Lecture 10 Multimedia Communications and Services on the Internet

8/6/2019 Lecture 10 Multimedia Communications and Services on the Internet

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Course Conducted by Shuvodip Das,Lecturer, Department of ETE,Prime University.

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Imagey

Images are of 3 types -y Computer Generated Image/ Computer Graphics/Graphics.

y Digitized Image of documents and

y Digitized Image of picture.

All three types of images are displayed in the form of a 2D matrix of individual

picture elements known as pixels or pels.

Each type of image is represented and created differently.

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Computer Generated Image/ Computer

Graphics/Graphicsy Software or program to create computer graphics.

y easy-to-use tools to create computer graphics that arecomposed of objects including lines, arcs, squares, rectangles,circles, ovals, diamonds, stars and so on.

y Facilities to change shape, size or colour.

y A computers display screen can be considered as being madeup of a 2D matrix of individual picture elements-pixels-each

of which can have a range of colours associated with it. Forexample, VGA (Video Graphics Array) is a common type ofdisplay and consists of a matrix of 640 horizontal pixels by480 vertical pixels with, for example, 8 bits per pixel whichallows each pixel to have one of 256 different colour.

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Graphics/Graphics

Graphics principles : a) example screen format; b) some simple object examples; c)effect of changing position attribute; d) solid objects. 4


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Graphics/Graphics

y

Each object has a number of attributes associated with it.yAttributes:

y Shape: a line, a circle, a square and so on.

y Size: in terms of pixel positions and its border

coordinates, the colour of the border, its shadow and soon.

Editing an object involving simply changing selected attributesassociated with the object.

Colour Fill: The pixels enclosed by its border can all be assignedthe same colour called colour fill.

Rendering: Colour filling process of creating solid object is

called Rendering. 5


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Representation ofComputer Graphics

y

Fo

rmsof

representatio

nof

co

mputer graphics/graphics:y a high level version (source code of a high level program) and

y actual pixel image of the graphic (bit map format).

y Standardization forms ofrepresenting graphics :y GIF (Graphical Interchange Format)

y TIFF (Tagged Image file format)

y SRGP (Simple Raster Graphics Package)

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Digitized Documentsy Example: An example of a digitized document is that produced by the scanner

associated with a facsimile (fax) machine.

y The scanner associated with a fax machine operates by scanning each complete page

from left to right to produce a sequence of scan lines that start at the top of the page and

end at the bottom. The vertical resolution of the scanning procedure is either 3.85 or 7.7

lines per milimeter which is equivalent to approximately 100 or 200 lines per inch. Aseach line is scanned, the output of the scanner is digitized to a resolution of

approximately 8 picture elements or pels.

y Fax machine uses single binary digit to represent each pel, a 0 for a white pel and a 1

for a black pel. For a typical page it produces a stream of about two million bits. Theprinter part of a fax machine then reproduces the original image by printing out the

received stream of bits to a similar resolution. Single binary digit per pel is used for fax

machines that are best suited to scan bitonal (black and white) images such as printed

documents comprising mainly textual information. 7


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Digitized Documents

Fig: Facsimile machine principles : a) schematic b) digitization format

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Digitized Picture

y In the case of scanners which are used for digitizing continuous-

tone mono-chromatic images-such as a printed picture or scene-

normally more than a single bit is used to digitize each pel. Good

quality black and white picture can be obtained by using 8 bits per

pel. This yields 256 different levels of gray per element-varying

between black and white-which gives a substantially improved

picture quality over a facsimile image when reproduced.

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Co our Princip ey A whole spectrum of colours (colour gamut)- can be produced by using different

proportions of the three primary colours Red (R), Green (G) and Blue (B).

yColour Mixing techniques:

y Additive colour mixing technique and

y Subtractive colour mixing technique.

y Additive colour mixing technique:

y This technique is used for producing a colour image on a black surface. Here black is

produced when all three primary colours are zero. An additive color model involveslight emitted directly from a source or illuminant of some sort. The additive

reproduction process usually uses red, green and blue light to produce the othercolors. Combining one of these additive primary colors with another in equalamounts produces the additive secondary colors cyan, magenta, and yellow.Combining all three primary lights (colors) in equal intensities produces white.Varying the luminosity of each light (color) eventually reveals the full gamut of thosethree lights (colors).

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Colour Principle: Additive colour mixing technique

y Computer monitors and televisions use a system called

optical mixing and cannot be considered additive lightbecause the colors do not overlap. The red, green, andblue pixels are side-by-side. When a green colorappears, only the green pixels light up. When a cyan

color appears, both green and blue pixels light up.When white appears all the pixels light up. Because thepixels are so small and close together our eyes blendthem together, having a similar effect as additive light.

Another common use of additive light is the projectedlight used in theatrical lighting (plays, concerts, circusshows, night clubs, etc.).

