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A Dynamic Texture-Based Approachto Recognition of Facial Actionsand Their Temporal Models

Sander Koelstra, Student Member, IEEE, Maja Pantic, Senior Member, IEEE, andIoannis (Yiannis) Patras, Member, IEEE

Abstract—In this work, we propose a dynamic texture-based approach to the recognition of facial Action Units (AUs, atomic facialgestures) and their temporal models (i.e., sequences of temporal segments: neutral, onset, apex, and offset) in near-frontal-view face

videos. Two approaches to modeling the dynamics and the appearance in the face region of an input video are compared: an extendedversion of Motion History Images and a novel method based on Nonrigid Registration using Free-Form Deformations (FFDs). The

extracted motion representation is used to derive motion orientation histogram descriptors in both the spatial and temporal domain. PerAU, a combination of discriminative, frame-based GentleBoost ensemble learners and dynamic, generative Hidden Markov Models

detects the presence of the AU in question and its temporal segments in an input image sequence. When tested for recognition of all27 lower and upper face AUs, occurring alone or in combination in 264 sequences from the MMI facial expression database, the

proposed method achieved an average event recognition accuracy of 89.2 percent for the MHI method and 94.3 percent for the FFDmethod. The generalization performance of the FFD method has been tested using the Cohn-Kanade database. Finally, we also

explored the performance on spontaneous expressions in the Sensitive Artificial Listener data set.

Index Terms—Facial image analysis, facial expression, dynamic texture, motion.

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1 INTRODUCTION

Awidely accepted prediction is that computingwillmoveto the background, weaving itself into the fabric of our

everyday living and projecting the human user into theforeground [1]. To realize this goal, next-generation comput-ing (a.k.a. pervasive computing, ambient intelligence, hu-man computing) will need to develop human-centered userinterfaces that respond readily to naturally occurring,multimodal human communication [24]. These interfaceswill need the capacity to perceive and understand humanusers’ intentions and emotions as communicated by socialand affective signals. Motivated by this vision of the future,automated analysis of nonverbal behavior, and especially offacial behavior, has attracted increasing attention in compu-ter vision, pattern recognition, and human-computer inter-action. Facial expression is one of the most cogent, naturallypreeminent means for human beings to communicateemotions, to clarify and stress what is said, to signalcomprehension, disagreement, and intentions, in brief, toregulate interactions with the environment and otherpersons in the vicinity [11]. Automatic analysis of facial

expressions therefore forms the essence of numerous next-generation-computing tools including affective computingtechnologies (i.e., proactive and affective user interfaces),learner-adaptive tutoring systems, patient-profiled personalwellness technologies, etc. [21]. In general, since facialexpressions can predict the onset and remission of depres-sion and schizophrenia, certain brain lesions, transientmyocardial ischemia, and different types of pain (acuteversus chronic), and can help identify alcohol intoxicationanddeception, the potential benefits from efforts to automatethe analysis of facial expressions are varied and numerousand span fields as diverse as cognitive sciences, medicine,education, and security [21].

Two main streams in the current research on automaticanalysis of facial expressions consider facial affect (emotion)detection and facialmuscle action (action unit) detection [25],[21], [41]. The most commonly used facial expressiondescriptors in facial affect detection approaches are the sixbasic emotions (fear, sadness, happiness, anger, disgust, andsurprise), proposed by Ekman and discrete emotion theor-ists, who suggest that these emotions are universallydisplayed and recognized from facial expressions. The mostcommonly used facial muscle action descriptors are theAction Units (AUs) defined in the Facial Action CodingSystem (FACS, [10]).

This categorization in terms of six basic emotions used infacial affect detection approaches, though quite intuitive,has some important downsides. The basic emotion cate-gories form only a subset of the total range of possible facialdisplays and categorization of facial expressions can there-fore be forced and unnatural. Boredom and interest, forinstance, do not seem to fit well in any of the basic emotion

IEEE TRANSACTIONS ON PATTERN ANALYSIS AND MACHINE INTELLIGENCE, VOL. 32, NO. XX, XXXXXXX 2010 1

. S. Koelstra and I. (Yiannis) Patras are with Queen Mary University ofLondon, Mile End Road, London E1 4NS, UK.E-mail: {sander.koelstra, i.patras}@elec.qmul.ac.uk.

. M. Pantic is with the Department of Computing, Imperial College, 180Queen’s Gate, London SW7 2AZ, UK. E-mail: [email protected].

Manuscript received 25 Sept. 2008; revised 29 May 2009; accepted 24 Dec.2009; published online 25 Feb. 2010.Recommended for acceptance by R. Ambasamudram.For information on obtaining reprints of this article, please send e-mail to:[email protected], and reference IEEECS Log NumberTPAMI-2008-09-0646.Digital Object Identifier no. 10.1109/TPAMI.2010.50.

0162-8828/10/$26.00 ! 2010 IEEE Published by the IEEE Computer Society

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

categories. Moreover, in everyday life, these prototypicexpressions occur relatively rarely; usually, emotions aredisplayed by subtle changes in discrete facial features, suchas raising of the eyebrows in surprise. To detect suchsubtlety of human emotions and, in general, to convey theinformation on facial expressions to the aforementionedapplications, automatic recognition of atomic facial signals,such as the AUs of the FACS system, is needed.

FACS was proposed by Ekman et al. in 1978 and revisedin 2002 [10]. FACS classifies atomic facial signals into AUsaccording to the facial muscles that cause them. It definesnine upper face AUs and 18 lower face AUs, which areconsidered to be the smallest visually discernible facialmovements. It also defines 20 Action Descriptors for eyeand head position. FACS provides the rules both for AUintensity scoring and recognition of temporal segments(onset, apex, and offset) of AUs in a face video.

Most of the research on automatic AU recognition hasbeen based on analysis of static images (e.g., [26]) orindividual frames of an image sequence (e.g., [3], [4], [18],[17]). Some research efforts toward using dynamic textures(DT) for facial expression recognition (e.g., [36], [43]) andtoward explicit coding of AU dynamics (e.g., with respect toAU temporal segments, like in [23], [35], or with respect totemporal correlation of different AUs like in [33]) have beenproposed as well. However, most of these previouslyproposed systems recognize either the six basic emotions(e.g., [43]) or only subsets of the 27 defined AUs. Except forgeometric feature-basedmethods proposed in [22], [23], [35],none of the existing systems attains automatic recognition ofAU temporal segments. Also, except for themethod based onMotion History Images proposed in [36], none of the pastworks attempted automatic AU recognition using a DT-based approach.

In this work, we present a novel DT-based approach toautomatic facial expression analysis in terms of all 27 AUsand their temporal segments. The novelties in this work are:

. We propose a new set of adaptive and dynamictexture features for representing facial changes thatare based on Free-form Deformations (FFDs).

. We introduce a novel nonuniform decomposition ofthe facial area to facial regions within which featuresare extracted. This is based on a quadtree decom-position of motion images and results in morefeatures being allocated to areas that are importantfor recognition of an AU and fewer features beingallocated to other areas.

. We combine a discriminative, frame-based Gentle-Boost classifier with a dynamic, generative HMMmodel for (temporal) AU classification in an inputface video.

