Evolution of Computer Graphics,

8/4/2019 Evolution of Computer Graphics,

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Evolution of computer Graphics,

Graphics Systems: Video Display Unit


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Video Display Units

Display Unit:

CRT

LCD

Plasma

Raster Scan Display:

Random Scan Display:

Color CRT:

Beam Penetration

Shadow Masking Technique


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Basic design of CRT.


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Working

A beam of electrons (cathode rays), emitted

by an electron gun,

passes through focusing and deflection

systems that direct the beam toward specified

positions on the phosphor coated screen.

The phosphor then emits a small spot of light

at each position contacted by the electron

beam and the light is emitted by the phosphor


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Operation of an electron gun with an

accelerating anode.


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Explanation

Heat is supplied to the cathode by directing a

current through a coil of wire, called the

filament, inside the cylindrical cathode

structure.

In the vacuum inside the CRT envelope, the

free, negatively charged electrons are then

accelerated toward the phosphor coating by ahigh positive voltage.


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Working

Intensity of the electron beam is controlled by setting voltage levelson the control grid, which is a metal cylinder that fits over thecathode.

1) A high negative voltage applied to the control grid will shut offthe beam by repelling electrons and stopping them from passing

through the small hole at the end of the control grid structure.2) A smaller negative voltage on the control grid simply

decreases the number of electrons passing through.

Since the amount of light emitted by the phosphor coating

depends on the number of electrons striking the screen, we controlthe brightness of a display by varying the voltage on the controlgrid. We specify the intensity level for individual screen positionswith graphics software commands


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The focusing system in a CRT is needed to

force the electron beam to converge into a

small spot as it strikes the phosphor.

Otherwise, the electrons would repel each

other, and the beam would spread out as it

approaches the screen


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Persistence

How long small spots continue to emit light

after the beam is moved. How long it takes to

the emitted light from the screen to decay to

one-tenth of its original intensity.

Lower persistence requires high refresh rate & it is

good for animation

High persistence is useful for displaying highlycomplex static picture.


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Resolution

Resolution is the number of pointes per inch or centimeterthat can be plotted horizontally & vertically.

The smaller the spot size, the higher the resolution.

The higher the resolution, the better is the graphics system

High quality resolution is 1280x1024

Intensity distribution


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Aspect Ratio

Another property of video monitors is aspectratio.

This number gives the ratio of vertical points to

horizontal points necessary to produce equal-length lines in both directions on the screen.

(Sometimes aspect ratio is stated in terms of theratio of horizontal to vertical points.)

An aspect ratio of 3/4 means that a vertical lineplotted with three points has the same length asa horizontal line plotted with four points.


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Addressability

Addressability is a measure of the spacing

between the centers of vertical and horizontal

lines.

The picture on a screen consists of intensified

points.

The smallest addressable point on the screen is

called pixel or picture element

In graphics mode there are 800x600


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We can see that the image consists of elements.

Such an image is called RASTER IMAGE or BITMAP.

When we zoom the image, its structure starts to appear.

Each element has its own colour


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However, some images do not seem to

consist of these elementsas they can

be zoomed smoothly.

Such image is called VECTOR IMAGE.


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The differences

Vector image

When zoomed its structure

continues to be smooth

Used for simple graphics

and drawings

Typical formats: EPS, AI,

CDR, WMF, DXF,

A special case: SVG

Raster image

When zoomed, its structureshows colouredelements

Used for photorealisticimages

Typical formats: JPEG,TIFF, GIF, PNG,


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Raster-Scan Displays1) The most common type of graphics monitor employing a

CRT is the raster-scan display, based on television

technology.2) In a raster-scan system, the electron beam is swept across

the screen, one row at a time from top to bottom. As theelectron beam moves across each row, the beam intensityis turned on and off to create a pattern of illuminated

spots.3) Picture definition is stored in a memory area called the

refresh buffer or frame buffer.

4) This memory area holds the set of intensity values for allthe screen points. Stored intensity values are then

retrieved from the refresh buffer and "painted" on thescreen one row (scan line) at a time .

5) Each screen point is referred to as a pixel or pel(shortened forms of picture element).


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Raster Scan Displays


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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 ofscreen positions.

A system with 24 bits per pixel and a screen

resolution of 1024 by 1024 requires 3megabytes of storage for the frame buffer.

On a black-and-white system with one bit per

pixeI, the frame buffer is commonly called abitmap.

For systems with multiple bits per pixel, the

frame buffer is referred to as a pixmap.


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Refreshing on raster-scan displays is carried out at the rateof 60 to 80 frames per second,

Using these units, we would describe a refresh rate of60frames per second as simply 60 Hz. At the end of each scanline, the electron beam returns to the left side of thescreen to begin displaving the next scan line. The return tothe left of the screen, after refreshing each scan line, iscalled the horizontal retrace of the electron beam.

And at the end of each frame (displayed in 1/80th to1/60th of a second), the electron beam returns (verticalretrace) to the top left comer of the screen to begin thenext frame.


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On some raster-scan systems (and in TV sets), eachframe is displayed in two passes using an interlacedrefresh procedure. In the first pass, the beam sweepsacross every other scan line from top to bottom.

