watermarking tech doc

7/30/2019 Watermarking Tech Doc

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WATERMARKING 08R91A12B3

1. INTRODUCTION

The enormous popularity of

the World Wide Web in the

early 1990's demonstrated

the commercial potential of

offering multimedia

resources through the digital

networks. Since commercial

interests seek to use the

digital networks to offer

digital media for profit, they

have a strong interest in

protecting their ownership

rights. Digital watermarking

has been proposed as one

way to accomplish this.

A digital watermark is a

digital signal or pattern

inserted into a digital image.

Since this signal or pattern is

present in each unaltered

copy of the original image,

the digital watermark may

also serve as a digital

signature for the copies. A

given watermark may be

unique to each copy (e.g. to

identify the intended

recipient), or be common to

multiple copies (e.g. to

identify the document

source). In either case, the

watermarking of the

document involves the

transformation of the

original into another form.

This distinguishes digital

watermarking from digital

fingerprinting, where the

original file remains intact

and a new created file

'describes' the original file's

content.

Digital watermarking is also

to be contrasted with public-

key encryption, which also

transform original files into

another form. It is acommon practice nowadays

to encrypt digital documents

so that they become un-

viewable without the

decryption key. Unlike

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encryption, however, digital

watermarking leaves the

original image (or file)

basically intact and

recognizable. In addition,

digital watermarks, as

signatures, may not be

validated without special

software. Further, decrypted

documents are free of any

residual effects of

encryption, whereas digital

watermarks are designed to

be persistent in viewing,

printing, or subsequent re-

transmission or

dissemination.

2. GENERAL FRAMEWORK

FORWATERMARKING

Watermarking is the

process that embeds data

called a watermark or digital

signature or tag or label into

a multimedia object such

that watermark can be

detected or extracted later to

make an assertion about the

object. The object may be animage or audio or video. A

simple example of a digital

watermark would be a

visible seal placed over an

image to identify the

copyright. However the

watermark might contain

additional information

including the identity of the

purchaser of a particular

copy of the material. In

general, any watermarking

scheme ,(algorithm) consists

of three parts.

The watermark.

The encoder

(insertion algorithm).

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The decoder and

comparator

(verification or

extraction or

detection algorithm).

Each owner has a unique

watermark or an owner can

also put different

watermarks in different

objects the marking

algorithm incorporates thewatermark into the object.

The verification algorithm

authenticates the object

determining both the owner

and the integrity of the

object.

2.1 Encoding Process

Let us denote an image by a

signature by S and the

watermarked image by I. E is

an encoder function, it takes an

image I and a signature S, and

it generates a new image which

is called watermarked image ,

Fig.2.1.encoder

It should be noted that the

signature S may be dependent

on image I. Following figureillustrates the encoding

process.

2.2 Decoding Process

A decoder function

D takes an image J.whose

ownership is to be

determined and recovers a

signature from the image. In

this process an additional

image I can also be included

which is often the original

and un- watermarked

version of J. This is due to

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Encoder

Origin

Watermark

edimage(I)

Signat

ure(S)


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the fact that some encoding

schemes may make use of

the original images in the

watermarking process to

provide extra robustness

against intentional and

unintentional corruption of

pixels. The extracted

signature will then be

compared with the owner

signature sequence by a

comparator function and a

binary output decision

generated. It is 1 if there is

match and 0 otherwise,

which can be represented as

follows.

Fig. Decoder

3. WATERMARKING TECHNIQUES

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decoder

Origin

al

compar

ator

Text

image(

Extract

ed

signatu

Ori

Sig

re(

Signat

ure(S)

Original

Signatu

re(S)


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The techniques

proposed so far can be divided

into two main groups of

according to the embedding

domain of the container image.

Spatial Domain

Approach

Frequency Domain

Approach

3.1 Spatial Domain

Approach

Several different

methods enable watermarking

in the spatial domain. The

simplest (too simple for many

applications) is just to flip the

lowest-order bit of chosen

pixels. This works well only if

the image is not subject to any

modification. A more robust

watermark can be embedded by

superimposing a symbol over

an area of the picture. The

resulting mark may be visibleor not, depending upon the

intensity value. Picture

cropping, e.g., (a common

operation of image editors), can

be used to eliminate the

watermark.

