Graphics display devices

COMPUTER GRAPHICS
Display Devices

DISPLAY DEVICES
 A display device is a device for presentation of
information, such as image or a text, for visual
reception, acquired, stored, or transmitted in
various forms
 When the input information is supplied as an
electrical signal, the display is called electronic
display.
 Electronic displays are available for presentation of
visual information.

 A CRT is a specialized vacuum tube in which images are produced
when an electron beam strikes a phosphorescent surface.
 Most desktop computers make use of CRTs.
 The CRT in a computer display is similar to the picture tube in a
television receiver.
 A CRT consists of several basic components as illustrated in fig.
 Heater element and cathode:
 Heat is supplied to the cathode by passing current through heater
element. The Cathode is cylindrical metallic structure which is rich in
electrons. On heating the electrons are released from cathode surface.
 Control Grid:
 The control grid is the next element which follows cathode. It almost
covers cathode leaving small opening for electrons to come out. Intensity
of the electron beam is controlled by setting voltage levels on the control
grid. A high negative voltage applied to the control grid will shut off the
beam by repelling electrons and stopping them from passing through the
small hole at the end of control grid structure. A smaller negative voltage
on the control grid will simply decrease the number of electrons passing
through the cathode. Thus, we can control the brightness of a display by
varying the voltage on the control grid.

 Accelerating Anode:
 Positively charged anodes, in the sequence of accelerating anodes,
accelerate the electrons towards phosphor screen.
 Focusing And Deflection Coils:
 Focusing and Deflection Coils are together needed to force the electron
beam to converge into a small spot, as it strikes the screen, otherwise
the electrons would repel each other and the beam would spread out as
it approaches the screen.
 Deflection coils produce an extremely low frequency electromagnetic
field that allows for constant adjustment of the direction of the electron
beam.
 There are two sets of deflecting coils: horizontal and vertical.
 Electrostatic focusing is commonly used in television and computer
graphics monitor.
 Phosphor Coating:
 Finally, when the accelerating electron beam collides on the phosphor
coating, a part of Kinetic Energy is converted into light and heat.
 When the electrons in the beam collide with the phosphor coating they
are stopped and their kinetic energy is absorbed by the phosphor,
resulting in the screen display.

DRAWBACKS
 CRTS produce crisp, vibrant images. But they do
have serious drawbacks:
 They are bulky.
 In order to increase the screen width in a CRT set,
you also have to increase the length of the tube (to
give the scanning electron gun room to reach all
parts of the screen).
 It has been claimed that the electromagnetic fields
emitted by CRT monitors constitute a health hazard
and can affect the functioning of living cells, and
people are moving towards other forms of digital
displays replacing them slowly.

RASTER SCAN DISPLAY
 A raster-scan display is the most common method of drawing images on a
CRT screen.
 In this method, horizontal and vertical deflection signals are generated to
move a beam all over the screen in a pattern for displaying any image.
 The electron beam is swept across the screen one row at a time from top to
bottom.
 The electron beam sweeps back and forth from left to right across the
screen.
 The beam is on, while it moves from left to right.
 The beam is off, when it moves back from right to left.
 This phenomenon is called the horizontal retrace, as shown by red lines in
the figure.
 As soon as the beam reaches the bottom of the screen, it is turned off and is
rapidly retraced back to the top to start again.
 This is called the vertical retrace.
 Raster-scan displays maintain the steady image on the screen by repeating
scanning of the same image.
 This process is known as refreshing of screen.

 Typically, a graphics display consists of three
components: frame buffer, display controller, and a TV
screen or monitor.

 Frame Buffer:
 The frame buffer stores an image as a matrix of
intensity values.
 It is an interface between what are usually relatively
slow graphics computation and the high data rate video
image display.
 In a typical personal computer, the frame buffer is
located on the graphics card that manages the video
subsystem of the computer.
 It basically used to be not much more than some extra
memory.
 Stored intensity values are then retrieved from the
refresh buffer and displayed on the screen one row at a
time.
 Each intensity value is represented by bit zero (0) or one
(1) in the frame buffer.

 Video or Display controller:
 The video or display controller has direct access to memory locations in the frame
buffer.
 It is responsible for retrieving data from the frame buffer and passing it to the
display device.
 It reads each successive bytes of data from frame buffer and converts this 0's and
1's in one line into a corresponding video signals, and this line is called a scan line.
 If the intensity is one (1) then the controller sends a signal to display a dot in the
corresponding position on the screen.
 If the intensity is zero (0) then no dot is displayed.
 This is the simplest way in which a black and white image is displayed.
 The display-controller repeats the read, convert, and fill operations at least 60
times per seconds that maintains a study picture on the screen (refresh rate).
 The frame buffer maps the screen into Cartesian coordinates.
 Generally, the screen coordinates are taken as a positive xy-plane.
 Hence the screen is continuously refreshed by scanning from maximum value of
ycoordinate down to y = 0.

 Intensity range for pixel positions depends on the capability of the
raster system.
 In a simple black-and-white system, each screen point is either on
or off, so only one bit per pixel is needed to control the intensity of
screen positions.
 For a bit level system, A bit value of 1 indicates that the electron
beam is to be turn ON at that position. A bit value of 0 indicates
that the beam intensity is to be turn OFF. Additional bits are
needed when color and intensity variations can be displayed.
 Up to 24 bits per pixel are included in high-quality systems, which
can require several megabytes of storage for the frame buffer,
depending on the resolution of the system.
 A system with 24 bits per pixel and a screen resolution of 1024 bv
1024 requires 3 Mega Bytes of storage for the frame buffer.
 Bitmap: On a black-and-white system with one bit per pixel, the
frame buffer is commonly called a Bitmap.
 Pixmap: Systems with multiple bits per pixel, the frame buffer are
often referred to as a Pixmap.

