2 Bai Toan Watermarking

7/30/2019 2 Bai Toan Watermarking

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Chng II :Bi ton watermarking

Watermarking khng nhn thy thng c dng cho nh (nhng d liuvo trong nh gc). Qu trnh x l nh khi qut c m t trong hnh 2.1. nh

gc c thm vo d liu k s to nn nh nhng watermark. m bo tnh

trong sut ca d liu nhng, s mo dng nh trong qu trnh nhng phi nh.

Hnh 2.1 M hnh giu dliu dng tng qut

2.1. Qu trnh xl watermark thy nhng kha cnh khc nhau trong bi ton watermarking, ty thuc

vo nhng ng dng ring bit v nhng iu kin cn ca ng dng, ta phi ci

tin m hnh watermarking tng qut v c ci nhn v nhng giai on lin tip ca

qu trnh x l watermarking bao gm nhng giai on nh sau:

Giai on nhng (embedding) Giai on trch (extraction)

B nhngnh nhng

watermark

Mo dng do

tn cng

B trchTrch ks

So snh (tnh

tng quan)

nh gc(Original

image)

Kho

nh gc

nh sautn cng

K s


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Giai on phn phi (distribution) Giai on quyt nh (decision)

2.1.1 Giai on nhng

Trong giai on nhng, nh chc bin i n min thun tin cho vic

nhng d liu. Lun vn ny la chn ng dng wavelets v cc php bin i

wavelets. Mt s phng php khc cng c th thc hin c nh DCT (Discrete

Cosine Transform) [6] hoc l DFT (Discrete Fourier Transform).

D liu k s (cn gi l thng ip) c th l d liu nh phn, mt nh nh

(logo) hoc l m

t gi tr

kh

i

u cho m

t b

sinh s

ng

u nhin gi

to ra m

t

chui s cng vi phn b xc nh (v dl phn b Gaussian hoc phn bu).

K tip, cc h s bin i c bin i sao cho ph hp vi d liu ca

thng ip. Tt hn l php bin i nh phi ph hp vi c tnh cm nhn v th

gic ca ngi HVS, nh vy s d dng hn khi gn vo tn hiu c nhng m

khng gy ra mo dng cm nhn. Lu phi la chn min bin i tn s v cc

h s ph hp vi nhu cu gn (chng hn nhl vng tn s thp v tn s gia

m gii thut ca Cox [16] la chn).Cui cng, php bin i ngc da trn cc h s c sa i hi

phc li nh c nhng.

Hnh 2.2 th hin m hnh tng qut ca giai on nhng d liu vo nh s.


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Hnh 2.2 M hnh tng qut giai on nhng dliu

2.1.2 Phn b

nh c nhng watermark sau sc em phn b, v d nha ln

mt trang web hoc bn cho khch hng. Ngy nay, s truyn thng trong k

nguyn s thng bao gm nn c tn hao trc khi truyn. Mc nh hng ca

nn trn d liu nhng s cp phn sau.

Trong qu trnh truyn v phn bnh c nhng watermark, khng ch c

mo dng do nn tc ng vo nh ch, m cn do li trong qu trnh truyn v

trong qu trnh x l nh. c bit l nhng php bin i hnh hc nh l php

xoay, chnh kch thc, ct b th tht s l gy hi cho nh nhng. Tt c cc thao

tc tc ng ln d liu nh nhng watermarkc xem nh l thao tc tn cng

ln thng tin nhng. Nhng s thay i xy ra trong sut qu trnh x l nh thng

thng c gi l tn cng trng khp ngu nhin, trong khi cc lai tn cngnhm lm yu, loi b hay thay i watermark th c gi l tn cng c .

Hnh 2.3 th hin m hnh phn phi nh sau khi c nhng thng tin

watermark.