Spatial watermarking can also

be applied using color

separation. In this way, the

watermark appears in only one

of the color bands. This renders

the watermark visibly subtle

such that it is difficult to detect

under

regular viewing. However, the

mark appears immediately

when the colors are separated

for printing. This renders the

document

useless for the printer unless

the watermark can be removed

from the color band. This

approach is used commercially

for journalists to inspect digital

pictures from a photo-

stockhouse before buying

unmarked versions.

3.2 Frequency Domain

Approach

Watermarking can be

applied in the frequency

domain (and other transform

domains) by first applying a

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transform like the Fast Fourier

Transform (FFT). In a similar

manner to spatial domain

watermarking, the values of

chosen frequencies can be

altered from the original. Since

high frequencies will be lost by

compression or scaling, the

watermark signal is applied to

lower frequencies, or better yet,

applied adaptively to

frequencies that contain

important information of the

original picture. Since

watermarks applied to the

frequency domain will be

dispersed over the entirety of

the spatial image upon inverse

transformation, this method is

not as susceptible to defeat by

cropping as the spatial

technique. However, there is

more a tradeoff here between

invisibility and decodability,

since the watermark is in effect

applied indiscriminately across

the spatial image. Table 1.

shows a small comparison

between the two different

techniques.

Spatial Domain Frequency Domain

Computation Cost Low High

Robustness Fragile More Robust

Perceptual Quality High Control Low Control

Capacity High (depend on the size of theimage)

Low

Example ofApplications

Mainly Authentication Copy Rights

Comparison between Watermarking Techniques

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4. DIGITAL WATERMARKING

Watermarking is

the process that embeds data

called a watermark, tag or

label into a multimedia

object such that watermark

can be detected or extracted

to make an assertion about

the object may an image or

video or audio may also be

text only. A watermark can

be perceived as an attribute

of the carrier (cover). It may

contain information such as

copyright, license, tricking

and authorship etc. Whereas

in case of steganography,

the embedded message may

have nothing to do with the

cover. Digital watermarking

differs from digital

fingerprinting.

4.1. Introduction Of

Digital water

marking

Digital Watermarking, an

extension of Steganography,is a promising solution of

content copyright protection

in the global network. It

imposes extra robustness on

embeddedinformation.Digita

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l watermarks dont leave a

noticeable mark on the

content and not affect its

appearance. These are

imperceptible and can be

detected only by proper

authorities. Digital

watermarks are difficult to

remove without noticeably

degrading the content and

are a covert means in

situations where

cryptography fails to

provide robustness.

4.2 Types Of Digital

Watermarks

Watermarks and

watermarking techniques

can be divided into various

categories in various ways.

The watermarks can be

applied in spatial domain.

An alternative to spatial

domain watermarking is

frequency domain

watermarking. It has been

pointed out that the

frequency domain methods

are more robust than the

spatial domain techniques.

Watermarking techniques

can be divided into four

categories according to the

type of document to be

watermarked as follows.

Image

Watermarking

Video

Watermarking

Audio

Watermarking

Text Watermarking

According to the human

perception, the digital

watermarks can be divide

into three different types as

follows.

Visible watermark

Invisible-Robustwatermark

Invisible-Fragile

watermark

Dual watermark

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Fig. TYPES OF DIGITALWATERMARK

Visible watermark is a

secondary translucent

overlaid into the primary

image. The watermark

appears visible to a casual

viewer on a careful

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inspection. The invisible-

robust watermark is embed

in such a way that an

alternation made to the pixel

value is perceptually not

noticed and it can be

recovered only with

appropriate decoding

mechanism. The invisible-

fragile watermark is

embedded in such a way that

any manipulation or

modification of the image

would alter or destroy the

watermark. Dual watermark

is a combination of a visible

and an invisible

watermark .In this type of

watermark an invisible

watermark is used as a back

up for the visible watermark

as clear from the following

diagram.

An invisible robust

private watermarking

scheme requires the original

or reference image for

watermark detection;

whereas the public

watermarks do not. The

class of invisible robust

watermarking schemes that

can be attacked by creating a

counterfeit original is called

invertible watermarking

scheme.