RANDOM SCAN DISPLAY
 A CRT, as a random-scan display unit, has an electron beam directed
only to the parts of the screen where a picture is to be drawn.
 Random-scan monitors draw a picture one line at a time.
 These are also referred to as vector displays (or stroke-writing or
calligraphic displays).
 The component of a picture (lines and curves) can be drawn and
refreshed by a random-scan system in any specified order.
 A pen plotter operates in a similar way and is an example of a random
scan, hardcopy device.
 The refresh rate, on a random-scan system, depends on the number of
primitives like lines to be displayed.
 A picture definition is stored as a set of line-drawing commands in an
area of memory called a refresh display file (or a refresh buffer).
 To display a specified picture, the system cycles through the set of
commands in the display file, drawing each component line one by one.
 After all line-drawing commands have been processed, the system
cycles back to the first line command in the list and repeats the
procedure of scan, display, and retrace.

 Random-scan displays are designed to draw all the component
lines of a picture 30 to 60 times each second.
 High -quality vector systems are capable of handling approximately
100,000 short lines at this refresh rate.
 It is important to note that the faster refreshing of the set of lines
could burn out the phosphor.
 Therefore, when a small set of lines are to be displayed, each
refresh cycle is delayed to avoid greater refresh rates, typically 60
frames per second.
 Random-scan systems are designed for line-drawing applications;
hence cannot display realistic shaded scenes.
 Vector displays generally have a higher resolution than raster
systems, as picture definition is stored as a set of line-drawing
instructions instead of a set of intensity values for all screen points.
 These vector displays produce smooth line drawings, because the
CRT beam directly follows the line path.
 A raster system, in contrast, produces jagged lines that are plotted
as discrete point sets.

COLOR CRT MONITORS
 A CRT monitor displays color pictures by using a
combination of phosphors that emit different-
colored light. By combining the emitted light from
the different phosphors, a range of colors can be
generated. The two basic techniques for
producing color displays with a CRT are
 1. The Beam-Penetration method.
 2. The Shadow-Mask method.

BEAM-PENETRATION METHOD
 The beam-penetration method for displaying color pictures has
been used with random-scan monitors.
 Two layers of phosphor, usually RED and GREEN, are coated
onto the inside of the CRT screen, and the displayed color
depends on how far the electron beam penetrates into the
phosphor layers.
 A beam of slow electrons excites only the outer RED layer.
 A beam of very fast electrons penetrates through the RED layer
and excites the inner GREEN layer. At intermediate beam
speeds, combinations of red and green light are emitted to show
two additional colors, ORANGE and YELLOW.
 The speed of the electrons, and hence the screen color at any
point, is controlled by the beam-acceleration voltage.
 Advantage: Beam penetration has been an inexpensive way to
produce color in random-scan monitors,
 Disadvantage: only four colors are possible, and the quality of
pictures is not as good as with other methods.

SHADOW-MASK METHODS
 Shadow-mask methods are commonly used in raster scan systems (including color
TV) because they produce a much wider range of colors than the beam penetration
method.
 A shadow-mask CRT has three phosphor color dots at each pixel position.
 One phosphor dot emits a RED Light, another emits a GREEN light, and the third
emits a BLUE light.
 This type of CRT has three electron guns, one for each color dot, and a shadow-
mask grid just behind the phosphor-coated screen.
 1. Delta-Delta Shadow-Mask method, commonly used in color CRT systems. The
three 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. When the
three beams pass through a hole in the shadow mask, they activate a Dot Triangle,
which appears as a small color spot on the screen.
 The phosphor dots in the triangles are arranged so that each electron beam can
activate only its corresponding color dot when it passes through the shadow mask.
 2.In-Line arrangement in which the three electron guns, and the corresponding red-
green-blue color dots on the screen, are aligned along one scan line instead of in a
triangular pattern. This in-line arrangement of electron guns is easier to keep in
alignment and is commonly used I n high-resolution color CRTs.

 The color variations in a shadow-mask CRT by varying the
intensity levels of the three electron beams.
 By turning OFF the RED and GREEN guns, gets the color
coming from the BLUE phosphor.
 Other combinations of beam intensities produce a small light spot
for each pixel position, since our eyes tend to merge the three
colors into one composite.
 The color depends on the amount of excitation of the red, green,
and blue phosphors.
 White (or gray) area is the result of activating all three dots with
equal intensity.
 Yellow is produced with the green and red dots only.
 Magenta is produced with the blue and red dots.
 Cyan shows up when blue and green are activated equally.
 In some low-cost systems, the electron beam can only be set to
on or off, limiting displays to Eight Colors.

RED GREEN BLUE COLOR
0 0 0 Black
0 0 1 Blue
0 1 0 Green
0 1 1 Cyan
1 0 0 Red
1 0 1 Magenta
1 1 0 Yellow
1 1 1 White

FLAT PANEL DISPLAY
 Although most graphics monitors are still constructed with CRTs, other
technologies are emerging that may soon replace CRT monitors. The
term flat-panel display refers to a class of video devices that have
 1. Reduced volume
 2. Weight
 3. Power requirements compared to a CRT.
 The flat panel display (FPD) technology is becoming increasingly
common in a wide variety of consumer devices that include cellular
phones, digital cameras, liquid crystal display (LCD) televisions,
computer displays, and personal digital assistants (PDAs).
 These FPDs are lighter and much thinner than traditional television and
video displays that use a CRT.
 Current uses for flat-panel displays include small TV monitors,
calculators, pocket video games, laptop computers, armrest viewing of
movies on airlines, as advertisement boards in elevators, and as
graphics displays in applications requiring rugged, portable monitors
and pocket notepads.

Graphics display devices

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