IMAGE compression.ppt

7/30/2019 IMAGE compression.ppt

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SOURCE CODING: IMAGE AND

VIDEO


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Image and Video Formats GIF,TIFF, SIF, CIF, QCIF

Image compression: READ, JPEGVideo Compression: Principles-I,B,P

frames, Motion estimation, Motion

compensation, H.261, MPEGstandard


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All types of images are displayed ( and printed )

in the form of a two dimensional matrix ofindividual picture elements known as pixels orsome time pels.

For example VGA ( video graphics array ) is a

common type of display and , so we show in fig ,consists of a matrix of 640 horizontal pixels by480 vertical pixels with, for example 8 bits perpixel which allows each pixel to have one of 256different colors.


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The graphics interchange format is used extensively with the Internet forthe representation and compression of graphical images


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Although colour images comprising 24-bit pixels are supported GIFreduces the number of possible colours that are present by choosing

256 entries from the original set of 224 colours that match closely to the

original image

Hence instead of sending as 24-bit colour values only 8-bit index to

the table entry that contains the closest match to the original is sent.This results in a 3:1 compression ratio

The contents of the table are sent in addition to the screen size and

aspect ratio information

The image can also be transferred over the network using theinterlaced mode.


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The LZW can be used to obtain further levels of compression


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GIF also allows an image to be stored and subsequently

transferred over the network in an interlaced mode; useful overeither low bit rate channels or the Internet which provides a variable

transmission rate

1/8 and 1/8 of the

total compressed

image


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The compression image data is organized so that the

decompressed image is built up in a progressive way as the data

arrives

Further and

remaining of the

image


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RGB - 24 or 48 bits,

16 bits are used for each R,G,B colors

For TIF files, most programs alloweither no compression or LZWcompression

Code number indicates particularformat

Code 1- uncompressed format

Codes 2,3,4- digitized format -


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Chrominance Components

Composite Video Signal for Transmission

- Ys, Cb, and Crsignals are combined together and signaldifferences are scaled down before transmission

In PAL

- Y = 0.299 R + 0.587 G + 0.114 B

- U(Cb) = 0.493(B-Y) = -0.147R-0.289G+0.437B

- V(Cr ) = 0.877(R-Y) = 0.615R-0.515G-0.1B

In NTSC

- Y = 0.299 R + 0.587 G + 0.114 B

- I(Cb) = 0.74(R-Y)-0.27(B-Y) = 0.599R-0.276G-0.324B

-Q(C

r) = 0.48(R-Y)+0.41(B-Y) = 0.212R-0.528+0.311B


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Advantages of DV Easy to store in computer

Easy to edit and integrate with other types

Easy to digitize three RGB component signals

The resolution of eyes are less sensitive for color than it is forluminance. Hence, two chrominance signals can tolerate areduced resolution

Transmission bandwidth is achieved by using the luminance

and two color difference signals, instead of the RGBsignals directly.

CCIR-601 Recommendations: standard for the digitizationof video pictures


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4:2:2 format(CCIR-601)

Recommendation for use in TV studio Three component (analog) video signals may have bandwidths

up to 6Mhz for the luminance 12Mhz sps

less than 3Mhz for the two chrominance signals 6 Mhz sps

In reality, 13.5M sps for luminance, 6.75 M sps for the twochrominance signals

In NTSC(525-line) system, total line sweep time 63.56sec =retrace time 11.56 sec + an active line sweep time 52 sec

In PAL(625-line) system, total line sweep time 64sec =retrace time 12 sec + an active line sweep time 52 sec

Line sampling rate:

52 10-6 13.5 106= 702 samples/line

In reality, 720 samples/line

Line sampling rate:

52 10-6 6.75 106= 351 samples/line

In reality, 360 samples/line

4Y samples for every 2Cb and 2Cr samples(4:2:2)

Orthogonal sampling

Y

Cb

Cr


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4:2:2 Format Bit Rate & Storage (NTSC 525-line)

The number of active (visible) lines: 480

The number of samples per line: 720

Resolution of luminance Y = 720 480

Two chrominance signals Cb = Cr= 360 480

Line sampling rate: 13.5sps for Y & 6.75sps for both Cb & Cr

Bits per sample: 8 bits

Bit rate per line = 13.5 106 8 + 2 (6.75 106 8) = 216Mbps

Bits per line = 720 8 + 2 (360 8) = 11.52Kbits

Bits per frame = 480 11.52 = 5.5296Mbits

Bits for 1.5 hrs Video assuming 60 refresh rate = 5.5296 60 1.5 3600

= 223.9488GBytes

PAL625

-line

576

720

72057

6 36057

6

576

6.63555Mbit

s

6.63555 5

0


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4:2:0 Format used in Digital Broadcast Applications

interlaced scanning with the absence of chrominancesamples in alternative lines

525-line system

Y = 720 480(the same as 4:2:2 format), Cb = Cr= 360 240

625-line system

Y = 720 576, Cb = Cr= 360 288

bit rate per line: 13.5 106 8 + 2 (3.375 106 8) = 162Mbps

HDTV Format

used in High-Definition Television (four times bit rate)