From application

point of view digital

watermark could be as

below.

Source based or

Destination based.Source-based

watermark are desirable for

ownership identification or

authentication where a

unique watermark

identifying the owner is

introduced to all the copies

of a particular image being

distributed. A source-based

watermark could be used for

authentication and to

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determine whether a

received image or other

electronic data has been

tampered with. The

watermark could also be

destination based where

each distributed copy gets a

unique watermark

identifying the particular

buyer. The destination

-based watermark could be

used to trace the buyer in the

case of illegal reselling.

5. APPLICATION OF DIGITAL

WATERMARKS

5.1 Visible Watermark

Visible watermarks can be

used in following cases:

Visible watermarking for

enhanced copyright

protection. In such

situations, where images are

made available through

Internet and the content

owner is concerned that the

images will be used

commercially (e.g.

imprinting coffee mugs)

without payment of

royalties. Here the content

owner desires an ownership

mark, that is visually

apparent, but which does not

prevent image being used

for other purposes.

visible watermarking used to

indicate ownership originals.

In this case images are made

available through the

Internet and the content

owner desires to indicate the

ownership of the underlying

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materials

(librarymanuscript), so an

observer might be

encouraged to patronize the

institutions that own the

material

5.2 Invisible Robust

Watermark

Invisible robust

watermarks find application

in following cases.

Invisible watermarking

to detect misappropriated

images. In this scenario, the

seller of digital images is

concerned, that his, fee-

generating images may be

purchased by an individualwho will make them

available for free, this would

deprive the owner of

licensing revenue.

Invisible

watermarking as evidence of

ownership. In this scenario,

the seller that of the digital

images suspects one of his

images has been edited and

published without payment

of royalties. Here, the

detection of the sellers

watermark in the image is

intended to serve as

evidence that the published

image is property of seller.

5.3 Invisible Fragile

Watermarks

Following are the

applications of invisible

fragile watermarks.

Invisible watermarking

for a trustworthy camera. In

this scenario, images are

captured with a digital

camera for later inclusion in

news articles. Here, it is the

desire of a news agency to

verify that an image is true

to the original capture and

has not been edited to falsify

a scene. In this case, an

invisible watermark is

embedded at capture time;

its presence at the time of

publication is intended to

indicate that the image has

not been attended since it

was captured.

Invisible

watermarking to detect

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alternation of images stored

in a digital library. In this

case, images (e.g. human

fingerprints) have been

scanned and stored in a

digital library; the content

owner desires the ability

to detect any alternation of

the images, without the need

to compare theimages to the

scanned materials.

5.4 Attacks On

Watermarks

A watermarked

image is likely to be

subjected to certain

manipulations, some

intentional such as

compression and

transmission noise and some

intentional such as cropping,

filtering, etc. They are

summarized in Lossy

Compression: Many

compression schemes like

JPEG and MPEG can

potentially degrade the

datas quality through

irretrievable loss of

data.Geometric Distortions:

Geometric distortions are

specific to images videos

and include such operations

as rotation, translation,

scaling and cropping.

Common Signal ProcessingOperations: They include

the

Following D/A conversion

A/D conversion

Resampling

Requantization

Dithering distortion

Recompression

Linear filtering such

as high pass and low

pass filtering.

Addition of a

constant offset to the

pixel values

Addition of Gaussian

and Non Gaussian

noise

Local exchange of

pixels

other intentional

attacks:

Printing and

Rescanning

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Watermarking of

watermarked image

(rewatermarking)

Collusion: A number

of authorized

recipients of the

image should not be

able to come

together (collude)

and like the

differently

watermarked copies

to generate an un-

watermarked copy of

the image (by

averaging all the

watermarked

images).

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Fig.Attacks on watermarking

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6. CHARACTERISTICS

OFWATERMARKS

6.1 Characteristics ofVisible

Watermarks

A visible watermark

should be obvious in both

colors and monochrome

images. The watermark

should spread in a large orimportant area of the image

in order to prevent its

deletion by clipping.

The watermark should be

visible yet must not

significantly obscure the

image details beneath it.

The watermark must

be difficult to remove.