Bin i nh(FFT, DCT,

DWT, ..

k s

nh gcCc h sbin i

Bin i ngcnh (FFT, DCT,

DWT, ..

nh saunhng

watermark

Chun b dliu, sp xp

li

B Nhngd liu

Key

H thngnhn

(HVS)


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Hnh 2.3 M hnh phn phi nh watermarked

2.1.3 Giai on trch

Sau khi nh c nhng watermarkv tri qua nhng mo dng trong giai

an phn b (cc lai tn cng). Mt ai mun trch k sc nhng t d liu

ch. c th l ngi nhng watermark; khch hng nhn nh, hoc k th ba

(khng cphp). Trong trng hp u, kha b mt dng nhng watermark

v cnh gc c s dng, iu ny to iu kin thun li rt ln cho h thng

watermarking v vic xc nh watermark tng i d lm. Ta gi h thng xc

nhn s dng cha kha b mt (c nhn) v nh gc l h thng watermarking non-

oblivious, non-blind, private.

Nhng trng hp khc khng s dng cha kha c nhn hoc nh gc

trong qu trnh trch thng ip. H thng watermarking ny gi l h thng

watermarking kha cng khai (public key).

Gn y, s watermarking bt i xng [20] c a ra s dng nhng

cha kha khc nhau cho vic nhng v xc nh watermark.

S watermarking cho php trch thng ip khng i hi nh gc c

gi l s watermarking m (blind). Phng php trch hoc xc nh da vo

Nn c

tn hao

nhsau khi

nhng

JPEG JPEG2000

Truy n nhNhiu, x l

nh

Cc t n

cng

(collusion

Ci thin nhct, xoay

Cc thutton bit

Cc

watermark

ng dng

Cc thit b phthin cng cng

nh sautn cng

Kho cng khai


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mt vi d liu hoc c tnh xut pht t nh gc c gi l thut ton

watermarking bn m (semi-blind).

Tm li c 3 phng php trch watermark da vo mc s dng nh gc

Blind hoc obvilious; Semi-blind; Non blind hoc non-oblivious.

Hnh 2.4 M hnh trch nh watermark

Cn da vo s cn thit ca loi kha dng trong qu trnh watermarking ta

phn bit theo:

Kha c nhn (private key); Kha cng khai (public key); Cc s watermarking bt i xng khc.

D liu khiphc

Bin i thun(FFT, DCT,

DWT, ..

Trch

k s

nh gcCc h sbin i

(nh gc)

Trch d liu

H thngnhn

(HVS)

Cc h sbin i

b modng

Bi n i thun(FFT, DCT,

DWT, ..

nh b tncng


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2.1.4 Giai on quyt nh

Trong giai on ny, h thng watermarking phn tch d liu trch. Ph

thuc vo loi ng dng v bn cht ca thng ip, c thc mt s yu cu khcnhau.

Trong ng dng chng sao chp, ng ra ca h thng watermarking c th

sp xp cc cu tr li tn gin n phc tp. Trng hp n gin nht, kt qu

ch l s biu th c hay khng du (mark) ca ngi ch bn quyn c tm thy

trong d liu nh nhn c. Trong cc h thng phc tp th phi trch ra logo

c nhng hay l thng tin bn quyn nguyn bn c t vo d liu nh ch.

Thng thng cnphi c thng s nhm nh gi s ging nhau (mi tng

quan) gia d liu gc watermark (W) v d liu watermark sau khi trch c (W*)

l tng quan chun ha ca chui ngu nhin gi:

WW

WW

.

.*

*

=

Hoc khong cch Hamming ca thng ip nh phn, i {-1, 1},

i

N

iN .*

=

trch v xc nh s tn ti ca watermark, thng da vo ngng cho

trc , nu th watermark c xc nh, ngc li th watermark khng

c tm thy trong nh.

Watermark

trchLy ngng

Quyt nhtnh tng quan, so snh

K s K strch

Hnh 2.5 M hnh giai on pht hin watermark


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Hnh 2.5 trnh by m hnh giai on pht hin watermark.

