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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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