lighter and use less power than CRTs, and have replaced them in almost all uses.
Topic 9
Colour models: RGB and CMY
Type
Additive — light is added
Subtractive — ink absorbs light
Primaries
Red, green, blue
Cyan, magenta, yellow
Black and white
Black (0,0,0); white (1,1,1)
White (0,0,0); black (1,1,1)
Used in
Monitors, cameras, projectors
Printers (CMYK adds black ink)
RGB to CMY
C = 1 − R, M = 1 − G, Y = 1 − B
Example
Orange RGB (1, 0.5, 0) → CMY (0, 0.5, 1)
Topic 10
Lookup tables
- A colour lookup table (palette) stores a limited set of colours; the frame buffer holds an index into the table instead of the full colour value for each pixel.
Example
With 8 bits per pixel, each pixel holds an index from 0 to 255 into a table of 256 entries, each a 24-bit RGB colour — so 256 colours chosen from 16.7 million are displayed using one third of the memory of full 24-bit colour.
- Advantages: saves frame-buffer memory; colours can be changed instantly by editing the table (palette animation).
Key terms
- Pixel
- Smallest addressable point on a raster display
- Frame buffer
- Memory holding the intensity of every pixel
- Refresh rate
- Number of times per second the picture is redrawn
- Shadow mask
- Metal plate guiding three beams to colour dot triads
- Lookup table
- Palette of colours indexed by pixel values
Quick revision
- Active vs passive graphics; applications.
- Light pen, tablet, joystick, trackball, digitiser, scanner.
- CRT working; persistence, refresh, resolution.
- Raster vs random scan; frame buffer; beam penetration vs shadow mask; DVST; plasma, LED, LCD.
- RGB additive, CMY subtractive; conversion; lookup tables.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Distinguish active and passive graphics.
- Q2.What is a light pen?
- Q3.Distinguish raster and random scan displays.
- Q4.Calculate the frame buffer size for 800 × 600 with 8 bits per pixel.
- Q5.State one advantage and one drawback of DVST.
- Q6.Convert RGB (0, 1, 1) to CMY.
Long-answer questions
- Q1.Explain the applications of computer graphics and input devices.
- Q2.Explain raster and random scan display systems.
- Q3.Explain colour CRT monitors, DVST and flat-panel displays.
- Q4.Explain the RGB and CMY colour models and lookup tables.
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