Cc ng dng giu thng tin v nhn nh th c trch thng ip gc c

nhng. Do khng cho php thay i thng ip, v th vic s dng m sa i btbuc dng cho loi ng dng ny khi c s tc ng hay mo dng thng tin nhng.

o S watermarking cho vic xc nhn nh v xc minh s ton vn d liuch cn biu th cu tr li yes/ no s gi mo d liu nh.

o S watermarking cn trch thng tin nhng dng t s nh tn hiu trnnhiu nh gi cht lng ca nh watermark, PSNR (peak to signal to

noise ratio) nh ngha nh sau:

)(255

log102

10 dBMSE

PSNR = (2.1)

Vi sai s trung bnh qun phng MSE (mean square error) c nh

ngha l :

( )2

1 1

'

',,

1

= =

=

m

i

m

j

jijihh

mnMSE (2.2)

Trong {hi, j

} v {hi, j

} l cc mc xm ca nhng pixels trong nh gc v nh

watermark tng ng. PSRN cng ln hn th cho cht lng nh tt hn. Tng

qut, mt nh du watermarkc th chp nhn c nu PSNR ln hn 30dB.

Thm vo tng quan c dng nh gi bn vng ca k thut

watermarking.

( )( )

( ) ( )22

y-yx-x

y-yx-xnCorrelatio

= (2.3)

2.2. Mi quan h vi mt m (crytography)Php ghi mt m ln nh c coi l mt k thut m ha cho vic truyn d

liu qua knh truyn thng trong iu kin m k th ba khng thc v hiu d

liu. K thut mt m xo trn d liu, nu b pht hin, ngi c cng khng th


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hiu d liu. V vy, mt m tr thnh mt trong nhng cng c chnh cho s

bo mt, iu khin truy xut, xc nhn nh, k s v s an ton cho thng ip.

Ngy trang k (steganography) chn thng ip b mt dng che giu dliu, in hnh l trong nh s hoc l lung video, che giu nh nhiu. Nu khng

c ng kha, gn nh l khng th trch thng ip c giu hoc l khng xc

nh c s c mt ca n. Steganography v bn cht l ngy trang mt thng

ip che giu s tn ti ca n v lm cho n khng thy c, ngha l che giu s

tht l c mt thng ip c gi km theo. Mt thng ip m ha c th gy ra

s nghi ngnhng vi mt thng ip khng thy c th khng? iu ny lm

tng thm gnh nng cho nhng ngi gii cc mt m, v phi xem xt nhng d

liu c v nh khng nghi ngg l c nhng nhng thng ip steganography hay

khng?

2.3. H thng nhn ngi (HVS: human visual system)Vng mc ca chng ta phn tch tn hiu cm nhn c thnh nhng thnh

phn khc nhau, mi thnh phn tc ng vo v no qua nhng knh ring bit.

Mi thnh phn c nhng c tnh sau :

Vng khng gian trong nh. Tn s ca nh. Hng ca tn hiu (ngang, dc, cho).

Khi hai tn hiu c nhng c tnh thnh phn tng t nhau, chng tc ng

cng mt knh trn v no nhng mc nh hng ty thuc vo hiu ng mt n.

Mt ns xut hin khi ngngpht hin c tng ln do s c mt ca mt tn

hiu khc mnh hn trn nn c nhiu tn hiu mang c tnh tng t nhau.

Mt n tng phn: kh nng pht hin mt tn hiu i vi s hindin ca mt tn hiu khc.


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nhy tn (frequency sensitivity): nhy ca mt ngi vi ccsng sin tn s khc nhau.

nhy chi (luminance sensitivity) : ngng pht hin nhiu trongvng phng.

Ngng JND (Just-noticeable-difference): ngng da trn bt k sthay i h s tng ng khng nhn thy.

M hnh cm nhn trong min tn s cth c thc hin nhsau:

t nhng b lc thng di hng vo nh gc t mt lng nnglng nh c trong mi thnh phn tn s khng gian. Tnh ton

ngng mt n (masking threshold) da trn nng lng cc b.

iu chnh t l nng lng watermark trong mi thnh phn (gi swatermarkc phn tch cng mt t l) n di ngng mt n.

thit k h thng watermarking s ti u, taphi khai thc HVS

2.3.1 nhy tng phn (contrast sensitivity)

nhy tng phn (cn gi l cng nhy) m t hay bo trc tnh

trng c th thy c nhiu. Gi s rng mt p ng vi chi (vng phng) I0,

mc ch l xc nh s khc bit chi ti thiu I gia vng trung tm v vng

xung quanh, s khc bit ti thiu ny gi l JND.

