Seeing With OpenCV

SeeingWithOpenCV
FollowThatFace!
. o do
d e s c r i b e di n l a s t m o n t h ' sa r t i c l e T
rithmin moredetail
C a m s h i f tw a s o r i g i n a l l yd e v e l o p e d
f o r h a n d s - f r egea m i n g .l t ' s d e s i g n e dt o
"lightweight"
so the
be veryfast and
computer can do other tasks while
t r a c k i n g .S i n c ei t w a s d e v e l o p e da s a
t l s oh a sa n
g a m i n gi n t e r f a c eC
, a m s h i fa
( l i m i t e d )a b i l i t y t o d e t e c t c h a n g e si n
view,
s h o u l d e ro, r t u r n st o w a r d s
t h e f r o n t a lf a c ed e t e c t o rw i l l n o l o n g e r
r i o t c r t i t c n v n r r ' dn e p dt o h a n d l et h a t
s i t u a t i o na, s w e l l .
Fortunately,OpenCV includes
s p e c i a l i z e dc o d e f o r t r a c k i n g a f a c e
e f f i c i e n t l y ,u s i n g c o n t i n u i t y b e t w e e n
f r a m e st o h e l p f i n d t h e b e s t m a t c hf o r
t h e f a c ei t ' sf o l l o w i n g .
T h e a l g o r i t h mt h a t O p e n C Vu s e s
for face tracking is called Camshift
C a m s h i f tu s e s c o l o r i n f o r m a t i o n ,b u t
r a t h e rt h a n r e l y i n go n a s i n g l ec o l o r ,i t
t r a c k sa c o m b i n a t i o no f c o l o r s .S i n c ei t
tracks by color, it can follow a face
t h r o u g h o r i e n t a t i o nc h a n g e st h a t t h e
. h es i d e b a r ,
H a a rd e t e c t o rc a n ' th a n d l eT
h e a d p o s i t i o n s, u c ha s t i l t i n gt h e h e a d
t o o n e s i d e .C o u l dy o u u s e t h a t a b i l i t y
t o c o m m u n i c a t ew i t h y o u r r o b o t ?
"Come
M a y b et w o f a s t h e a dt i l t s m e a n
h e r e ,r o b o t ! "
F i g u r e 1 s h o w s O p e n C V ' sf a c e
t r a c k e ri n a c t i o n- f o l l o w i n ga f a c e a s
i t t i l t s t o o n e s i d ea n d d u r i n g a t u r n t o
p r o fi l e .
Lastmonth'sarticlein this t h a t , h o w e v e ry, o u ' d n e e dt o d e c i d ei f
how to tt hh ee fsaacme yeofua dc ee .t el fc tt eh dei ndeeat ec hcftroarmf i nedi ss
seriesexplained
and configurem o r e t h a n o n e f a c e i n a f r a m e ,y o u ' d
implement
n e e d t o d e c i d ew h i c h d e t e c t i o ni s t h e
Thismonth, o n e y o u ' r e t r a c k i n g . F i n a l l y ,i f a
facedetection.
l'll showyou howto use p e r s o n ' s h e a d t i l t s t o wp raorfdi lse o n e
to trackataceonce
OpenCV
it.
you'vedetected
FaceTrackingin
OpenCV
T r a c k i n ga f a c e i s m o r e d r f f i c u l t
t h a n t r a c k i n ga s t r o n g l y - c o l o r eodb j e c t .
S k i nr e f l e c t st h e a m b i e n tl i g h ti n s u b t l e ,
c h a n g i n g w a y s a s a p e r s o n ' sh e a d
t u r n so r t i l t s .