. This is the second DT-based method for AUrecognition proposed. We compare our method tothe earlier method [36] and show a clear improve-ment in the performance.

An early version of thiswork appeared in [16]. The outlineof the paper is as follows: Section 2 provides an overview ofthe related research. Section 3 presents the two utilizedapproaches to modeling dynamics and the appearance in theface region of an input video (MHI and FFD), and explains

the methodology used to detect AUs and their temporalsegments. Section 4 describes the utilized data sets and theevaluation study and discusses the results. Section 5concludes the paper.

2 STATE OF THE ART

2.1 Facial Features

Existing approaches to facial expression analysis can bedivided into geometric and appearance-based approaches.Dynamic texture recognition can be seen as a generalizationof appearance-based approaches. Geometric features in-clude shapes and positions of face components, as well asthe location of facial feature points (such as the corners ofthe mouth). Often, the position and shape of thesecomponents and/or fiducial points are detected in the firstframe, and then tracked throughout the sequence. On theother hand, appearance-based methods rely on skin motionand texture changes (deformations of the skin) such aswrinkles, bulges, and furrows. Both approaches haveadvantages and disadvantages. Geometric features onlyconsider the motion of a number of points, so one ignoresmuch information present in the skin texture changes. Onthe other hand, appearance-based methods may be moresusceptible to changes in illumination and differencesbetween individuals. See [25], [40] for an extensive over-view of facial expression recognition methods.

2.1.1 Geometric-Feature-Based Approaches

Approaches that use only geometric features mostly rely ondetecting sets of fiducial facial points (e.g., [26], [23], [35]), aconnected face mesh or active shapemodel (e.g., [13], [7], [5],[17]), or face component shape parameterization (e.g., [31]).Next, the points or shapes are tracked throughout the videoand the utilized features are their relative and absoluteposition, mutual spatial position, speed, acceleration, etc. Ageometric approach that attempts to automatically detecttemporal segments of AUs is the work of Pantic andcolleagues [22], [23], [35]. They locate and track a numberof facial fiducial points and extract a set of spatiotemporalfeatures from the trajectories. In [22] and [23], they use arule-based approach to detect AUs and their temporalsegments, while in [35], they use a combination of SVMs andHMMs to do so. Using only the movement of a number offeature points makes it difficult to detect certain AUs, suchas AU 11 (nasolabial furrow deepener), 14 (mouth cornerdimpler), 17 (chin raiser), 28 (inward sucking of the lips) (seealso Fig. 1), the activation of which is not apparent frommovements of facial points but rather from changes in skintexture. Yet, these AUs are typical for facial expressions of

2 IEEE TRANSACTIONS ON PATTERN ANALYSIS AND MACHINE INTELLIGENCE, VOL. 32, NO. XX, XXXXXXX 2010

Fig. 1. Apex phases of eight AUs of the FACS system.


emotions such as sadness (see EMFACS [10]), and forexpressions of more complex mental states, includingpuzzlement and disagreement [11], which are of immenseimportance if the goal is to realize human-centered, adaptiveinterfaces. On the contrary, our appearance-based approachis capable of detecting the furrows and wrinkles associatedwith these AUs and is therefore better equipped to recognizethem.

2.1.2 Appearance-Based ApproachesSystems using only appearance-based features have beenproposed in, e.g., [18], [3], [4], [14], [2], [20], [36]. Severalresearchers have used Gabor wavelet coefficients as features(e.g., [14], [42], [38]). Bartlett et al. [3], [18], [4] have trieddifferent methods, such as optical flow, explicit featuremeasurement (i.e., length of wrinkles and degree of eyeopening), ICA, and the use of Gabor wavelets. They reportthat Gabor wavelets render the best results [18]. Othertechniques used include optical flow [2] and Active Appear-ance Models [20]. Tian et al. [31], [32] use a combination ofgeometric and appearance-based features (Gabor wavelets).They claim that the former features outperform the latterones, yet using both yields the best result.

2.1.3 Dynamic-Texture-Based Approaches

An emerging new method of appearance-based activityrecognition is known as Dynamic Texture recognition. A DTcan be defined as a “spatially repetitive, time-varying visualpattern that forms an image sequence with certain temporalstationarity” [6]. Typical examples of DTs are smoke, fire,sea waves, and talking faces. Many existing approaches torecognition of DTs are based on optical flow [28], [19]. Adifferent approach is used in [30]. Instead of using opticalflow, they use system identification techniques to learngenerative models. Recently, Chetverikov and Peteri [6]published an extensive overview of DT approaches.

The techniques applied to the DT recognition problemcan also be used to tackle the problem of facial expressionrecognition. Valstar et al. [36] encoded face motion intoMotion History Images. This representation shows asequence of motion energy images superimposed on asingle image, detailing recent motion in the face. Anextended version of MHI-based facial expression recogni-tion is proposed in this work as well. In this work, videosare temporally segmented by manually selecting the startand endpoints of an AU activation and a single MHI iscreated from six frames distributed equidistantly betweenthese points. In our implementation, an MHI is created for atemporal window around each frame without any manualinput. Also, while their method uses a multiclass classifier,we train separate binary classifiers for each AU, andtherefore, we can detect any combination of AUs.

Zhao and Pietikainen [43], [44] use volume local binarypatterns (LBPs), a temporal extension of local binarypatterns often used in 2D texture analysis. The face isdivided into overlapping blocks and the extracted LBPfeatures in each block are concatenated into a single featurevector. SVMs are used for classification. The approachshows promising results, although only the six prototypicemotions are recognized and no temporal segmentation isperformed. They normalize the face using the eye positionin the first frame, but they ignore any rigid head movementthat may occur during the sequence. In addition, instead ofour learned class (AU)-specific quadtree placement methodfor feature extraction regions, they use fixed overlappingblocks distributed evenly over the face. To the best of ourknowledge, our method is the only other DT-based methodfor facial expression analysis proposed so far.

3 METHODOLOGY

Fig. 2 gives an overview of our system. In the preprocessingphase, the face is located in the first frame of an input videoand head motion is suppressed by an affine rigid faceregistration. Next, nonrigid motion is estimated betweenconsecutive frames by the use of either Nonrigid Registra-tion using FFDs or Motion History Images (MHIs). For eachAU, a quadtree decomposition is defined to identify faceregions related to that AU. In these regions, orientationhistogram feature descriptors are extracted. Finally, acombined GentleBoost classifier and a Hidden MarkovModel (HMM) are used to classify the sequence in terms ofAUs and their temporal segments. In the remainder of thissection, the details of each processing phase are described.

3.1 Rigid Face Registration

In order to locate the face in the first frame of the sequence,we assume that the face is expressionless and in a near-frontal position in that frame and use the fully automaticface and facial point detection algorithm proposed in [37].This algorithm uses an adapted version of the Viola-Jonesface detector to locate the face. The 20 facial characteristicpoints and a facial bounding box are detected by usingGabor feature-based boosted classifiers.