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Colour Principle: Subtractive colour mixing technique

y SubtractiveColour Mixing Technique:

y With this technique white can be produced when the three chosen primary colours

cyan (C), magenta (M) and yellow (Y) are all zero. It is effective for producing imageon a white surface.

y The subtractive primary colors are cyan, magenta and yellow (CMY). These are thethree colors used in printer ink cartridges.

Subtractive ColorsCyan, Magenta and Yellow

Absorbs Creates

Blue + Green Red Cyan

Red + Blue Green Magenta

Green + Red Blue Yellow

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VideoDisplay DevicesDisplay Hardware

Video display devices

Input devicesy Locator Devices

y Keyboard devices

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Display Technologies

Cathode Ray Tubes (CRTs)

y Most common display device today

y Evacuated glass bottle

y

Extremely high voltagey Heating element (filament)

y Electrons pulled towardsanode focusing cylinder

y Vertical and horizontal deflection plates

y Beam strikes phosphor coating on front of tube

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Electron Gun

y Contains a filament that, when heated, emits astream of electrons

y Electrons are focused with an electromagnet into a

sharp beam and directed to a specific point of the faceof the picture tube

y The front surface of the picture tube is coated withsmall phosphor dots

yWhen the beam hits a phosphor dot it glows with abrightness proportional to the strength of the beamand how often it is excited by the beam

yThe picture is repeatedly repainted (refresh CRT)

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CRT Monitor

Electron Guns

Red Input

Green

Input

Blue Input

Deflection

Yoke

Shadow Mask

Red, Blue,

and Green

Phosphor Dots

CRT

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List the properties ofCRT1. Persistence

2. Resolution

3. Addressability4. Aspect ratio

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Resolution

The maximum number of points that can be displayed

without an overlap on a CRT

is referred to as the resolution. The smaller the spot size, the higher the resolution.

The higher the resolution, the better is the graphics system

High quality resolution is 1280x1024

The intensity distribution of spots on the screen have

Gaussian shape. Adjacent points will appear distinct as long as their

separation is greater than the diameter at which each spothas intensity of about 60% of that at the center of the spot.

Intensity distribution

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Addressability Addressability is the number of individual dotsper inch (d.p.i.) that can be created. If the address

of the current dot is (x, y) then the next dot willbe (x + y), (x + y + 1)

The picture on a screen consists of intensified

points. The smallest addressable point on the screen iscalled pixelor picture element

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Aspect ratio

This number gives the ratio between vertical pointsand horizontal points necessary to produce equal

length lines in both directions on the screen. Aspectratio = means that a vertical line plotted with 3points is equal in length to a horizontal line plotted

with 4 points.

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Display Technologies: CRTs1. Raster Scan Displays

2. Vector Displays

3. Liquid Crystal Displays (LCDs)4. Plasma Panel

5. Organic LEDArrays

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Raster Scan Displays

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Raster Scan Displays (1)y Raster: Arectangular array of points or dots

y Pixel: One dot or picture element of the raster. Itsintensity range for pixels depends on capability of thesystem

y Scan line: Arow of pixels

y Picture elements are stored in a memory called framebuffer

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Raster Scan Displays (2)

y Frame must be refreshed to draw new images

y As new pixels are struck by electron beam, othersare decaying

y Electron beam must hit all pixels frequently toeliminate flicker

y Critical fusion frequency

y Typically 60 times/sec

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R

aster ScanD

isplays (3)

y Intensity of pixels depends on the system forexample black and white screens each point can beon or off thus it needs one bit of memory to

represent each pixel.y To paint color screen additional bits are needed. If

three bits are used, then number of different colorsare 2*2*2.

y Aspecial memory is used to store the image withscan-out synchronous to the raster. We call thistheframe buffer.

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Raster Scan Displays (6)

Raster CRT pros:

y Allows solids, not just wire frames

y Leverages low-cost CRT technology (i.e., TVs)

y Bright! Displayemits light

Cons:

y Requires screen-size memory array

y

Discrete sampling (pixels)y Practical limit on size

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Frame Buffers

y Aframe buffer may be thoughtofas computer memoryorganizedas a two-dimensional arraywith each (x,y) addressable locationcorresponding toone pixel.

y Bit Planes or Bit Depth is the numberofbits corresponding to

each pixel.

y A typicalframe buffer resolution might be

y 640 x 480 x 8

y 1280 x 1024 x 8

y 1280 x 1024 x 2428


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Monochrome Display(Bit-map Display)

El tron

Gun

1 bit

2 l v l

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3-Bit Color Display

3

red

green

blue

COLOR: black red green blue yellow cyan magenta white

R

G

B

0

0

0

0

0

0

0

0

0

0

0

0

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True ColorDisplay

24 bit planes, 8 bits per c

olor gun.224 = 16,777,216

Green

Red

B ue

N

N

N

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