I n p r i n c i p l ey, o u c o u l dt r a c ka f a c e
b y l o c a t i n gi t o v e r a n d o v e r I n e v e r y
frame, using the Haar detector
Jv
tvu
TheCamshiftDemo
T h e O p e n C V s a m p l e sd i r e c t o r Y
c o n t a i n sa p r o g r a m c a l l e d c a m s h r f t d e m o .Y o u c a n g e t s o m eg o o d h a n d s a n d a n i n t u i t i v ef e e l f o r
on experience
the Camshifa
t l g o r i t h mw i t h t h i s d e m o
"How
O p e n C V ' s p r o g r a m .H e r ea r e t h e s t e p sf o r d o i n g
Works," t h a t :
FaceTracker
e x p l a i n st h i s a l g o 1 ) P l u gi n a w e b c a m .
F I G U R E1 . O p e n C V ' s
face tracker in action, 2 ) L a u n c ht h e p r o g r a mc a l l e dc a m s h i f t It's able to follow a d e m o i n t h e s a m p l e sd i r e c t o r y .
face as it tilts to one
s i d e a n d d u r i n ga t u r n ;
t rectangle
3 ) U s ey o u r m o u s et o s e l e c a
to profile.
c e n t e r e dt i g h t l yo n y o u r f a c e .
yindow
4 ) C l i c k i n t h e v i d e o - d i s p l aw
and type the letter b. (The display
s h o u l dc h a n g et o l o o k s o m e t h i n gl i k e
t h e v i e w i n F i g u r e2 . )
2. Totunethe Camshiftparameters
FIGURE
smin drtd rmin" run the camshiftdemoI
programin the samplesdirectory.These
parametersare easierto set if you toggle
to the backprojectionview by clickingin
the view window, then typing b,
i l
36 srnvoo3.eoo7
t.t
- : e n C V ' sf a c et r a c k e ru s e sa n a l g o r i t h mc a l l e dC a m s h i f t , FIGURE
A. Two examplesof
' - ' : c o n s i s tos f f o u r s t e P s :
th€ color histogram that
Camshiftusesto represenla
face,
.z-:e a color histogramto representthe face.
in OPenCV.
, :- ate a "faceprobability"for each pixelin the incoming backprojection"
built-in
method
Ihere's
a
'-':'::
t h a t i m p l e m e n t si t , c a l l e d
^-.^^f
^n^^r,^l^^
..
.:_-_!!L^'
- 'i the location
of the face rectanglein eachvid e o f r a m e . c v ( d . c d o c K P r o l e c .
Figure C shows the
f a c e - p r o b a b i l i t yi m a g e i n
- : : , l a t e t h e s i z ea n d a n g l e ,
o n e v i d e o f r a m ea s C a m s h i f t
my face. Blackpixels
tracks
'row
'l :
each stePworks:
have the lowest probabilitY
white,the high.'eate a histogram.Camshift representsthe face it's v a l u g a n d
pixels
lie somewherein the middle.
est.
Gray
^
S
( a l s oc a l l e da b a r c h a r to) f c o l o rv a l u e s .
-.
a sa h i s t o g r a m
'a A shows two example histogramsproduced by the
- =^ rft demo programthat shipswith OpenCV.The heightof 3) Shift to a new location. With each new video frame,
"shifts"its estimateof the face location,keepingit
-- colored bar indicateshow many pixels in an image Camshift
"hue."
Hue is one of three valuesdescribing centeredover the area with the highestconcentrationof
: . ldve that
, e s c o l o r i n t h e H S V( H u e ,S a t u r a t i o nV,a l u e c) o l o r m o d e l . bright pixels in the face-probabilityimage.lt finds this new
' -ore on color and color models, see "Ihe World of f,ra"utlon
OVttarting at the previow to.u-tionand computing
the cenler of gravityof the face-probabilityvalueswithin a
Magazine,November'05,)
:' SERVO
- the imageregionrepresentedby the top histogram,a rectangle.lt then shifts the rectangleso it's right over the
^ h u e i s m o s tc o m m o n ,a n d a s l i g h t l ym o r e l a v e n d e h
r u e center of gravity. lt does this a few times to center the
a rectangle well. The OpenCV f unction cvcars rif - r )
shows
histogram
The
bottom
:-e next most common,
location.