To suppress intersequence variations (i.e., facial shapedifferences) and intrasequence variations (i.e., rigid headmotion), registration techniques are applied to find adisplacement field T that registers each frame to a neutralreference frame while maintaining the facial expression:

T ! Tinter " Tintra: #1$

The intrasequencedisplacement fieldTintra ismodeledas asimple affine registration. The facial part of each frame in thesequence is registered to the facial part of the first frame to

KOELSTRA ET AL.: A DYNAMIC TEXTURE-BASED APPROACH TO RECOGNITION OF FACIAL ACTIONS AND THEIR TEMPORAL MODELS 3

Fig. 2. Outline of the proposed method.


suppress minor head motions. This is done using a gradientdescent optimization, with the squared sum of differences(SSDs) of the gray-level values as a distance metric.

The intersubject displacement field Tinter is again mod-eled as an affine registration. A subset of 9 of the 20 facialpoints detected in the first frame that are stable (i.e., theirlocation is mostly unaffected by facial expressions) isregistered to a predefined reference set of facial points.This predefined set of reference points is taken from anexpressionless image of a subject that was not used in therest of the experiments. The displacement field Tinter isapplied to the entire image sequence to eliminate inter-subject differences in facial shape.

The Tintra and Tinter registrations are performed sepa-rately since Tinter is a geometric registration of two sets offiducial facial points, whereas Tintra is an appearance-basedregistration based on the minimization of the sum of squaresof the motion-compensated image intensities. Therefore, wecannot combine the two registrations. Let us also note herethat intrasequence transforms (i.e., from a frame to theprevious one) are, in general, smaller and therefore moreeasily estimated than the combined transform to a globalreference frame. However, once estimated, Tinter and Tintra

are combined and applied as a single transformation. Anillustration of the two steps and the facial points used isgiven in Fig. 3.

3.2 Motion Representation

Most existing approaches base their classification on eithersingle frames or entire videos. Here, we use overlappingsliding windows of different sizes and classify each windowin terms of depicted AUs and their temporal segments. Inany given frame, each AU can be in one of four differenttemporal segments: neutral (inactive), onset, apex, or offset.Different AUs have different onset and offset durations.Therefore, it is useful to have a flexible ! (size of temporalwindow) and consider several sizes. The onset of AU 45(blink), for instance, has an average duration of 2.4 frames (inthe utilized data sets). On the other hand, the offset of AU 12(smile) lasts 15.4 frames on average. A temporal window oftwo frames is well suited to find the onset of AU 45, but it ishard to detect the onset of AU 12 using such a window.Therefore, several window sizes are tested, ranging from2 frames to 20 frames. The 96.4 percent of all onsets/offsets inour data set last 20 frames or less, so this size suffices to easilycapture most activations.

To represent the motion in the face due to facialexpressions, two different methods of Motion HistoryImages and Nonrigid registration using Free-form Deforma-tions have been investigated, which will now be discussedin detail.

3.2.1 Motion History Images

MHIs were first proposed by Davis and Bobick [8]. MHIscompress the motion over a number of frames into a singleimage. This is done by layering the thresholded differencesbetween consecutive frames one over the other. In doing so,an image is obtained that gives an indication of the motionoccurring in the observed time frame.

Let t be the current frame and let ! be the temporalwindow size. Then, MHI!t consists of the weighted layeredbinary difference images for each consecutive two frames#t% !

2 ; t%!2 & 1$; . . . ; #t& !

2 % 2; t& !2 % 1$. A binary differ-

ence image for the pair #t; t& 1$ is denoted with dt and isdefined as

dt#x; y$ ! b1 jg#x; y; t$ % g#x; y; t& 1$j > "0 otherwise

!" #; #2$

where g#'; '; t$ is the frame t filtered by a Gaussian filter ofsize 2, " is a noise threshold set to 4 (this means that twopixels must differ four gray levels to be classified asdifferent), and b is a binary opening filter applied to thedifference image to remove the remaining isolated smallnoise spots with an area smaller than 5 pixels. g was variedbetween 0 and 10, " was varied between 1 and 20, b wasvaried between 0 and 20. The parameters were varied on asmall set of videos and the values as used above gave thebest results for recognition.

Using weighted versions of these binary differenceimages, the MHI is then defined as

M!t ! 1

!max

s#f#s& 1$dt%!

2&sj0 ( s ( !% 1g$: #3$

That is, the value at each pixel of the MHI is the weight ofthe last difference image in the window that depicts motion,or 0 if the difference images do not show any motion.

In the original implementation by Davis [8], motionvectors are retrieved from the MHI by simply taking theSobel gradient of the image. This will, however, only givemotion vectors at the borders of each gray-level intensity inthe image. This works well in the case where the MHIsshow smooth and large motion, but in our case, the motionis usually shorter and over a smaller distance, leading tofewer smooth gradients in the image. Applying the Sobelgradient in such a case leads to a very sparse motionrepresentation. The approach taken here is as follows: Foreach pixel that is not a background pixel (i.e., pixels whereM!

t is 0 since no motion was detected), we search in itsvicinity for the nearest pixel of higher intensity (withoutcrossing through background pixels). The direction inwhich a brighter pixel lies (if there is one) is the directionof motion in that pixel. In the case that multiple brighterpixels are found at the same distance, the pixel closest to thecenter of gravity of those pixels is chosen. This gives us adense and informative representation of the occurrence andthe direction of motion. This is illustrated in Fig. 4.

3.2.2 Nonrigid Registration Using FFDs

This method is an adapted version of the method proposedby Rueckert et al. [29], which uses an FFD model based onb-splines. The method was originally used to register breastMR images, where the breast undergoes local shapechanges as a result of breathing and patient motion.


Fig. 3. An illustration of the rigid registration process. Also shown are the10 facial feature points used for registration.


Let !t denote the gray-level image of the face region atframe t, where !t#x; y$ is the gray-level intensity at pixel#x; y$. Given a pixel #x; y$ in frame t, let #x; y$ be theunknown location of its corresponding pixel in frame t% 1.Then, the nonrigid registration method is used to estimate amotion vector field Ft between frames t and t% 1 such that:

#x; y$ ! #x; y$ & Ft#x; y$: #4$

To estimate Ft, we select a U ) V lattice "t of controlpoints with coordinates #t#u; v$ in !t, evenly spaced withspacing d. Then, nonrigid registration is used to align "t

with !t%1, resulting in a displaced lattice "t%1 ! "t & "$.Then, Ft can be derived by b-spline interpolation from "$.To estimate "t%1, a cost function C is minimized. Rueckertet al. [29] use normalized mutual information as the imagealignment criterion. However, in the 2D low-resolution caseconsidered here, not enough sample data are available tomake a good estimate of the image probability densityfunction from the joint histograms. Therefore, we use theSSDs as the image alignment criterion, i.e.,

C#"t%1$ !X

x;y

#!t#x; y$ % !t%1#x; y$$2: #5$

The full algorithm for estimating "t%1 (and, therefore, "$)is given in Fig. 5. We can calculate Ft using b-splineinterpolation on "$.