Hnh 2.6: M t nhy tng phn

Mi quan h gia cng xung quanh I v tng phn tng ng ti

thiu c nh ngha l I/I. Vi nhy gia thang, cng tngphn xp


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x gi tr hng s, trong khi vng cao v thp cng tng phn li tng ln,

ngha l JND ln hn. Phn s tngphn xp x trung tm gi l phn s Weber-

Fencher (theo nh lut Weber). Phn s Weber-Fencher nm trong khong 1-3%,

ngha l chi JND tm khong bng 0.01 0.03 ln chi xung quanh.

Hnh 2.7: M t hm mt chi.

2.3.2 nhy tn s khng gian (spatial frequency sensitivity)

Tn s khng gian c nh hng quan trng n nhy HVS. Mt ngi

th nhy hn vi nhiu tn s thp. Ngc li nhiu tn s cao th t nhy hn. p

ng tn s ca HVS th khng u, l do nhiu hin tng gy ra v d nh vt

qu nhy trong vng tng phn cao, c bit l bin. Mt hin tng c

gi l hiu ng Mach band (c minh ha trong hnh 2.7). nh trnh by s thay

i chi theo chiu ngang. Mc d mi bc c mt chi u, chng ta vn

cm thy cng bn trong mi bc l khng u, ngha l sng hn bn tri

v ti hn bnphi bin gia hai bc.

Hnh 2.8:Hiu ng Mach band


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2.3.3 Hiu ng mt n (Masking)

Khi c thnh phn l c giu trong nh, kt cu mt n c dng nhm

gim kh nng cm nhn nhng thnh phn l , ngha l s giu thng ip

nhng im t nhy i vi mt ngi. Cng vi mt nh b mo dng do nhiu,

chng ta c th nhn thy rng nhiu th d nhn bit trong vng phng hn l vng

nhm.

2.3.4 Tm tt v HVS

- Tn s cao mt ngi kh cm nhn hn l tn s thp.

- nhng mt thng tin trong watermarkcng bn cng tt, chng taphinhng n mc thp hn JND. Ngha l phi lu tm n nhy tngphn v

hiu ng mt n.

- HVS t nhy i vi nhng thay i trong vng nh c sng cao. Ta c

th giu i cc watermarkti nhng vng ny.Ngoi ra, mt ngi li t nhy vi

knh mu xanh bin nn watermarkc nhng trong knh mu ny c th c nng

lng cao hn so vi watermarkc nhng trong knh mu khc.

2.4. Cc thuc tnh cn c ca watermark

Ty theo ng dng v mc ch watermarking, nhng iu kin cn khc

nhau pht sinh nhng vn thit k khc nhau bao gm nhng iu kin sau:

2.4.1 Tnh bo mt (Security)

Trong hu ht cc ng dng, chng hn nh bo v bn quyn, tnh bo mt

ca thng tin nhng cn phi c bo m, tnh hiu qu ca mt thut ton khng

th da vo gi nh l nhng k tn cng khng bit c qu trnh nhng

watermark vo ti liu a phng tin c thc hin nh th no. Tuy nhin, gi

nh li c dng nh gi an ton ca cc sn phm thng mi s dng

watermarking c gi tr trn th trng. Vi mt ng dng watermarking, mt khi

bit c cch lm vic ca b nhng v b d, k tn cng s d dng loi b


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watermark. Hn na, mt s k thut s dng d liu gc trong qu trnh d tm v

rt trch (thng th cc gii php loi ny khng kh thi trong thc t).

i vi cc h thng i hi tnh an ton, th mt kha bo mt c thcs dng trong qu trnh nhng v trch. Hai mc bo mt c thc xc nh.

Vi mc bo mt cao nht, nhng ngi s dng khng c php khng th

c, gii m watermark nhng cng nh khng th pht hin d liu c cha ng

watermark hay khng, v thng tin nhng khng thc c nu khng c kha b

mt.

Trong mt s s c thng dng nhiu watermark cng vi c kha b

mt (secret key) v kha cng khai (public key). C th ni mt hoc mt vi publickey vi private key v nhng vo public/private watermark.