:a in which the most common hue is the rightmostbin. implementsthe stepsfor shiftingto the new
with
to
correspond
rectangle
process
the
of
shifting
This
. rue is almos! but not quite, red,
"Mean
the center of gravityis based on an algorithmcalled
h i f t , " b y D o r i n C o m a n i c i u ,I n f a c t , C a m s h i f ts t a n d s f o r
-atculate face probability - simpler than it sounds! fhe S
" C o n t i n u o u sA
l yd a p t i v eM e a nS h i f t . "
::gram is created only once, at the start of tracking.
"face-probability"
value to
:e'wards, it's used to assigna
4) Calcutate size and angte. The OpenCV method is
that follow'
pixel
video
frames
the
in
-- rrrl6g€
"ContinuouslyAdaptive" and not just "Mean Shift"
Faceprobability"soundsterribly complicatedand heav- called
* athematical,
but it's neither!Here'show it works.FigureB because it also adjusts the size and angle of the face
- ^'s the barsfrom a histogramstackedone atop the other. rectangleeach time it shifts it, lt does this by selectingthe
':e' stacking
t h e m ,i t ' sc l e a rt h a t t h € r i g h t m o sbt a r a c c o u n t s s c a l e a n d o r i e n t a t i o n F I G U R E
C. The normal and face- :oout 457oof the pixels in the region.That meansthe that are the best fit to probabilityviewsas Camshift
tracks
:caoility that a pixel seleded randomlyfrom this region t h e f a c e - p r o b a b i l i t y m y f a c e . I n t h e f a c e - p r o b a b i l i t y
"faceprob- pixels inside the n€w view, black pixelshave the lowest
: - d fall into the rightmostbin is 45%.That'sthe
'- i,' for a pixelwith this hue.The samereasoningindicates
:: tf'e face probabilitYfor
'z rext histogram
b i nt o t h e
:-: ls about 2O7o,since it
-:rutts for about 207oof
stack's total
-h€ight.
- :: s all there isto it..
As new video frames
' .e, the hue value for
':. pixel is determined.
'': r that,the facehistogram
, ..;sed to assign a face
: c a b i l i t yt o t h e p i x e l .T h i s
"histogram
:cess is called
-:JRE B. To see what
..e probability" means/
t h e b a r si n
:grre stacking
: histogram on€ atoP
'-e other. The probability
. . ; o c i a t e dw i t h e a c hc o l o ri s
' z p e r c e n lt h a t c o l o r b a r
: : rtributes to the total
-:ght of this stack.
r;."ii."
iu.ii"si"
il:#2i1,"f"'f,lf,fil;f.',?
;?];?
%
3V
SERVOO3.2OO1
1- ////
Constants
2 const char * DISPLAY_WINDOUI= "Di-splayWindow";
3 #def ine OPniCV_R@I "C: /Proqrarn Files/@enCv/1.0"
A
5 ////
cLoi:p.l variables
* pvideoFramecopy = 0;
5 Ipllmage
1
B void main( inc argc, char** argTv )
g {
10
11
L2
13
L4
15
1 6
L]
fB
L9
CvRect. * pFaceRect = 0;
if(
linitAllO
) exitprogram(-1);
video frames until
a face
// Capture and display
// is detected
while( I )
{
/ / Ir,ok for a face in the next video frame
capEureVideoFrameO;
pFaceRect = detectFace(pvideoFramecopy);
20
2I
22
23
24
25
26
21
28
29
30
31
32
33
3 4
35
36
17
?A
// Showthe display image
cvSlor./Tfiage( DISPI,AY-WINffi,i. pVideoFrarreCoEl ) ;
if( (char)27==cvlvaitKey(1)) exitprogram(0);
loop when a face is detected
// *it
if(pFaceRect) break;
)
// inLtLalize tracking
startTracking(pVideoFrameCogl, pFaceRect);
/ / Track the detected face using Camshift
while( I )
{
CvBolD faceBox;
/ / dcr lhe next
video
^ . n t s 1 , r 6 l / i A a ^ F r. a a m e ( l ;
frame
39
40
/ / track the face in the new video frame
4L
faceBox = track(pVideoFrameCopy);
42
43
// outli-ne face ellipse
44
cvEllipseBox(pvideoFramecopEr,faceBox,
45
C V _ R G B ( 2 5 5 , 0 , 03) ., C V _ A A ,0 ) ;
pVideoFrarpCoEf );
46
cvShovulrnage(DISPIAY_WINDC&\I,
4'1
if( (char)27==cwlaitKq/(l) ) break;
4 8 )
49
50
exitProgram(0);
51 )
5) Adjustthe slidersfor sminand vmin
untilthe ellipseis well positioned
and
the background
is mostlyblack.