For a pixel at location #x; y$, let #t#u; v$ be the controlpoint with coordinate #x0; y0$ that is the nearest controlpoint lower and to the left of #x; y$, i.e., it satisfies:

x0 ( x < x0 & d; y0 ( y < y0 & d: #6$

In addition, let #$#u; v$ denote the vector that displaces#t#u; v$ to #t%1#u; v$. Then, to derive the displacement forany pixel #x; y$, we use a b-spline interpolation between its16 closest neighboring control points (see Fig. 6). This givesus the estimate of the displacement field Ft:

Ft#x; y$ !X3

k!0

X3

l!0

Bk#a$Bl#b$#$#u& k% 1; v& l% 1$; #7$

where a ! x% x0; b ! y% y0 and Bn is the nth basis functionof the uniform cubic b-spline, i.e.,

B0#a$ ! #%a3 & 3a2 % 3a& 1$=6;B1#a$ ! #3a3 & 6a2 & 4$=6;B2#a$ ! #%3a3 & 3a2 & 3a& 1$=6;B3#a$ ! a3=6:

To speed up the process and avoid local minima, we usea hierarchical approach in which the lattice density is beingdoubled at every level in the hierarchy. The coarsest lattice"0

t is placed around the point c ! #cx; cy$ at the intersectionof the horizontal line that connects the inner eye corners,and the vertical line passing through the tip of the nose andthe center of the upper and bottom lip. Then,

"0t ! #u; v$ u 2 *cx % 2id; . . . ; cx & 2id+

v 2 *cy % 2id; . . . ; cy & 4id+

$$$$

! %; #8$

where id is the distance between the eye pupils (i.e., "0t

consists of 35 control points). New control points areiteratively added in between until the spacing becomes0:25id (approximately the size of a pupil), giving 1,617 con-trol points. This has proven sufficient to capture mostmovements and gives a good balance between accuracy andcalculation speed.

Having estimated Ft, we now have a motion vector fielddepicting the facial motion between frame t% 1 and t, fromwhich orientation histogram features can be extracted. Forfeature extraction, we actually consider the motion vector


Fig. 4. Illustration of the estimation of a motion vector field from an MHI.(a) Original MHI. (b) For each pixel, the closest neighboring brighterpixel is found (without crossing background pixels). (c) This process isrepeated for each pixel, resulting in the motion vector field shown here.

Fig. 5. The nonrigid registration algorithm. % is a stopping criterion and &is the step size in the recalculation of control point positions. The valuesfor both are taken from [29].

Fig. 6. Illustration of the B-spline interpolation showing an image !t andthe control point lattice "t, as well as the estimated "t%1 aligned with!t%1. To estimate the new position #x; y$ of the point at #x; y$, only the16 control points shown in a lighter red color are used.


field sequence F !t over a sliding window of size ! around

frame t.Fig. 7 shows an example of the MHI and FFD methods.

Figs. 7a and 7b show the first and last frames of thesequence. Fig. 7c shows the resulting MHI M!

t , where ! isset such as to include the entire sequence. It is quite easy forhumans to recognize the face motion from the MHI. Fig. 7dshows the motion field sequence F !

t from the FFD methodapplied to a rectangular grid. The face motion (Fig. 7f) isless clear to the human eye from this visualization of thetransform. However, when we transform the first frame byapplying F !

t to get an estimate of the last frame, thesimilarity is clear, as shown in Fig. 7e. In addition, one cansee that between Figs. 7a and 7b, the subject shows a slightsquinting of the eyes (AU6). While this is invisible in theresulting MHI (Fig. 7c), it is visible in the motion fieldderived from FFD (Fig. 7d), indicating that the FFD methodis more sensitive to subtle motions than the MHI method.

3.3 Feature Extraction

3.3.1 Quadtree Decomposition

In order to define the face subregions at which features willbe extracted, we use a quadtree decomposition. Instead ofdividing the face region into a uniform grid (e.g., as in [43])or manually partitioning the face, a quadtree decompositionis used to divide the regions in such a manner that areasshowing much motion during the activation of a specificAU are divided in a large number of smaller subregions,while those showing little motion are divided into a smallnumber of large subregions. This results in an efficientallocation of the features. We note that different features(i.e., different quadtree decompositions) are used for theanalysis of different AUs.

Some AUs are very similar in appearance but differgreatly in the temporal domain. For instance, AU 43 (closedeyes) looks exactly like AU 45 (blink) but lasts significantly

longer. Therefore, we also use a number of temporal regionsto extract features. Let #a;s be the collection of all slidingwindows of size ! around the frames depicting a particularAU a in a particular temporal segment s in the training set.We then use a quadtree decomposition specific to each AUand the segments onset and offset on a set of projections of#a;s to decide where to extract features to recognize thetarget AU and its target temporal segment.

Three projections of each window are made, showing themotion magnitude, the motion over time in the horizontaldirection, and the motion over time in the vertical direction:

P !mag#x; y$ !

X

t

u#x; y; t$2 & v#x; y; t$2; #9$

P !tx#t; x$ !

X

y

u#x; y; t$2; #10$

P !ty#t; y$ !

X

x

v#x; y; t$2; #11$

where u#x; y; t$ and v#x; y; t$ are the horizontal and verticalcomponents of the motion vector field sequence F !

t . Theseprojections are then summed over all windows in #a;s to getthe final projections used for the quadtree decomposition:

P#a;smag#x; y$ !

X

!2#a;s

P !mag#x; y$; #12$

P#a;s

tx #t; x$ !X

!2#a;s

P !tx#t; x$; #13$

P#a;s

ty #t; y$ !X

!2#a;s

P !ty#t; y$: #14$

These three images then undergo a quadtree decompositionto determine a set of 2D regions ((x; y), (t; x), and (t; y)-regions) where features will be extracted. The definedprojections show us exactly where much motion occurs fora particular AU and a particular temporal segment andwhere there is less motion. The quadtree decompositionalgorithm is outlined in Fig. 8. The splitting threshold ' wasset to 0.1, meaning that a region in the quadtree will be split


Fig. 7. Example of MHI and FFD techniques. (a) First frame. (b) Lastframe. (c) M!

t . (d) F !t applied to a grid. (e) F !

t applied to first frame.(f) Difference between (b) and (e).

Fig. 8. The quadtree decomposition algorithm. ' is the threshold forsplitting and ( is the minimum region size.


if the region accounts for 10 percent of the total motion inthe frame. This gives a reasonable balance between havingtoo large regions, so the detail is lost, and too many smallregions, where the features become less effective as facialfeatures do no longer always fall in the same region. Theminimum region size ( is defined to be 0:25id, where id isthe interocular distance. In other words, the minimumregion size is about the size of a pupil. Extracting features insmaller regions will not be very informative due to smallvariations in facial feature locations in different subjects.Some examples of motion magnitude images and theresulting quadtree decompositions are shown in Fig. 9.We can see in Fig. 9e that for AU46R (right eye wink), mostof the features will be extracted in the eye area, where allthe motion occurs.

In#a;s, some frames also show the activation of other AUsthan a. Usually, the activation of other AUs does not occurfrequently enough to significantly alter the decomposition.However, in some cases, AUs co-occur very frequently andthe decomposition shows some of the motion of the co-occurring AU. It may then happen that some featurescorresponding to the co-occurring AU are then selected toclassify a.

3.3.2 Features

After generating the quadtree decompositions, we extractthe features for the sliding window around each frame inthe data set. We consider the u#x; y; t$ and v#x; y; t$components from F !

t in the subregions determined by thequadtree decomposition of P

#a;smag#x; y$. In each subregion,

11 features are extracted from the components: an orienta-tion histogram of eight directions, the divergence, the curl,and the motion magnitude.