Rather, removing a

watermark should be more

costly and labor intensive

than purchasing the image

from the owner. The

watermark should be applied

automatically with little

human intervention and

labor.

6.2 Characteristics of

Invisible Robust

Watermark

The invisible watermark

should neither be Noticeable

to the viewer nor should

degrade the quality of the

content. An invisible robust

watermark must be robust to

common signal distortions

and must be resistant to

various intentional

tamperings solely intended

to remove the watermark.

Retrieval of watermarkshould unambiguously

identify the owner. It is

desirable to design a

watermark whose decoder is

scalable with each

generation of computer.

While watermarking high

quality images and art works

the amount of pixel

modification should be

minimum. Insertion of

watermark should require

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little human intervention or

labor.

6.3 Characteristics of

Invisible Fragiles

Watermarks

The invisible

watermark should neither be

noticeable to the viewer nor

should degrade the quality

of the content.

An invisible fragile

watermark should be readily

modified when the image

pixel values have been

altered. The watermark

should be secure. This

means that it is impossible

to recover the changes, or

regenerate the watermark

after image alternations,

even when the watermarking

procedure, and/or the

watermark itself is known.

For high quality images, the

amount of individual pixelmodification

should be as small as

possible.

6.4 Characteristics

Of Video

Watermarks

The presence of

watermark should not cause

any visible or audible effects

on the playback of the

video. The watermark

should not affect the

compressibility of the digital

content. The watermark

should be detected with high

degree of reliability. The

probability of false detection

should be extremely small.

The watermark should be

robust to various intentional

and unintentional attacks.

The detection algorithm

should be implemented in

circuitry with small

extra cost.

6.5. Characteristics

Image

Watermarking

There are plenty of image

watermarking techniques

algorithms available in

current literature. In this

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section we will discuss a

few of them.

M.Kankanhalli, have

developed a visible

watermarking technique.

They divide the host image

into different blocks, find

the DCT of each block.

Then they classify the

blocks into six different

classes in the increasing

order of noise sensitivity,

such as edge block, uniform

with moderate intensity,

uniform with high or low

intensity, moderate busy,

busy and very busy. Each

block is then assigned.

The watermark is robust to

common signal and

geometric distortion such as

A/D and D/A conversion,

resampling, quantization,

compression, rotation,

translation, cropping and

scaling. The watermark is

universal in the sense that it

can be applied to all three

media. Retrieval of the

watermark unambiguously

identifies the owner and the

watermark can be

constructed to make

counterfeiting almost

impossible. The

watermarking technique has

the disadvantage that it

needs the original image for

its extraction. It is also not

clear whether the watermark

is robust to photocopying.

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Fig.visibleimage watermarking

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7. THE WATERMARK TECHNOLOGY

Digital watermarking,

an extension of

steganography, is a

promising solution for

content copyright protection

in the global network. It

imposes extra robustness on

embedded information. To

put into words, digital

watermarking is the art and

science of embedding

copyright information in the

original files. The

information embedded is

called watermarks.

Digital watermarks dont

leave a noticeable mark on

the content and dont affect

its appearance. These are








situations where


provide robustness.

The content is

watermarked by converting

copyright information into

random digital noise using a

special algorithm that is

perceptible only to the

content creator. Digital

watermarks can be read only

by using the appropriate

reading software. These are

resistant to filtering and stay

with the content as long as

Originally purposely

degraded.

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Digital watermarks dont

leave a noticeable mark on

the content and dont affect

its appearance. These are








situations where


provide robustness.

The content is

watermarked by converting

copyright information into

random digital noise using a

special algorithm that is

perceptible only to the

content creator. Digital

watermarks can be read only

by using the appropriate

reading software. These are

resistant to filtering and stay

with the content as long as

Originally purposely

degraded.

While the later

technique facilitates access

of the encrypted data only

for valid key holders but

fails to track any

reproduction or

retransmission of data after

decryption. On the other

hand, in digital

watermarking, an

identification code (symbol)

is embedded permanently

inside a cover image which

remains within that cover

invisibly even after

decryption process. This

requirement of

watermarking technique, in

general, needs to possess the

following characteristics:

imperceptibility for

hidden information,

redundancy in

distribution of the

hidden information

inside the cover

image to satisfy

robustness in water

mark extraction

process even from

truncated(cropped)

image .