6) RepeatStep 4 to toggle back to
normal view, then use Camshiftto
trackyourface.
TuningCamshift
As mentioned above, Camshift
usesa combination
of colorsto track
faces. In the representation
that
Camshiftuses,color is undefinedfor
pixelsthat havea neutralshade(white,
38 sEnvoo3.9oo7
not onlymy face,but halfthe roomas
well!Thereasonfor the oversized
face
detectionis clearlyvisiblein the faceprobabilityview. Backgroundpixels
with a nearlyneutralshadecontributed
too muchnoisewhen vrnindfld smin
gray,or black).Color wereat theirdefaultvalues.
The middle and right views in
canbe computed
for
pixels that are
Figure2 showthe effectof increasing
almost neutral,but
first snj-n,then rzmin.In the right-hand
theircolorvaluesare view, noisy pixelshave been largely
unstable,and these eliminated,but the face region still
pixels contribute producesa strongsignal.Trackingis
noisethat interferes now quitegood,and the ellipseis well
with tracking.
oositioned.
Camshift uses
two parameters
smin ood vmin - to
screenout thisnoise.
These parameters
OoenCVincludessourcecodefor
camshiftdemo,
definethresholds
for
but it's not easy to
ignoringpixelsthat
adapt, since it combinesuser-input
are too close to
handlersand view togglingwith the
neutral.rrminsetsthe
stepsfor facetracking.
"almost
for
lf you're programmingin C++,
threshold
ratherthan in C, you could use the
black,"and smi-nfor
"almostgray."
class,defined in
These CvCamShiftTracker
Again,however,this class
two thresholdlevels cvaux.hpp.
is fairly complex, with many
will need to be
to C++
interfaces,
and is onlyavailable
adjusted for your
programmers.
setup to get good
results
withCamshift.
To make the Camshifttracker
Camshift also moreaccessible,
l'vewrittena wrapper
usesa third parame- for it in C with four maininterfaces:
ter called r,rmax,to
1) createTrackerO pre-allocates
set a thresholdfor
pixelsthat are too
internaldatastructures.
bright. But smin
has the side effect 2) releaselracker ( ) releasesthese
of also eliminating resources.
pixelsthat are close
3) startrrackinsO initiatestracking
to white, so you
f rom an image plus a rectangular
shouldn't need to
tweak r,'maxto get
region.
FIGURE3. The main
program listing for
detectinga face in a
live video stream,then
tracking it using the
Camshift
wrapperAPl.
good results.
The easiestway to selectgood
valuesfor your setupis with camshiftdemo.As suggested
in the preceding
section,it's easierto set theseif you
toggle the viewingmode by clicking
the view window and typingb. (This
viewis the calledthe "facealternative
"backprojection"
probability,"
view.
or
It'sexplained
in the sidebar.)