For the temporal regions determined by the decomposi-tions of P

#a;s

tx #t; x$ and P#a;s

ty #t; y$, we first determine theprojections P !

tx#t; x$ and P !ty#t; y$ for the test frame in

question. For each subregion in the projections, we extractthree features: the average absolute motion, the averageamount of positive (i.e., left, upward) motion, and theaverage amount of negative (i.e., right, downward) motion.

3.4 Classification

We use the GentleBoost algorithm [12] for feature selectionand classification.Advantages ofGentleBoost overAdaBoostare that it converges faster and ismore reliable when stabilityis an issue [12]. For each AU and each temporal segmentcharacterized by motion (i.e., onset and offset), we train adedicated one-versus-all GentleBoost classifier. Since ourdata set is rather unbalanced (over 95 percent of the frames inthe database depict expressionless faces), we initialize theweights such that both the positive and the negative classescarry equal weight. This prevents that all frames areclassified as neutral. The GentleBoost algorithm is used toselect a linear combination of features one at a time until theclassification no longer improves by adding more features.This gives a reasonable balance between speed and complex-ity. The number of features selected for each classifier rangebetween 19 and 93, with an average of 74 features selected.Table 1 gives an overview of the number of selected featuresfor several AUs.

The first three selected features for some of the classifiersare shown in Figs. 10 and 11. In the images, for each featureselected from the P

#a;smag-projection, a neutral face image is


Fig. 9. Quadtree decompositions: (a), (b), (c), and (d) Onset of AU 12

(smile); (e), (f), (g), and (h) Onset of AU 46R (right eye wink). Shown for

each AU are (a), (e) example frames and the three projections

(b), (f) P#a;smag , (c), (g) P

#a;s

tx , (d), (h) P#a;s

ty . Overlaid on each projection is

the resulting quadtree decomposition.

TABLE 1Original Number of Features and Number of Features Selected

by GentleBoost Per AU When Trainedon the Entire MMI Data Set with a Window Size of 20 Frames

Fig. 10. First three selected features for onset of AU 1 (inner browraiser), window size 8, and superimposed on a neutral frame. (a) P

#1;8mag :

divergence. (b) P#1;8mag : divergence. (c) P

#1;8mag : divergence.

Fig. 11. First three selected features for onset of AU 43 (closed eyes),window size 8, and superimposed on a neutral frame. (b) depicts theabsence of upward motion in shown y-area of frame t& 2. (a) P

#43;8mag :

divergence. (b) P#43;8

ty : no upward motion. (c) P#43;8mag : divergence.


overlaid to indicate the location of the region. The selectedfeatures correspond reasonably well to the intuitivelyinteresting features/regions for each AU. The P

#a;smag-projec-

tion is the most important (and most often selected)projection since most information is available in the spatialdomain. This is also the reason why the problem of facialexpression recognition can be solved (to a certain extent)using static images (e.g., [26]). However, for some AUs, theinformation in the spatial magnitude projection is insuffi-cient to distinguish them from other AUs. One example isAU 43 (closed eyes), which only differs fromAU 45 (blink) inthe temporal domain. Since AU 45 is much more common,an AU 43 detector that does not take the temporal domaininto account would detect many false positives. Fig. 11shows that a temporal feature is the second most importantone in the detection of the onset of AU 43. The feature inquestion measures the amount of upward motion in theeyelid area for the next two frames. If the depicted AU wasAU 45, then the next two frames after any of the onset framesshould show upward motion as the eye would be openingagain. In AU 43, however, the next two frames after any ofthe onset frames will show no motion as the eyes will still beclosed. Thus, the absence of upward motion in this area in aperiod of two frames after an onset frame is a very good wayto tell AU 43 apart from AU 45 onset segments.

Each onset/offset GentleBoost classifier returns a singlenumber per frame indicating the confidence that the framedepicts the target AU and the target temporal segment. Inorder to combine the onset/offset GentleBoost classifiersinto one AU recognizer, a continuous HMM is used. Themotivation for using an HMM is to use the knowledge thatwe can derive from our training set about the priorprobabilities of each temporal segment of an AU and itsduration (represented in the HMM’s transition matrix).Hence, an HMM is trained for the classification of each AU.

HMMs are defined by ) ! f$; B;%g, where $ is thetransition matrix, B is the emission matrix, and % is theinitial-state probability distribution. These are all estimatedfrom the training set, where the outputs of the onset andoffset-GentleBoost classifiers are used to calculate theemission matrix B for the HMM by fitting a Gaussian tothe values of both outputs in any temporal state. Then, theprobability for each state can be calculated given the outputof the GentleBoost classifiers in a particular frame.

The HMM has four states, one corresponding to each ofthe temporal segments. The initial probabilities % show thatthe sequences in our data set usually start in the neutralsegment (i.e., no AU is depicted), but on rare occasions, theAU is already in one of the other states. Based on the initialprobabilities %, the transition probabilities $, and emissionprobability matrix B, the HMM decides the mostly likelypath through the temporal segment states for the inputimage sequence, using the standard Viterbi algorithm. Thisresults in the classification of the temporal segment for eachframe in the tested image sequence.

The HMM facilitates a degree of temporal filtering. Forinstance, given that the input data temporal resolution is25 fps and given the facial anatomy rules, it is practicallyimpossible to have an apex followed by a neutral phase andthis is reflected in the transition probabilities $. Also, the

HMM tends to smooth out the results of the GentleBoostclassifiers (for instance, short incorrect detections areusually filtered out). However, it only captures the temporaldynamics to a limited degree since it operates under theMarkov assumption that a signal value at time t is onlydependent on the signal value at time t% 1. For example,the HMM does not explicitly prevent onsets that last onlyone frame (even though in most AUs, the minimum onsetduration is much longer). Yet, it does model these dynamicsimplicitly through its use of transition probabilities betweenthe states.

An example of the learned transition probabilities $ forone HMM, trained to recognize AU 1, is given in Fig. 12.The transition probabilities say something about the stateduration. For instance, the transition probability for neutral! neutral is very high since the duration of a neutral state isusually very long (it is as long as the video itself when thevideo does not contain the target AU). The normal sequenceof states is neutral ! onset ! apex ! offset ! neutral.However, the transition probabilities show that, althoughhighly unlikely, transitions apex ! onset or offset ! apex dooccur. This is typical for spontaneously displayed facialexpressions which are characterized by multiple apexes[11], [23]. As both utilized data sets, the MMI and the Cohn-Kanade data set, contain recordings of acted (rather thanspontaneously displayed) facial expressions, the occurrenceof multiple apexes is rare and unlikely. In the SALspontaneous expression data set, on the other hand,multiple apexes occur quite frequently. However, especiallyin the MMI data set and especially by brow actions (AU1,AU2), smiles (AU12), and parting of the lips (AU25), somerecordings seem to be capturing spontaneous (uncon-sciously displayed) rather than purely acted expressions.

