基于皮肤模板和改进HMM的自动人脸识别系统(3)
theellipse
are
indicatedby
across.
Fig.4
Detect
facewithdifferentorientations
图4不同方向的人脸
2
ImprovedHMMforfacerecognition
2.1
HMMbasedfacerecognition
TheHMMis
a
finitesetofstates.eachofwhichisassociat-
edwith
a
multidimensionalprobabilitydistribution[131.Transi.
tionsamongthe
states
are
govemedby
aset
ofprobabilities
called
transitionprobabilities.Ina
particular
state
anoutcome
or
ohser-
vation
can
begeneratedaccording
to
the
associatedprobability
distribution.InimplementationofHMMbasedfacerecognition,a
faceimageis
dividedinto
a
seriesofoverlappingimageblocks.
%e
observationsequencefortheHMM
can
be
generatedby
con-
catenating
the
observation
vectors
extractedfrom
each
image
block.Nefian
et
al[10]apply2D
DCT
on
each
image
blockand
onlythelowfrequencycoefficientsarie
extractedto
produceohser-
vationvectors.7nleobservationsequenceextractedfrom
atest
im-
ageisinput
toallofthetrainedHMMs
associatedwitheachper-
son
andtheconditionalprobabilitygivenbyeachHMMisalso
calculated.ne
identityoftheinputfaceis
determinedbythe
HMMwhich
gives
thehighest
probability.SinceDWT
iswidely
believedtobeadvantageous
over
DCTinrepresentingfeatures,
DWTwillbeusedforobservationvectorextractioninthe
next
section.2.2
ImprovedHMMusingDWTWaveletahavemany
advantages
over
othermathematicaltransformssuchas
theFourier
transfo咖or
DCT.Inusing
wave-
lets
to
produceobservationvectors,thealgorithmworkbyscan
万
方数据ningthefaceimagefromleft
to
right
andtoptobottomusing
a
P
x
Lsized
blockandperformingwaveletmulti resolutionanalysis
at
each
imageblock.Theimageblockisdecomposed
to
a
certain
levelandthecoefficients
at
thelowestlevel
or
theenergyofthe
subbandsare
extracted
to
formobservationvectorsfortheHMM.
2D
J_levelwaveletdecompositiononall
image,[P,L]represents
theimageby3J+1subbands
[口J,{司,孝,霉}.『=1,…,.,],whereD,is
a
lowresolution
approximationoftheoriginalimage,and钟arc
detail
images
con-
miningthedetailsofthe
image
at
different
scales(巧)and
often—
rations(&).Waveletcoefficients
in谚,毋and霉correspond
to
vertical
hi.ghfrequencies(horizontaledges),horizontalhighfre-
quencies(verticaledges),andhighfrequencies
inbeth
direc-
tions,respectively.Fig5showsthe2levelwaveletdecompositionofanimagerepresenting
an
eye.Forthis2levelsdecomposition
in
total7
subbands{a2,d:,《,前,以,《,d;}are
generated.
Notethattheimagehasbeenrescaledforvisualinspection.Inthispaper,theobservation
vector
is
producedbyconcatenating
each
lOW
ofaJ.Oncetheobservation
vectors
are
extracted,theobservationsequences
are
generatedandgiven
to
theHMMfor
processing.
(a)An
eye
imageblock蚴Wavelet
decomposition
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