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one or more keys to

achieve

cryptographic

security of hidden

content.

Besides these

general properties, an ideal

watermarking system should

also be resilient to insertion

of additional watermarks to

retain the rightful

ownership. The perceptually

invisible data hiding needs

insertion of watermark in

higher spatial frequency of

the cover image since

human eye is less sensitive

to this frequency

component. But in most ofthe natural images majority

of visual information are

concentrated on the lower

end of the frequency band.

So the information hidden in

the higher frequency

components might be lost

after quantization operation

of lossy compression. This

motivates researchers in

recent times to realize the

importance of perceptual

modeling of human visual

system and the need to

embed a signal in

perceptually significant

regions of an image,

especially if the watermark

is to survive lossy

compression.In spatial

domain block based

approach, this perceptually

significant region is

synonymous to low variance

blocks of the cover image.

It is found in the

literature that the robust

watermarking systems

proposed so far can only

withstand some of the

possible external attacks but

not all. While spatial domain

watermarking, in general, is

easy to implement on

computational point of view

but too fragile to withstand

large varieties of external

attacks. On the other hand,

frequency or Transformed

domain approach offers

robust watermarking but

most cases

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implementationneed higher

computational complexity.

Moreover the transform

domain technique is global

in nature (global within the

block in block based

approach) and cannot

restrict visual degradation of

the cover image. But in the

spatial domain scheme,

degradation in image quality

due to watermarking could

be controlled locally leaving

the region of interest

unaffected. The present

paper describes a

computationally efficient

block based spatial domain

watermarking technique for

a two level watermark

symbol. The selection of the

required block is based on

variance of the block and

watermark insertion exploits

average brightness of the

blocks.

7.1 Insertion Of

Watermark

In the present work, a

block based spatial domain

algorithm is used to hide

copyright mark (invisible

logo) in the homogenous

regions of the cover image

exploiting average

brightness.

The cover image is

partitioned into non-

overlapping square blocks of

size (8X8)pixels. A block is

denoted by the location of

its starting pixel (x, y). If the

cover image is of size

(NXN), total (N/8XN/8)

number of such block is

obtained for watermark

insertion. Next, all such

blocks are arranged in

ascending order based on

their variance values. The

variance () of a block of

size (M X N) is denoted by

m-1 n-1

=1/mn[(,y)-]

(1)

x=0 y=0

where

m-1 n-1

=1/mn[(,y)]

(2)

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x=0 y=0

is the statistical average

value of the block.

The blocks having

small variance values may

be called as homogenous

blocks and, of course, the

smallness in variance value

depends on the

characteristics of image to

be watermarked. If the

Watermark symbol is a (N X

N) binary image, only N

homogeneous blocks are

sufficient to insert one

watermark pixel in each

such homogenous block. A

two level map of size

(N/8XN/8) _is constructedbased on the location of

homogenous blocks in the

cover image assigning each

homogeneous block of the

cover image by value 1

while all other blocks by

value 0. This two level

map later modified as multi

level image, also called as

secret image (s), is used for

extraction of watermark

pixels. The formation of

multilevel image from two

level maps is described.

In the proposed

scheme, one watermark

pixel is inserted in each

homogenous block. Before

insertion, the binary

watermark is spatially

dispersed using a chaotic

system called tours auto

Orphism. Basically, the

tours auto Orphism is a kind

of image independent

permutation done by using

pseudo random number of

suitable length. This pseudo

random number is generated

using Linear Feedback

Shift Register. The

pseudorandom number in

the present case is of length

256 and the spatially

dispersed watermark data

thus obtained is denoted by

L1.a J

From the two level

image formed in step 2,

desired blocks Of the

cover image are selected and

statistical average value of

these blocks are used for

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watermark insertion. Let for

one such block this average

value and its integer part are

denoted by A and A=A

respectively. Now one pixel

from L1 replaces a

particular bit (preferably

Least Significant Bit planes)