Figure 2 shows the effect of
Initially,
in the
adjusting
sminondvrnj-n.
firstframe,thesewereat theirdefault
values.At these levels,Camshiftdisplayeda verylargeellipse
that included
Ihe SimpleCamshift
Wrapper
I
J
'
)
J
J
G
aa
a
t
:
:s
4) tracko tracksthe object in this
region f rom f rame to f rame using
Camshift.
r'6
!:lo
r:l
Therearetwo additional
interfaces
vmin and
for settingthe parameters
smrn:
!l
.1"5 I
1) setvmino
2) setsrnin()
0t
rraa
-
TheCamshiftwrapperis onlineat
www.cog notics.com/opencv/down
loads/camshift-wra pper/i ndex.html.
I
FIGURE
4. ThehelperfunctionsinirAll ( )
and exitProgramO handle program
initializationand cleanup.
Face
Combinins
-and
Detection Tracking
!
\
'l
or
io
Jt
he
you neededto
In camshiftdemo,
manuallyinitializetrackingwith the
mouse.For a roboticsapplication,it
wouldbe muchnicerto initialize
tracking automatically,
usinga face detection that the Haardetectorreturned.
(Seelastmonth'sarticlefor detailson
implementing
facedetection.)
Thissectionshowshow to do that
usingthe Camshiftwrapperdescribed
above.The programdescribedhere
detectsa face in a livevideostream,
then tracks it with Camshift.The
sourcefor code for the complete
program,called"TrackFaces,"is also
availableonline at www.cognotics.
com/opencv/down loads/camshiftwrapper/index.html.
1
2
3
4
5
6
7
8
9
10
1tL2
13
I4
15
l6
L]
t-8
L9
20
21,
22
23
24
25
26
21
28
initAll
t-nt
o
t
if{ linicCapture{} ) return 0;
if ( !initFaceDet (OPn{CV_R@T
" /daLa/haarcascades/haarcascade_fronLaI f ace-def aulc . xn] " ) )
return 0;
Sfarr'rn mpqq^da fplls
rqpr how ro l-eoin and how tO exit
( \\ni*******************************************\n"
//
pfintf
"To
exit,
"then
inside
click
press
the
the
video
display,\n"
ESC key\n\n"
"Press <mJIER>to begin"
\\\n********************************************\n/ ),
fdof./cl-din\.
// CrcarP fhe disnlaw windOw
cvNamedwindow( DISPLAY_WINmW, I
/ / IntLialtze
n:nfr
rrol/i
);
t.racker
donE
r^ma
/ I '
if ( lcreateTracker(pVideoFramecopy) ) return 0;
//
Sef Camshift n:r:metefs
set\iinin(60);
setSmin(50);
return
1;
ne
rn
I ))
rry
.++
ker
)t e s
rese
krng
't - 'l-:' r
t hl s
r snl g
The Main Program
Figure3 showsthe mainprogram
listingfor detectinga face in a live
videostream,thentrackingit usingthe
CamshiftwrapperAPl.(Thisportionis
in TrackFaces.c
There
in the download.)
arethreemainprogramsegments:
1) Detecta face.
2) Startthe tracker.
3) Trackthe face.
1) Detect a face. Lines 15-27
molementa looo to examinevideo
'ramesuntil a face is detected.The
invokes
:all to capLureVideoFrameO
a helper method to bring in the
-ext videoframeand createa copyof
: (Recallfrom
Part1 of thisseries
that
: s neversafeto modifythe original
. Jeo image!)The working copy is
.:rred as pVideoPramecopy,
declared
:: rrne
6.
races
and
r-? aI
lown
html.
. Start the tracker.When a face is
:=:ected,
the codeexitsthisloop(line
- a and startsthe tracker(line 30),
',; ng it the facerectangle
from the
",:'detector.
tTGURE 5. The helper function
'::reVideoFrame0. At line '11,the
:all to cvFlipO flips the imageupside
or,vn if th" orisin fi"ld ir 0.
1 void exitProgram(int
code)
2 {
in this file
3
// Release resources allocated
4
cvDestroyWindow ( DISPI"AY-WINDOW) ;
5
cvReleaselmage ( &pvideoFrameCopy ) ;
6
1
in oLher projecc
// Release resources allocated
.l^eoa^nfrrroll.