4 EXPERIMENTS

4.1 Data SetsThe first data set consists of 264 image sequences taken fromthe MMI facial expression database [27] (www.mmifacedb.com). To the best of our knowledge, these data are thelargest freely available data set of facial behavior record-ings. Each image sequence used in this study depicts a(near-)frontal view of a face showing one or more AUs. Theimage sequences are chosen such that all AUs underconsideration are present in at least 10 of the sequencesand distributed over 15 subjects. The image sequences last,on average, 3.4 seconds and were all manually coded for thepresence of AUs. Ten-fold cross validation was used, withthe folds divided such that each fold contains at least oneexample of each AU. Temporal window sizes ranging from4 to 20 frames were all tested independently and thewindow size that yielded the best result was chosen.


Fig. 12. The states and transition probabilities for an HMM trained onAU 1. Initial probabilities are denoted below the state names. Transitionswith probability 0 are not shown.


To test the generalization performance of the system, wehave also evaluated the proposed FFD-based method on theCohn-Kanade (CK) data set [15], arguably the most widelyused data set in the field. We only tested the system onthose AUs for which more than 10 examples existed in theCK data set. This resulted in examples of 18 AUs shown in143 sequences in total. The original CK data set only hasevent coding for the AUs (stating only whether an AUoccurs in the sequence, not a frame-by-frame temporalsegment coding). Here, we have used frame-by-frameannotations provided by Valstar and Pantic [34] based onthe given event coding.

Finally, we also tested the method on the SensitiveArtificial Listener (SAL) data set containing displays ofspontaneous expressions [9]. The expressions were elicitedin human-computer conversations through a “SensitiveArtificial Listener” interface. Subjects converse with one offour avatars, each having its own personality. The idea is forsubjects to unintentionally and spontaneously mirror theemotional states of the avatars. Ten subjects were recordedfor around 20 minutes each. The speech sections wereremoved from the data, leaving 77 sequences that depictspontaneous facial expressions. For four subjects, the datahave been FACS-coded on a frame-by-frame basis, for theother six subjects only event coding exists. Since our methodrequires frame-by-frame annotations to train the classifiers,we used the data of four subjects for training and tested onthe remaining six subjects. We only tested ourmethod on the10 AUs for which there were at least five training examples.

4.2 Results

Fig. 13 shows two typical results for AU 27 (mouth stretch).As can be seen in Fig. 13a, the GentleBoost classifiers yield

good results and the resulting labeling is almost perfect for! ! 20. For ! ! 2, the GentleBoost classifiers yield lesssmooth results (Fig. 13b). Even so, the HMM filters out thejitter very effectively.

4.2.1 Event Coding

Table 2 gives the results for all AUs tested with the MHIand the FFD technique on the MMI data set (per AU, thewindow width ! that gave the highest F1-score ismentioned). The F1-measure is a weighted mean of theprecision and recall measures. In the manual labeling of thedata set, AU 46 (wink) has been split up into 46L and 46Rsince the appearance differs greatly depending on whicheye is used to wink. Similarly, AU 28 (lip suck) is scoredwhen both lips are sucked into the mouth, and AU 28B andAU 28T are scored when only the lower or the upper lip issucked in. This gives us a total of 30 classes, based on the27 AUs defined in FACS. As can be seen in Table 2, bothtechniques have difficulties with subtle AUs (i.e., 5 (upperlid raiser), 7 (eye squint), and 23 (lip tightener)). Theseproblems possibly stem from the method of extractingmotion statistics over larger regions. If the regions are toolarge, these subtleties are easily lost (however, having theregions too small generates errors relating to the rigidregistration and intersubject differences). Possibly, geo-metric approaches are better equipped to handle these AUs(e.g., AU5 and AU7) since their activation is clearlyobservable from displacements of facial fiducial pointsand no averaging of the motion over regions is needed.

It is clear that, overall, the FFD technique producessuperior results to those obtained for the MHI-based


Fig. 13. Example classification results. (a) The output of the Gentle-Boost-classifiers: AU 27, ! ! 20. (b) The true and estimated frame labels(as predicted by the HMM): AU 27, ! ! 2. ! is the used temporal windowsize.

TABLE 2Results for 27 AUs (30 Classes) on 264 Sequences from the

MMI Data Set for the MHI and the FFD Method


approach. Therefore, in the remainder of this work, only theFFD-based approach is investigated further. One reason forthe inferior performance of the MHI-based approach is thatonly intensity differences above the noise threshold areregistered in the MHI. For instance, if the mouth cornermoves (e.g., in AU12), only the movement of the corner ofthe mouth is registered in the related MHI. More subtle andsmoother motion of the skin (e.g., on the cheeks) is notregistered in the related MHI (see Fig. 7). In the FFDmethod, however, we will see the entire cheek deform as aresult. Also, in MHIs, earlier movements can obscure latermovements (e.g., in AU 28) and fast movements can showup as disconnected regions that do not produce motionvectors (e.g., in AU 27).

Ingeneral, theF1-measure is reasonablyhigh formostAUswhen the FFD technique is applied, but there is still room forimprovement. In particular, there are many false positives.Most of these occur in AUs that have a similar appearance.The AUs performing below 50 percent are AUs 5 (upper lidraiser), 7 (eye squint), 20 (lip stretcher), 22 (lip funneller), 23(lip tightener), and 28T (upper lip inward suck). For most ofthese AUs, the reasons for the inaccurate performance lie inthe confusion of the target AU with other AUs. For instance,the onset of AU7 (eye squint) is often confusedwith the onsetofAU45 (blink), theoffset ofAU5 isvery similar to theonset ofAU45 (and vice versa), and AUs 20, 23, 24, and 28T are oftenconfused with each other since they all involve downwardmovement of the upper lip.

Another cause of some false positives is a failure of theaffine registration meant to stabilize the face throughoutthe sequence. Out-of-image-plane head motions, for in-stance, if not handled well, result in some classifiersclassifying rigid face motions as nonrigid AU activations.

We partially address this issue for spontaneous expressionsin Section 4.2.3 by incorporating the results of a facial pointtracker in the rigid registration process. However, weshould note that for very large out-of-plane rotations, affineregistration is not sufficient. The use of 3D models seems apromising direction. However, they require the construc-tion of a 3D model that might be difficult to obtain frommonocular image sequences.

Though most AUs perform best with the largest windowsize tested, it is clear from the results that AUs with shorterdurations, such asAU45, benefit froma smallerwindowsize.

Fig. 14 shows the results for all AU classifiers for alltested window widths for the FFD technique. Overall, wesee that the F1-measure improves as the temporal windowincreases. Exceptions include AUs with particularly shortdurations, such as 7 (eye squint), 45 (blink), 46L (left eyewink), and 46R (right eye wink).

4.2.2 Temporal Analysis

We were also interested in the timing of the temporalsegment detections with respect to the timing delimited bythe ground truth. This test was run using the optimalwindow widths as summarized in Table 2. Only sequencesthat were correctly classified in terms of AUs wereconsidered in this test. Four different temporal segmenttransitions can be detected: neutral ! onset, onset ! apex,apex! offset, and offset! neutral. Fig. 15 shows the averageabsolute frame deviations per AU and temporal segmenttransition. The overall average deviation is 2.46 frames. The44.12 percent of the detections are early and 38.18 percent arelate. The most likely cause of late detection is that most AUsstart and end in a very subtle manner, visible to the humaneye but not sufficiently pronounced to be detected by the


Fig. 14. F1-measure per AU for different window sizes for the FFD method.

