Unit 1

Computer GraphicsComputer Graphics
Module-IModule-I
Graphics SystemsGraphics Systems
Ankit GargAnkit Garg

Objective
 Video Display Device
 CRT
 Types of Display System
 Color Generation Technique
 Input/Output Device
 Numerical based on display System
 Application of Computer Graphics
2

3
Video Display DevicesVideo Display Devices
 The primary output device in a graphical
system is the video monitor.

4
Cathode-ray tubes (CRT)Cathode-ray tubes (CRT)
Base
Connector
Pins
Y deflect
x deflect
Phosphor
Electron
Gun
Focusing
System
Control
grid
voltage

5
1. The electron gun emits a beam of electrons
(cathode rays).
2. The electron beam passes through focusingfocusing and
deflectiondeflection systems that direct it towards
specified positions on the phosphor-coated
screen.
3. When the beam hits the screen, the phosphor
emits a small spot of light at each position
contacted by the electron beam.
Because the light emitted by the phosphor fades very
quickly some method is needed for maintaining the
screen picture.
1. Redraw the picture by quickly directing the
electron beam back over the same screen points.
Basic operations of a CRTBasic operations of a CRT
Steps

6
Phosphor PersistencePhosphor Persistence
Definition:Definition: The time from the initial light output to
the moment when has decayed to its 10%.
There are different kind of phosphors for use in a
CRT. Besides color, a major difference is their
persistence – how long they continue to emit light
after the CRT beam is removed.
A phosphor with low-persistence is useful in
animation.
A high-persistence phosphor is useful for displaying
highly complex, static pictures.
Graphics monitors are usually constructed with a
persistence in the range from 10 to 60 microseconds.

7
Developed in the early seventies.
It is today's dominant hardware technology.
Almost all graphics systems are raster-based.
A picture is produced as an array – the rasterraster – of
picture elements.
This elements are called PixelsPixels or Pels (Picture
Elements). A pixel corresponds to a location, or
small area, in the image.
Collectively, the pixels are stored in a part of
memory called the refresh bufferrefresh buffer or frame bufferframe buffer.
Raster Scan DisplaysRaster Scan Displays

8
ResolutionResolution
The maximum number of points (pixels) that can be
displayed without overlap on a screen is referred to as the
resolution, and determines the detail that can be seen in an
image.
A more precise definition is the number of points per
centimeter that can be plotted horizontally and vertically,
although it is often simply stated as the total number of
points in each direction (i.e. 1280 × 1024).
The physical size of a graphics monitor, on the other hand,
is given as the length (in inches) of the screen diagonal.

9
 The aspect ratio gives the ratio of vertical points
to horizontal points necessary to produce equal-
length lines in both directions on the screen. So
4:3 (most common) means that a vertical line
plotted with 4 points has the same length as a
horizontal line plotted with 3 points.

10
The Frame BufferThe Frame Buffer
x
y
0
0 800
600
Display surfaceFrame buffer
pixel at address (x,y) spot at (x,y)
at (800,600)

11
Frame
Buffers
 A frame buffer may be thought of as computer memory organized
as a two-dimensional array with each (x,y) addressable location
corresponding to one pixel.
 Bit Planes or Bit Depth is the number of bits corresponding to each
pixel.
 A typical frame buffer resolution might be
– 640 x 480 x 8
– 1280 x 1024 x 8
– 1280 x 1024 x 24

12
Monochrome Display
(Bit-map Display)
Electron
Gun
1 bit
2 levels

13
3-Bit Color Display
3
red
green
blue
COLOR: black red green blue yellow cyan magenta white
R
G
B
0
0
0
1
0
0
0
1
0
0
0
1
1
1
0
0
1
1
1
0
1
1
1
1

14
True Color Display
24 bit planes, 8 bits per color gun.
224
= 16,777,216
Green
Red
Blue
N
N
N

15
Refresh RateRefresh Rate
 Definition:Definition: The number of times per second the image is
redrawn.
 The entire contents of the frame buffer are displayed on
the CRT at a rate high enough to avoid flicker. This rate
is called the refresh raterefresh rate.
 For a human to see a steady image on most CRT displays,
the same path must be retraced, or refreshed, by the beam
at least 60 times per second.
 Current raster-scan displays perform refreshing at the rate
of 60 to 80 frames per second, although some systems
now have refresh rates of up to 120 frames per second.
 Refresh rates are described in units of cycles per second,
or Hertz (Hz), where a cycle corresponds to one frame
(i.e. a refresh rate of 60 frames per second = 60 Hz).

16
Refresh-rate for films and TVRefresh-rate for films and TV
 On films, below 24 frames per second, we can perceive a
gap between successive screen images.
– Old silent films show flicker because they where photographed at
a rate of 16 frames per second.
– When sound systems were developed in the 1920s, motion
picture film rates increased to 24 frames per second removing
flickering.
– Today TV refresh rate is 25 frames per second in Europe and 30
frames per second in the USA.

17
The depth (or intensity) of the frame
buffer, defined as the number of bits that
are used for each pixel, determines
properties such as how many colors can be
represented on a given system.
1-bit-deep frame buffer allows 21
colors (black
and white)
8-bit-deep frame buffer allows 28
(=256) colors
In full color systems (also called RGB-color
systems), there are 24 (or more) bits per pixel in
order to display sufficient colors to represent
most images realistically.