B
9
closeFaceDet ( ) ;
-^t
^-^^m-^^t,^/ \ 10
lElgoJg1!aLAYr
\ / i
11
L2
exit (code) ;
L3 ]
files
with a
a rectangle
datatyperepresents
rotation angle. The call to
cvEJ-lipseBox0at lines44-45 draws
the ellipsedefinedby thisbox.
3) Trackthe face.Lines33-48contain
the face-tracking
loop.Eachcallto the
wrapper'strackO method(line 41)
invokesCamshiftto find the face
locationin the currentvideoframe.The
Camshiftresult is returnedas an
OpenCVdatatypecalledcveoxzn.This
HelperFunctions
In additionto the mainprogram,
1 rrnid
^ ^ n t s , , r 6 \ I i d a ^ t r r . . -a]neo
2 {
3
/ / Capture the next frame
4
lplIrnage * pvideoFrarne = nextVideoFrameo;
C
In\Iido^trr^mo
if/
I
owirDr^dr:ml-11.
6
'7
t
B
//
Q
it
f^
Pr
^!,.^^,,
v v v v y j
Llif1
L 2 \
fha
dicnlrrr
ufJPrsJ
im:aa
inrzorfinc
ir
if
nccdcd
lpVideoFrameCopy )
n\/i danFr^maf-^n/
J
I n
a
nnz
tvPl
rt
if(
Luuvl
-
r\r,'LLwJ
/
\
^\/r
y v r v v v r
i ^ ^ E r . m ar t 4 , , v ,
.\raro:f
nr/i
FTm:ao
uY!
rlaatr-:ma.an,,
y v 4 s ! v r r w , ! ! ! v t s J ,
6==pVideoFramecopy->origin
/errCci-Qi
\!
zc ln\/i
vvvev+g
0
dcotrr:mel
R
? \ .
) ;
) cvFlip(pvideoFrameCopy,0,0)
;
SERVO
03.e00739
detectFace ( )
. OpenCVon Sourceforge
http :// sourcef orge.na/ Proleds/
opencvllbrary
a,= .,
. Official OpenCVusergrouP
hllp: / / lech.groups.yahoo.com/
group/Open€V
"Computer
. G.R.Bradski,
videoface
trackingfor usein a perceptualuser
interface,"lntel TechnologyJournal,
Q9 1998.
. D. Comaniciuand P.Meer, "Robust
Analysisof FeatureSPaces:Color
fmageSegmentation,"CVPR,1997.
. The Simple Camshift \VraPPer
www.cognotics.com/opencv/down
loads/camshilt-wr apger/ index. htmI
. Sourcecode in this articlecan be
downloaded from:
www. cognotics. com/ ope ncv/ sewo
T r a c k F a c ec s a l s o c o n t a i n s h e l p e r
f u n c t i o n sf o r i n i t i a l i z a t i oann d c l e a n u p
i:,i t;---i )
a n d
e x t , t i l ql al
1I .
T h e s ea r e s h o w n i n F i g u r e4 .
A t l i n e 2 1 i n r - . : r . . .. 1 , t h e c a l l
Camshrft wraPper's
to
the
': i:
function Prea l l o c a t e st h e w r a p p e r ' si n t e r n a ld a t a
to prestructures1t'snot necessarV
ffClne 6. Th; detecipaceO function.
deliver image pixelsstarting at the
Themin-neighborsparameteris setto 6 to
b o t t o m , r a t h e rt h a n a t t h e t o p , o f t h e
false
a
detection.
the
chance
of
reduce
i m a g e T h er r ' : r . n f i e l di n d i c a t ewsh i c h
r o w o r d e ' t h e l p l l m a g eu s e s . S o m e
a l l o c a t ten e t r a c k i n gd a t a ,b u t d o i n gs o
O p e n C Vf u n c t i o n sw i l l o n l y w o r k c o r speeds the transition from face
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