4/3 1440 1152 pixels(50/60 Hz refresh rate) & 16/9 wide-screen1920 1152 pixels(25/30 Hz) with # of visible lines per frame 1080


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SIF (Source Intermediate Format), 4:1:1 Format

used in Video Cassette Recorders (VCRs)

progressive (non-interlaced) scanning since it is intended forstorage applications

Half of 4:2:0 format: Subsampling & Temporal Resolution

Uses half the refresh rate -temporal resolution

Frame refresh rate- 30 Hz for 525 line system and 25 Hz for 625 line system.

525-line system

Y = 360 240, Cb = Cr= 180 120

625-line system

Y = 360 288, Cb = Cr= 180 144

bit rate per line

6.75

106

8 + 2

(1.6875

106

8) = 81Mbps


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CIF (Common Intermediate Format), 4:1:1 format

used in Video Conferencing applicationsalso known as FCIF (Full Common Intermediate Format),

spatial resolution of the SIF 625-line system plustemporal resolution of the SIF 525-line system

Y = 360 288, Cb = Cr= 180 144

refresh rate: 30 Hz

bit rate per line: 6.75 106 8 + 2 (1.6875 106 8) =81Mbps

many variants for videoconferencing using desktop PCsor ISDN/PSTN

say, typically 4 or 16 64Kbps channels used

4CIF: Y = 720 576, Cb = Cr= 360 288

16CIF: Y = 1440 1152, C = C = 720 576


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QCIF (Quarter CIF), 4:1:1 Format

used in Video Telephony applications

half spatial resolution of the CIF andeither half or quarter temporal resolution of the CIF

Y = 180 144, Cb = Cr= 90 72

refresh rate: 15 or 7.5 Hz

bit rate per line:3.375 106 8 + 2 (0.84375 106 8) = 81Mbps

a lower version is typically used for single 64Kbps channelISDN or PSTN with modems: sub-QCIF(SQCIF)

Y = 128 96, Cb = Cr= 64 48


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PC Video Digitization

DigitizationFormat

System Spatial Resolution TemporalResolution

4:2:2 525-line625-line

Y = 640 480, Cb = Cr= 320 240

Y = 768 576, Cb = Cr= 384 288

60Hz50Hz

SIF 525-line625-line

Y = 320 240, Cb = Cr= 160 240

Y = 384 288, Cb = Cr= 192 144

30Hz25Hz

CIF Y = 384

288, Cb = Cr= 192

144 30HzQCIF Y = 192 144, Cb = Cr= 96 72 15/7.5Hz

- Video capture board or S/W required- All PC monitors use progressive (non-interlaced) scanning


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Since FAX machines are used with public carrier

networks, the ITU-T has produced standards relating to

them

These are T2(Group1), T3 (Group2), T4 (Group3)

(PSTN), and T6 (Group 4) (ISDN)

Both use data compression ratio in the range of 10:1

The resulting codewords are grouped into termination-

codes table (white or black run-lengths from 0 to 63

pels in steps of 1) and the make-up codes table

(contains in multiples of 64 pels)

Since this codeword uses two sets of codeword it is

known as the modified Huffman codes


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ITU T Group 3 and 4facsimile conversion

codes: termination-

codes

Termination code table


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ITU T Group 3and 4 facsimile

conversion codes:

make-up codes

Make-up of 64

codewords


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Each scanned line is terminated with an EOL code. In

this way the receiver fails to decode a word it starts tosearch for an EOL pattern

If it fails to decode an EOL after a preset number of

lines it aborts the reception process and informs the

sending machine

A single EOL precedes the end of each scanned line

and six consecutive EOLs indicate the end of each page

The T4 coding is known as one-dimensional coding


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The modified-modified relative element addressdesignate coding explores the fact that most scanned

lines differ from the previous line by only a few pels

E.g. if a line contains a black-run then the next line will

normally contain the same run pels plus or minus 3 pels

In MMR the run-lengths associated with a line are

identified by comparing the line contents, known as the

coding line (CL), relative to the immediately preceding

line known as the reference line (RL)

The run lengths associated with a coding line are

classified into three groups relative to the reference line


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This is the case when the run-length in the reference line (b1b2) overlaps

the next run-length in the coding line(a1a2) by a maximum of plus or minus

3 pels

This is the case when the run-length in the reference line(b1b2) is to the left

of the next run-length in the coding line (a1a2), that is b2 is to the left of a1

Pass mode

Vertical mode


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This is the case when the run-length in the reference line (b1b2) overlaps

the run-length (a1a2) by more than plus or minus 3 pels

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