Raster Scan Displays
– Raster: A rectangular array of points or dots
– Pixel: One dot or picture element of the raster.
Its intensity range for pixels depends on
capability of the system
– Scan line: A row of pixels
– Picture elements are stored in a memory called
frame buffer
18

19
Electron beam “paints” the picture on screen one
line at a time.
Vertical
retrace
Horizontal
retrace
Scan line
000000000000000000000
000000000111000000000
000000111111111000000
000111111111111111000
000111110000011111000
000111111111111111000
000111111000111111000
000111111000111111000
000111111000111111000
000111111000111111000
000111111111111111000
000000000000000000000

20
They are based on TV technology
Refresh rate = 60 to 80 frames per second.
Note: Below 24 frames/second, eye detects flicker.
Each screen point is visited every refresh cycle.
Their capability to store intensity information for
each screen point makes them well suited for the
realistic display of scenes containing shading and
color patterns.
The frame-buffer with 1-bit intensity is called a
bitmap.
The frame-buffer with multiple-bits intensity is called
a pixmap.

21
Random-Scan (Vector) displaysRandom-Scan (Vector) displays
Vector stands for line.
Developed in the mid-sixties and in common use until the
mid-eighties.
The electron beam is directed only to parts of the screen
where the picture is to be drawn.
MoveTo (300,800)
LineTo (700,800)
LineTo (500,300)
LineTo (300,800)

22
Random Scan DisplaysRandom Scan Displays
Picture is stored as a set of point- and line-drawing
commands with (x,y) or (x,y,z) endpoint coordinates, as
well as character-plotting commands.
Refresh rate depends on the number of lines to be
displayed. To avoid flicker it must be at least 30 times per
second (30 Hz).
They are designed to draw all the component lines of a
picture 30 to 60 times per second – more than 60 could
burn the phosphor.
High quality vector systems are capable of handling
approximately 100,000 lines at this refresh rate.
They are designed for line drawing applications and cannot
display realistic shaded images.

23
Raster Scan Vs Random ScanRaster Scan Vs Random Scan
Random scanRaster scan

2 Basic Techniques for Color Display
Beam-Penetration
Method
Shadow-Mask
Method

25
Beam-Penetration Method
 Used with random scan monitors
 The screen has two layers of phosphor: usually red and green
 The displayed color depends on how far the electron beam penetrates
through the two layers.
 A beam of slow electrons excites only the outer of the red layer, a beam
of fast electrons penetrates through the red layer and excites the inner
green layer, and at intermediate beam speeds, combinations of the two
colors are emitted to show other colors (yellow & orange).

26
Shadow-Mask Method
Color CRTs have
– Three electron guns
– A metal shadow mask to differentiate the
beams

 The Shadow mask in the previous image is known as the delta-delta
shadow-mask.
 The 3 electron beams are deflected and focused as a group onto the
shadow mask, which contains a series of holes aligned with the
phosphor-dot patterns.
 The 3 beams pass through a hole in the shadow mask and activate a dot
triangle, which appears as a small color spot on the screen.

Beam Penetration VS Shadow
mask
28

Controlling Colors in shadow mask
 Different colors can be obtained by varying the intensity levels of the
three electron beams.
 Example: Simply turning off the red and green guns, we get only the
color coming from the blue phosphor.
 Yellow = Green + Red
 Magenta = Blue + Red
 Cyan = Blue + Green
 White is produced when all the 3 guns possess equal amount of
intensity.

30
Display Processor
 Also called either a Graphics Controller or Display Co-Processor.
 Specialized hardware to assist in scan converting output
primitives into the frame buffer.
 The display processor is used to convert digital information from
the CPU into analog value needed by the display device.
 A major task of display processor is to perform a process called
scan conversion. It is the process of separating contiguous
graphics objects as a collection of ellipse, rectangles and
polygons.

31
Video
ControllerCycles through the frame buffer, one scan line at a time. Contents of the
memory are used the control the CRT's beam intensity or color.
X address
Y address
Pixel
value(s)
Raster scan
generator
Data
Horizontal
and vertical
deflection
signals
Intensity
or color
Linear
address
Set or increment
Set or decrement
M
e
m
o
r
y

32
Input DevicesInput Devices
 Keyboard
– Entering non-graphical data (i.e. text)
Each device is more suitable for certain tasks than for others

33
 Optical Detector Mouse
• Measure distance traveled by counting lines on a
special pad.
 Mouse
• Used to select a location to the screen.
• The motion of the roller at the bottom of the mouse is
converted into signals sent back to the computer.

34
 Trackball
• Similar in use to the mouse.
• Popular with portable computers because they
can be incorporated directly into the keyboard.
• There exist various pressure-sensitive devices
in keyboards that perform similar functions to
the mouse and trackball.

35
 Data Tablet
– It has rows and columns of wires embedded
under its surface. The position of the stylus is
determined through electromagnetic
interactions between signals traveling through
the wires and sensors in the stylus.
 Touch-Sensitive Screens
– Have many of the same properties as Data
Tablet.

36
 Light pen
– If it is positioned on the face of the CRT at a
location, a signal is sent to the computer.
– Not very popular as other devices.

37
 Joystick
– The motion of the screen in two orthogonal
directions is encoded and integrated to identify
a screen location.
– Variable sensitive device – well suited for
flight simulators and games.

38
 Spaceball
– 3D input device
– Stick does not move but it has pressure
sensors (rotations).
– 3 independent twists (translations).

39
 Z-mouse
– 3D input device
– It has three buttons, a thumbwheel on the
sided, a trackball on the top, and a standard
mouse ball underneath.

40
 Data Glove
– Used to grasp a “virtual” object.
– Sensors detect hand and finger motions.

41
 Digitizers
– 2D or 3D input devices.
– Interactive selection of coordinate positions on
an object.
– Wireframe models – rectangular grid

Unit 1

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Unit 1