Then after the vertical retrace, the beam sweeps outthe remaining scan lines . Interlacing of the scan linesin this way allows us to see the entire screen displayedin one-half the time it would have taken to sweep

across all the lines at once from top to bottom.Interlacing is primarily used with slower refreshingrates


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Random Scan Displays

When operated as a random-scan display unit, aCRT has the electron beam directed only to theparts of the screen where a picture is to bedrawn.

Random scan monitors draw a picture one line ata time and for this reason are also referred to asvector displays (or stroke-writing or calligraphicdisplays)

A pen plotter operates in a similar way and is anexample of a random-scan, hard-copy device


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Pi t d fi iti i t d t f li


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Picture definition is now stored as a set of line

drawing commands in an area of memory referred

to as the refresh display file. Also called the display

list, display program, or simply the refresh buffer.

Random-scan systems are designed for line drawing

applications and cannot display realistic shaded

scenes.

Since picture definition is stored as a set of Line

drawing instructions and not as a set of intensity

values for all screen points, vector displays generallyhave higher resolution than raster systems


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CRT monitor

A CRT monitor displays color pictures by using

a combination of phosphors that emit

different-colored light.

The two basic techniques for producing color

displays with a CRT are the

beam-penetration method

shadow-mask method.


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Beam Penetration

The beam-penetration method for displaying color pictureshas been used with random-scan monitors. Two layers ofphosphor, usually coated onto the inside of the CRT screen,and the displayed color depends on how far the electronbeam 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 redlayer 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 hencethe screen color at any point, is controlled by the beam-acceleration voltage.


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Shadow-masking

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


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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.


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Shadow Masking


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We obtain color variations in a shadow-mask CRT by varying the intensitylevels of the three electron beams. By turning off the red and green guns,we get only the color coming from the blue phosphor.

Other combinations of beam intensities produce a small light spot for eachpixel position, since our eyes tend to merge the three colors into onecomposite.

The color we see depends on the amount of excitation of the red, green, andblue phosphors.

A white (or gray) area is the result of activating all three dots with equalintensity. Yellow is produced with the green and red dots only, magenta isproduced with the blue and red dots, and cyan shows up when blue andgreen are activated equally. In some low-cost systems, the electron beam

can only be set to on or off, limiting displays to eight colors. Moresophisticated systems can set intermediate intensity levels for the electronbeams, allowing several million different colors to be generated.


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Direct-View Storage Tubes

It stores the picture information as a charge

distribution just behind the phosphor-coated

screen.

Two electron guns are used in a DVST. One,

the primary gun, is used to store the picture

pattern;

the second, the flood gun, maintains the

picture display.


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Advantages/ Disadvantages

no refreshing is needed, very complex pictures

can be displayed at very high resolutions

without flicker.

Disadvantages of DVST systems are that they

ordinarily do not display color and that

selected parts of a picture cannot he erased.

To eliminate a picture section, the entire

screen must be erased and the modified

picture redrawn.


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Flat Panel Displays

We can separate flat-panel displays into twocategories: emissive displays and non emissive

displays. The emissive displays (or emitters) are

devices that convert electrical energy into light.

Plasma panels, thin-film electroluminescent displays,and Light-emitting diodes are examples of emissive

displays.

Non emissive displays (or non emitters) use optical

effects to convert sunlight or light from some other

source into graphics patterns. The most important

example of a non emissive flat-panel display is a

liquid-crystal device.


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Plasma Panels

Plasma panels, also called gas-discharge displays, are constructed by filling

the region between two glass plates with a mixture of gases includes

neon.

A series of vertical conducting ribbons is placed on one glass plane, and a

set of horizontal ribbons is built into the other glass panel.

Firing voltages applied to a pair of horizontal and vertical conductors

cause the gas at the intersection of the two conductors to break down into

a glowing plasma of electrons and ions.

Picture definition is stored in a refresh buffer, and the firing voltages are

applied to refresh the pixel positions (at the intersections of the

conductors) 60 times per second.


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Output Devices

Stereoscopic viewing glasses: the user wears them to perceivestereoscopic view of 3D scenes displayed on screen Used in screen-based Virtual Reality (VR)

Has high resolution

Limited head-movement

Head-mounted display (HMD): two small TV screens areembedded in a rack and placed in front of the two eyes. It allows full-freedom head movement,

and gives the feel of immersion

Widely used in Virtual Reality (VR)

A tracking system is used to report

the position of HMD in 3D space.

Plotter

Printer


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Output Devices

Wide Screen


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Input Devices

Keyboard Mouse

Trackball: a 2D input device, usually used ona mouse or a lap-top computer.

Space ball: hand held, non-movable. It uses astrain gauge to detect pull, push, and twistapplied to the ball, and translate them into3D locations. Used for navigation in virtualenvironments, CAD, etc.

Head Mounted Display: Although it is

primarily a display device, it can also trackposition and orientation

Joystick: similar to the space ball. Can bemovable and non-movable.


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Input Devices

Data glove: a glove with sensors. Used tocontrol a virtual hand for grasping, dropping,and moving an object in a virtual environment.

Image scanner: input still picture, photo, orslides as images into computer.

Touch panel: highly transparent andembedded over a display surface.

Digital camera: directly stores photo shots asimages on a diskette.

Digital video recorder: input a video clip indigital form; often used for tele-conferencing.

Laser range scanner: input discrete andscattered points on a 3D surface model fromwhich a digital one can be built.


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Input Devices

Motion Capture: input full-body,

facial, hand movements

Documents

Evolution of Computer Graphics,