2.4.2 Tnh khng nhn thy (Invisibility)

Mt trong nhng iu kin quan trng l tnh trong sut hay khng nhn thy

ca watermark v c tnh ny c lp vi cc mc ch v cc ng dng khc

nhau. Thnh phn l xut hin trong qu trnh x l watermarking khng nhng gy

phin phc khng mong mun m cn lm suy gim hay ph hy cc gi tr thng

mi ca d liu. V vy, mt iu quan trng l thit k watermark khai thc cc

hiu qu cm nhn ca mt ngi cng nh ca tai ngi cc i ha nng

lng watermark m vn khng vt qu mc ngng cm nhn c. C hai vn

lin quan, mt l nhng nh gi mc bin dng ca d liu v nhng qu

trnh x l tc ng n d liu nhng watermark, chng hn nh trong k thut

watermarking i vi nh, mc cm nhn ca watermark s tng ln khi nh

c phng i ln.

V hnh i vi mt: Nhng nh nghin cu gn y c gng giuwatermark sao cho n kh b pht hin(khng hin th trn nn nh) v khng

nh hng nhiu n cht lng mt cng nh ni dung d liu cha. Tuy

nhin, yu cu ny mu thun vi mt s yu cu khc, chng hn nh tnh bn


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vng l mt yu cu quan trng khi i u vi cc thao tc tn cng

watermarking, c bit l thut ton nn c tn hao. Thng thng, cc bit t c

ngha i vi mt ngi sc u tin dng cha watermark. t c

mc ch ny, chng ta phi kho st cc tnh cht ca h thng mt ngi HVS

(i vi nh) v h thng thnh gic con ngi HAS (i vi m thanh) trong

qu trnh nhng v d tm watermark.

V hnh i vi thng k: Mt c nhn bt hp php khng th d tmwatermark bng cc phng php thng k. V d nhiu tc phm k thut s

c nhng cng mt watermark sao cho khi thc hin tn cng da trn thng

k th khng ti no trch c watermark. Mt gii php kh thi l s dng

watermark ph thuc ni dung.

2.4.3 Tnh bn vng (Robustness)Ty thuc vo cc mc ch ng dng ca h thng, bn vng mong

mun nh hng n qu trnh thit k.

Thng thng, mt watermark bn vng phi i u c vi cc php

bin i, bao gm nhng php bin i tn hiu ph bin nh php bin i A/D,D/A, nn c tn hao. i vi nh s v video, watermark phi chng li c cc

php x l nh ph bin nh php lc, co gin nh, ci tin tng phn, xn,

quay nh, hoc nn nh.

V d, trong k thut watermarking dng cho nh, nu chng ta cn k thut

bn vng vi nn JPEG t l cao (loi b trn nh nhng thng tin t c tnh nhn

thy nht lm gim kch thc ca nh m vn m bo c cht lng nh) th

vic dng mt k thut nhng trong min bin i c hiu qu hn l mt k thutnhng trong min khng gian. Tng t nh vy, nu k thut thch ng vi nhng

bin i hnh hc nh l php xoay, chnh kch thc khng theo t l th gii thut

thc hin trong min khng gian ph hp hn.


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Ngoi ra, ngi ta cn nhn thy rng, tnh bn vng ch c tht c nu

watermarkc t nhng vng c ngha nhn thy nhiu nht ca mt bc

nh. Tuy nhin, watermark s kh b pht hin nu nh n c t nhng vng t

c ngha nhn thy nht. Tnh phc tp ca k thut watermarking th hin hai

tnh cht mu thun ln nhau ny.

Cn lu rng, tnh bn vng tht s bao gm hai vn ring bit l:

- Watermark phi tn ti trong d liu d c xy ra mo dng hay khng.

- Liu my d tm watermark c pht hin ra n hay khng.[

2.4.4

Kh nng ti thng tin (Payload capacity)Lng thng tin c th cha trong watermark ty thuc vo cc ng dng

khc nhau. Chng hn, i vi ng dng chng sao chp, thng tin watermark ch

cn mt bit l .

2.4.5 Chng minh quyn shu tht s c th thc hin quyn shu hp php, cn phi xc nh ai l ngi

nhng watermark vo d liu trc tin trong trng hp c nhiu watermarkc

nhng. iu ny c th thc hin thng qua vic t ra cc trngi thit k nh tnh

kho ca watermark,...