Fig. 15. Average detection offsets per AU and temporal segment transition.


system. Early detections usually occur when a largertemporal window width is used, where the AU’s segmentin question is already visible in the later frames of thewindow but it is not actually occurring at the frame underconsideration (this can also be seen in Fig. 13a). In general,AUs of shorter duration also show smaller deviations. Also,the transitions that score badly are usually subtle ones. Thehigh deviations for apex ! offset in AUs 6 (cheek raiser andlid compressor) and 7 (eye squint) can be explained byconsidering that these transitions are first only slightlyvisible in the higher cheek region before becoming apparentin the motion of the eyelids. Since the eyelid motion is muchclearer, our method targets that motion andmisses the cheekraising in the start of the transition. Similarly, the offset !neutral transition in AU 14 (mouth corner dimpler) hasalmost all of the motion in the first few frames, and thencontinues very slowly and subtly. Our method picks up onlythe first few frames of this transition.

Another way to look at the temporal analysis results is toanalyze them per window size and transition type. Fig. 16illustrates that it shows the proportion of early, timely, andlate detections for all correctly detected transitions perwindow size. It also shows the mean absolute frame offsetper transition and per window size (this is depicted by thenarrow bar, placed on the right side of each of the main barsin the graph). Interestingly, for the neutral ! onset and apex! offset transitions, the most accurate results are obtainedfor the lowest window size and the results deteriorate as thewindow size increases. For the other two transitions, thelower window sizes are actually less accurate and the bestresults are obtained at window sizes 8 and 12. This behaviormight be explained by a few factors. First, most motionoccurs in the beginning of the onset and offset segments, withthe endings of those segments containing slower, moresubtle motions. Hence, the transitions indicating the end ofmotion (onset! apex and offset! neutral) are detected earlysince the subtle motion at the end of the onset and offsetsegments remains undetected by the system. The transitionsindicating the start of motion (neutral ! onset and apex !offset) are quite unlikely to be early, simply because there isno prior motion which could be classified as the transition inquestion. The results change as the window size increases.This is due to the smoothing effect discussed earlier, due towhich the start of motion is detected earlier and the end ofmotion is detected later.

4.2.3 Spontaneous ExpressionsWe performed tests on the SAL data set, containing77 sequences of spontaneous expressions, mostly smilesand related expressions. We tested for the 10 AUs thatoccurred five or more times. We trained on the sequences of4 of the 10 subjects that were annotated frame-by-frame forAUs, and tested on the data of the other six subjects thatwere annotated per sequence.

The data set contains relatively large head motions andmoderate out-of-plane rotations. We note that in the datasets used in this paper, all facial fiducial points were visibleat all times. If that is not the case, one could train a differentset of classifiers for each facial viewpoint.

The results for the SAL data set are given in Table 3. Theobtained classification rate is 80.2 percent, which is lowerthan the results on the posed data sets (89.8 percent on CKand 94.3 percent on MMI). However, we achieve asatisfactory average F1-score of 75.5 percent, which is, infact, higher than for the MMI (65.1 percent) and CK(72.1 percent) data sets. The worst performance is reportedfor AUs 2, 7, and 10. AUs 2 and 10 are much exaggerated inposed expressions, and therefore, harder to detect in subtlespontaneous depictions. AU 7 here is also often confusedwith AU 45, just as in the MMI data set. The best performingAUs are 12, 25, and 6. In fact, these AUs perform muchbetter than in the MMI data set. This can be explained by thefact that many more training samples were available here,indicating that more training examples can greatly benefitthe performance. In addition, these AUs also occur more


Fig. 16. Percentages of early/on time/late detection per transition and window size. Also shows average frame offset.

TABLE 3Results for Testing the System for 10 AUs on 77 Sequences

from the SAL Data Set for the FFD Method


frequently in the test set than in the MMI case, making thetest set less unbalanced compared to the other data sets. Wenote that here, the selected window sizes are much shorterthan for the MMI data set. A possible explanation for this isthat spontaneous expressions are generally less smooth anddepict multiple apexes interleaved with onset and offsetsegments. As a result, each segment occurs for a shortertime period.

4.2.4 Generalization Performance

To test the robustness and generalization ability of theproposed FFD method, we performed a smaller test on theCK data set [15]. We only tested on those AUs for which atleast 10 examples exist in the data set (18 AUs in143 sequences). The 10-fold cross-validation results areshown in Table 4. As a reference, the F1-scores for the MMIdata set are also repeated. The results achieved for the CKdata set are, on average, similar to those for the MMI dataset. AUs 2, 5, 12, 15, 20, 24, and 25 perform much better inthe CK data set. Possible explanations for the inferiorperformance of AU 10, 11, 14, and 45 lie in the differences inground truth labeling and the absence of offset segments inthe CK data set. The two data sets were labeled in differentways. More specifically, in the CK database, trace activa-tions (FACS intensity A) were also coded, whereas in theMMI data set, only AUs of FACS intensity B and higherwere considered. Trace activations (especially in AU 10, 11,and 14) involve very subtle changes in the facial skinappearance that remain undetected by our method.

Another difference between the results is that, for the CKdata set, lower window sizes are selected than for the MMIdata set. Since each sequence in the CK data set ends at theapex of the expression with the offset segments cut off, noGentleBoost classifiers could be trained for the detection ofoffsets and the HMM classification relies solely on the onsetdetections. Since the duration of onsets is generally shorterthan offsets, shorter window sizes tend to be selected. The

absence of offset phases, especially for fast AUs like AU 45,in which onset phases can often not be captured in morethan 1-2 frames and the detection relies heavily on thedetection of offset phases, explains the inferior performancefor such AUs. A possible explanation for better perfor-mance for AU 2, 5, 12, and 15 lies in the intensity of theseexpressions present in the CK data set. More specifically,facial expression displays constituting the CK data set areshorter and more exaggerated than is the case with datafrom the MMI data set. The better performance for AUs 24and 25 can be explained by the greater number of examplespresent in the CK data set.

We compare our results to those reported earlier byValstar and Pantic [34], the only other authors whoaddressed the problem of AU temporal segments recogni-tion. Valstar and Pantic use 153 sequences from the CK dataset, where we use 143. Their geometric feature-basedapproach gives, on average, very similar results. Interest-ingly, on this data set, the results of Valstar and Pantic aremuch better for AUs 4 and 7 (the related facial displays arecharacterized by large morphological changes which can beeasily detected based on facial point displacements) and theresults obtained by the FFD-based method are much betterfor AUs 15, 20, and 24 (which activations involve distinctchanges in skin texture without large displacements offacial fiducial points). Also, the method of Valstar andPantic is unable to deal at all with AUs 11 (nasolabialfurrow deepener), 14 (mouth corner dimpler), and 17 (chinraiser), the activation of which is only apparent fromchanges in skin texture and cannot be uniquely detectedfrom displacements of facial fiducial points only [26], [23].