in bit plane representation of

A for each homogenous

block. The selection of

particular bit in bit plane

representation may be

determined based on the

characteristics (busyness

/smoothness of regions) of

the block. The bit plane

selection is also governed by

global characteristics of thecover image besides the

local property of candidate

block, such as mean gray

value. For a block of low

variance (homogenous zone)

higher bit plane may be

chosen provided that the

mean gray level value of the

block is either less than T1

or greater than T2, where T1

and T2 are certain pre-

specified threshold values

with T1 should preferably

be close to 0 (minimum)

and T2 close to 255

(maximum). However, the

closeness of T1 and T2 to

0 and 255 respectively, is

relative, and is strongly

image dependent. Users may

choose the value of T1 and

T2 and also the proper bit

plane by checking the

degradation in the image

quality affected by the

insertion of the logo.

A multilevel secret

image is constructed by

inserting the value of bit

position selected for

different homogeneous

block located in the 1

position of the secret image.

This positional information

as gray value of the secret

image helps to extract

watermark pixel from the

proper bit position of the

mean gray value of the

block. Watermark insertion

keeps all pixels values of

each homogenous block

unchanged, increased or

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decreased by fixed value

(based on the appropriate bit

plane selection).

The choice of lower

order MSB plane (say 3rd or

higher from the bottom

plane) may result in more

robust watermarking at the

cost of greater visual

distortion of the cover

image. Further bit

manipulation is done to

minimize this aberration and

to counter the effect of

smoothing that may cause

possible loss of embedded

information. The process

effectively changes those

mean gray values of the

blocks that have been used

in watermark insertion.

Implementation is done by

estimating the tendency of

possible change in mean

gray value after the attack

like mean filtering. Larger

size of spatial mask such as

7x 7 is used to adjust

suitably the gray values of

all pixels of the block. The

use of spatial mask reduces

visual distortion on and

average fifty percent times.

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Fig. insertion of watermark

7.2 Watermark

Extraction

The extraction of

watermark requires the

secret image(s) and the key

(k) used for spatial

dispersion of the watermark

image. The watermarked

image under inspection with

or without external attacks is

partitioned into non-

overlapping block of size

8x8 pixels. Now from the

secret image, position of the

homogenous blocks are

selected and gray value of

the secret image indicates

the corresponding bitpositioning mean gray

values where watermark

pixel was

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inserted. Hence from the

secret image the mean gray

value of the blocks of the

watermarked

image/distorted

watermarked image is

calculated and watermark

pixel is extracted. The

spatially dispersed

watermark image thus

obtained is once again

permuted using the same

key (k) (pseudo random

number) and watermark in

original form is thus

obtained. This completes

watermark

extractionprocess.

Fig. watermark extraction

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8. CONCLUSION

Watermarking methods are

fast /robust and protect

against most forms of

manipulation. The

watermarking research is

progressing very fast and

numerous researchers from

various fields are focusing

to develop some workable

scheme. Different

companies also working to

get commercial products.

We hope some commercial

and effective schemes will

be available in future.

Digital watermarkingtechnique describes robust

and blind digital image

watermarking in spatial

domain, which is

computationally efficient.

Embedded watermark is

meaningful and

recognizable rather than a

sequence of real numbers

that are normally distributed

or a Pseudo-Noise sequence.

Proposed technique has been

tested over large number of

benchmark images as

suggested by watermarking

community and the results

of robustness to different

signal processing operations

are found to be satisfactory.

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9. REFERENCES

11. Hal Berghel,

Watermarking

Cyberspace, Comm. of

the ACM, Nov.1997,

Vol.40, No.11, pp.19-

24.

22. G. W. Braudaway, et.

al., Protecting Publicly

Available Images with a

Visible Image

Watermark, Proc.

SPIE Conf. Optical

Security and

Counterfeit Deterrence

Technique, Vol. SPIE-

2659, pp.126-132, Feb.

1996.

33. C.-T. Li and F.M. Yang.

One-dimensional

Neighborhood Forming

Strategy for Fragile

Watermarking. In

Journal of Electronic

Imaging, vol. 12, no. 2,

pp. 284-291, 2003.

4. R. Anderson. Information

Hiding. Proceedings

of the First Workshop

on Information Hiding,

LNCS-1174, Springer

Verlag, New York,

1996.

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