2.4.6 Tnh linh hot (activity)Trong cc ng dng thng mi, chi ph tnh ton cho vic nhng v trch rt

quan trng. Tuy nhin, trong mt sng dng, vic chn watermark ch c th thc

hin mt ln v offline. Do , chi ph nhng c th khng quan trng bng chi ph

d, vn thng phi xy ra theo thi gian thc. Hn na, chng ta bit rng tc

my tnh tng xp x gp i sau 18 thng, nhng g c tnh ton khng tha ng

ngy nay c th nhanh chng trthnh hin thc. Ngi ta mong i vic thit k

mt b d watermark tng thch vi mi th h my vi tnh. V d th h th nht

ca b d c th c chi ph tnh ton r nhng khng ng tin cy bng b d th


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h tip theo (c th c nhiu tnh ton hn) x l vi cc vn nh cc bin

dng hnh hc.

2.4.7 Kh nng ci tin v nhng nhiu watermarkCn phi cho php nhng nhiu watermark khc nhau trong cng mt nh

sao cho mi watermark c th d c bi mt ngi dng c cp quyn. c

trng ny rt hu dng trong cc ng dng du vn tay, trong thuc tnh tc

quyn c truyn t ngi s hu tc phm n cc tc phm khc. Hn na

chng ta c th ngn ngi khc thc hin watermarking cho mt tc phm c

ng du. Trong mt s trng hp, vic chnh sa hoc thay i mt watermark

sau khi nhng l cn thit. v d, trong trng hp cc a DVD, mt watermark c

thc dng ch cho s lng cc bn sao chp c php. Ngay sau khi vic

sao chp c thc hin, watermark trn a gc sc thay i gim s ln

cho php sao chp xung. Vic chnh sa watermark c th thc hin c bng

cch loi b watermark th nht v sau thm vo mt watermark mi hoc chn

mt watermark th hai sao cho c hai u c thc c, nhng chng ln ln

nhau. Kh nng th hai c chn la nhiu hn bi v mt watermark m c th

xo c th hin nhin khng phi l mt watermark bn vng v d dng b gimo.

2.4.8 Khi phc watermark cn hay khng cn dliu gcK thut watermarking s dng d liu gc trong qu trnh khi phc lm

tng tnh bn vng hn khng ch vi cc loi mo dng nh nhiu m cn vi cc

loi mo dng trong cc thao tc x l hnh hc v n cho php pht hin v o

ngc nhng mo dng ny. Trong nhiu ng dng, nh gim st hoc theo di d

liu, th vic truy xut d liu gc l khng th. Trong nhng ng dng khc nh

ng dng watermarking trong video, khng thc t khi dng d liu gc bi v phi

x l mt lng d liu ln.


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Trong khi nhng k thut watermarking nhng giai on u cn dng d

liu gc trong qu trnh khi phc th ngy nay c khuynh hng xy dng cc k

thut watermarking khng cn d liu gc khi khi phc (s dng key hoc ch mt

phn c trng ca d liu gc). V k thut ny c thc s dng trong nhiu

ng dng.

2.4.9 Tnh phthngWatermark s nn p dng cho c ba phng tin truyn thng ang c

ch l m thanh, hnh nh, video. iu ny c kh nng gip ch trong vn

watermarking cho nhng sn phm truyn thng a phng tin. c tnh ny cng

dn ti s thc thi ton b thut ton watermarking cho c m thanh, hnh nh,

video trn cng mt phn cng.

2.4.10Tnh r rngVic phc hi watermark nn xc minh mt cch r rng ch s hu.

Watermark khng nn cn n bt k s phin dch no nh tm csd liu cc

b m phin dch watermark nu bn thn n khng l chun quc t.

2.4.11 tin cy trong qu trnh d tm v rt trchNgay c khi khng c cc tn cng cng nh cc bin dng tn hiu, kh

nng khng d c watermark nhng (li phnh sai) hoc d sai watermark

khi m n khng tn ti trong thc t (khng nh sai) phi rt nh. Thng thng,

cc thut ton thng k cbn khng c kh khn g trong vic p ng nhng yu

cu ny. Tuy nhin mt kh nng nh vy phi c a ln hng u nu ng

dng watermarking lin quan n lut php, v c nh vy mi to s tin cy chc

chn cho cc phn quyt cui cng...