A cross-database test was also performed with the MMIand CK data set. Average results are shown in Table 5. Thetests were run on those AUs available in both data setsusing a temporal window size of 20 frames. The averageresult is slightly lower than the result for training andtesting on the MMI data set, but this is to be expected giventhe different coding styles and other differences betweenthe two data sets.

4.2.5 Comparison to Earlier WorkWe compared our method to earlier works that reportedresults on either the CK or the MMI data set. Table 7 givesan overview of these works. It is interesting to note thatmost works are image-based, which means that they derivethe classification per frame independently and do not taketemporal information into consideration. Additionally, itmeans that the results reported for those works are foundusing manually selected “peak” frames, that is, framesshowing the AU in question at maximum intensity. Incontrast, sequence-based approaches take the whole se-quence into account without prior information as to thelocation of the peak intensity.


TABLE 4Results for Testing the System for 18 AUson 143 Sequences of the CK Data Set

TABLE 5Results for Cross-Database Testing, 18 AUs


Table 6 shows results reported previously on the CK andMMI data sets. While the classification rate (the percentageof correctly classified frames/sequences) is the mostcommonly reported measure, it is also the one that is theleast informative. Especially in cases where the data set ishighly unbalanced, it can be misleading. For example, in oursubset of the CK data set, the percentage of true positivesequences is below 10 percent for most AUs. This means thatit is possible to report a 90 percent classification rate bysimply classifying every sequence as negative. Therefore, wereport the F1-measure, which gives a better understandingof the quality of the classifier. Our results in terms of theclassification rate on the CK data set are largely comparableto those reported in the other works, 89.8 percent versus 90.2and 93.3 percent. For the MMI data set, we outperform theother works. The main reason for the worse comparativeperformance on the CK data set is probably the absence ofoffset segments. In contrast, both the MMI and SAL data setscontain the offset segments, which can greatly help validatethe occurrence of AUs in our HMM classification scheme.

5 CONCLUSION AND FUTURE WORK

In this work, we have proposed a method based on nonrigidregistrationusing free-formdeformations tomodel dynamicsof facial texture in near-frontal-view face image sequences forthe purposes of automatic frame-by-frame recognition ofAUs and their temporal dynamics. To the best of ourknowledge, this is the first appearance-based approach tofacial expression recognition that can detect all AUs and theirtemporal segments. We have compared this approach to anextended version of the previously proposed approach basedon Motion History Images. The FFD-based approach wasshown to be far superior. On average, it achieved an F1-scoreof 65 percent on the MMI facial expression database,72 percent on the Cohn-Kanade database, and 76 percent onthe SAL data set (containing spontaneous expressions). Foreach correctly detected temporal segment transition, themean of the offset between the actual and the predicted timeof its occurrence is 2.46 frames. We have compared theproposed FFD-based method to that of Valstar and Pantic[34], [35], which is the only other existing approach torecognition of AUs. and their temporal segments in frontalview face images (using a geometric feature-based approachrather than an appearance-based approach). Comparableresults have been achieved for the CK facial expressiondatabase. The two approaches seem to complement each

other, with some AUs being better detected with oneapproach and someAUs being better detectedwith the otherapproach. This is in accordance to the previously reportedfindings suggesting that combining the appearance andgeometric feature-based approaches to facial expressionanalysis will result in an increased performance [31], [21].Attempting to fuse the two approaches therefore seems anatural extension of this work.

ACKNOWLEDGMENTS

The authors would like to thank Jeffrey Cohn of theUniversity of Pittsburgh for providing the Cohn-Kanadedatabase. The research of Sander Koelstra has receivedfunding from the Seventh Framework Programme undergrant agreement no. FP7-216444 (PetaMedia). This work hasbeen funded first in part by the EC’s Seventh FrameworkProgramme [FP7/2007-2013] under grant agreement no211486 (SEMAINE). The current research of Maja Pantic isfunded by the European Research Council under the ERCStarting Grant agreement no. ERC-2007-StG-203143 (MAH-NOB). The research of Ioannis Patras has been partiallysupported by EPSRC Grant No EP/G033935/1.

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[23] M. Pantic and I. Patras, “Dynamics of Facial Expressions—Recog-nition of Facial Actions and Their Temporal Segments from FaceProfile Image Sequences,” IEEE Trans. Systems, Man, and Cyber-netics, vol. 36, no. 2, pp. 433-449, Apr. 2006.

[24] M. Pantic, A. Pentland, A. Nijholt, and T. Huang, “HumanComputing and Machine Understanding of Human Behavior: ASurvey,” Lecture Notes on Artificial Intelligence, vol. 4451, pp. 47-71,Springer, 2007.

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[33] Y. Tong, W. Liao, and Q. Ji, “Facial Action Unit Recognition byExploiting Their Dynamic and Semantic Relationships,” IEEETrans. Pattern Analysis and Machine Intelligence, vol. 29, no. 10,pp. 1683-1699, Oct. 2007.

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Sander Koelstra received the BSc and MScdegrees in computer science from Delft Uni-versity of Technology, The Netherlands, in 2006and 2008, respectively. He is currently workingtoward the PhD degree in the School ofElectronic Engineering and Computer Science,Queen Mary University of London. His researchinterests lie in the areas of computer vision,brain-computer interaction, and pattern recogni-tion. He is a student member of the IEEE.



Maja Pantic received the MSc and PhD degreesin computer science from Delft University ofTechnology, The Netherlands, in 1997 and2001, respectively. She is a professor ofaffective and behavioral computing at both theComputing Department of Imperial CollegeLondon and the Computer Science Departmentof the University of Twente. She is the editor-in-chief of the Image and Vision Computing Journaland an associate editor for the IEEE Transac-

tions on Systems, Man, and Cybernetics Part B. She is a guest editor,organizer, and committee member for more than 10 major journals andconferences. Her research interests include computer vision andmachine learning applied to face and body gesture recognition,multimodal human behavior analysis, and context-sensitive human-computer interaction (HCI). She is a senior member of the IEEE.

Ioannis (Yiannis) Patras received the BSc andMSc degrees in computer science from theComputer Science Department, University ofCrete, Heraklion, Greece, in 1994 and 1997,respectively, and the PhD degree from theDepartment of Electrical Engineering, DelftUniversity of Technology (TU Delft), The Nether-lands, in 2001. He has been a postdoctoralresearcher in the area of multimedia analysis atthe University of Amsterdam, and a postdoctoral

researcher in the area of vision-based human machine interaction at TUDelft. Between 2005 and 2007, he was a lecturer in computer vision inthe Department of Computer Science, University of York, UnitedKingdom. Currently, he is a senior lecturer in computer vision in theSchool of Electronic Engineering and Computer Science, Queen MaryUniversity of London. He is/has been on the organizing committees ofIEEE SMC ’04 and Face and Gesture Recognition ’08, and was thegeneral chair of WIAMIS ’09. He is an associate editor of the Image andVision Computing Journal and the Journal of Multimedia. His researchinterests lie in the areas of computer vision and pattern recognition, withemphasis on motion analysis, and their applications in multimedia datamanagement, multimodal human computer interaction, and visualcommunications. Currently, he is interested in the analysis of humanmotion, including the detection, tracking, and understanding of facial andbody gestures. He is a member of the IEEE.

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