2.4.12Chng li sxuyn tcWatermark c th b tn cng bng cc php x l tn hiu vi nh duy

nht l loi b watermark. Lm sao watermark chng li s xuyn tc nh th


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cng l mt vn ng quan tm. y l mt s cch c trin vng t c

iu ny:

- Watermark kn: Mt watermark kn trong b gii m yu cu mt sthng tin v ti liu cha c watermark hoc chui nhiu gi ngu nhin cu

thnh watermark vn chc bit r bi ngi gi v ngi nhn.

-B m ho - gii m btng b: Nu vic loi b watermark yu cu o

ngc qu trnh m ha th chng ta nn lm cho b m ha trnn cng phc tp

cng tt, c bit nu watermark chc p t mt ln. Tuy nhin, nu nhng b

gii m phi chy trong thi gian thc th qu trnh gii m nn n gin hn qu

trnh m ha.

2.4.13Tnh kho ca watermarkMc d tnh bn vng thng c ch ra nh mt yu cu chnh cn phi

c, nhng mi quan tm ln li tp trung vo tnh kho ca watermark. Thut

ng kh o c dng vi nhng ngha khc nhau, ngha t nhin nht c

nh ngha l mt watermarkgi l kho nu cc ngi dng c cp quyn c

th xo n khi ti liu. Trong nhiu ng dng, tnh kho ny c th l mt c

trng mong i, bi v n c th cho php thay i tnh trng ca mt ti liu cho

trc theo lch s ca n m khng cn phi n qu nhiu bit thng tin trong n.

Tnh kho ca watermark cn c nh ngha theo cch khc, l kh

nng lm mt hiu lc tha nhn quyn shu c h trbi watermarking bng

cch s dng k thut cng ngho o li quy trnh watermarking. Mt m

hnh watermarking mun s dng thnh cng trong ng dng bo v quyn shu

th phi p ng c tnh khng kho ca watermark. Chng ta c th ni rng

mt watermark l kh o nu n c th to ra mt watermark ngc (false

watermark) v mt ti liu gi mo ti liu gc sao cho khi nhng false watermark

vo n, ta c th thu c mt ti liu ging hay gn ging vi ti liu gc c

ng du.


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Mt s tc gi [20] ch ra rng cc m hnh watermarking kho hay ta

kho t c dng trong cc ng dng thc t.

2.4.14Ti thiu ha sthay i cc pixelKhi thc hin watermark hnh nh cht lng cao v cc tc phm ngh

thut, s lng cc pixel ci bin nn l nh nht.

2.4.15Ti thiu ha scan thip ca con ngiTy thuc vo tng ng dng, k thut watermarking c nhng yu cu khc

nhau. Mt trong nhng thch thc ca nhng ngi nghin cu v lnh vc ny l

cc yu cu trong k thut ny thng mu thun vi nhau. Nhng yu cu lit k

trn l nhng yu cu thng thng m hu ht cc ng dng thc t phi t c.

Tm li, trong mt h thng watermarking, thng th cc iu kin ny c

mi quan h vi nhau. Nu nh mt h sc s dng nhm thay i mc

nhng thng tin watermark trong d liu nhng th h s ny c tm quan trng nh

hng n cht lng d liu sau nhng cng nh tnh bn vng ca h thng. C

th, khi h sc trng cho mc nhng cng ln th h thng cng bn vng,

nhng tc ng bn ngoi ln nh nhng snh hng xu n cht lng nh. Mt

khc, lng thng tin nhng cng nhiu th thng tin nh gc c th b thay i

nhiu. Trong nhiu trng hp nhng nhiu bit cho kh nng nh gi tt hn so

vi vic nhng mt bit.

Trong thc t, ba yu cu v tnh bo mt, tnh bn vng, v tnh khng nhn

thy c th xem nh mt dng tam gic, ngha l nu ci ny b bin i th hai ci

kia cng bnh hng.

Khng nhn thyBo mt

Bn vng

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2 Bai Toan Watermarking