detecting external disturbances on the camera lens in wireless

2982 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 59, NO. 11, NOVEMBER 2010

Detecting External Disturbances on the Camera Lensin Wireless Multimedia Sensor NetworksCesare Alippi, Fellow, IEEE, Giacomo Boracchi, Romolo Camplani, and Manuel Roveri

Abstract—Assessing the quality of images acquired by nodesof a wireless multimedia sensor network (WMSN) is a criticalissue, particularly in outdoor applications where external distur-bances such as the presence of water, dust, snow, or tamperingon the camera lens may seriously corrupt the acquired images.In this paper, we address the problem of determining when thelens of a microcamera mounted on a WMSN node is affectedby an external disturbance that produces blurred or obfuscatedimages. We show that such a problem can be solved by consideringchange detection tests applied to a measure of the blur intensity.Interestingly, both measuring the blur and the change detectiontest do not require any assumption about the content of theobserved scene, its nature, or the characteristics of the blur. Theproposed methodology is thus flexible and effective for a largeclass of monitoring applications. In particular, it is attractive whennodes cannot continuously acquire images (e.g., because of energylimitations) and background subtraction methods are not feasible.Two solutions, with different hardware costs, have been designed,which can detect disturbances on the camera lens at either/boththe node or/and the remote station level.

Index Terms—Change detection tests, degradation detection,digital image analysis, digital image processing, wireless multi-media sensor networks (WMSNs).

I. INTRODUCTION

IN RECENT years, wireless multimedia sensor networks(WMSNs) [1], [2] have gained an increasing interest in

the wireless sensor network (WSN) community, e.g., see ap-plications in [3]–[6]. Such networks differ from traditionalWSNs for the presence of audio and visual sensors that providephotos, videos, and audio streams. WMSN applications rangefrom tracking and surveillance to traffic control and environ-mental monitoring: not rarely, nodes are requested to operatein outdoor harsh environments. Noticeable examples of nodesprovided with visual sensors are given in [7]–[10].

This paper addresses an often-forgotten problem that regu-larly arises in WMSN nodes designed to work in a real environ-ment: the presence of external disturbances on the camera lensthat affect the acquired image. Possible sources of disturbances

Manuscript received January 21, 2010; revised March 4, 2010; acceptedMarch 5, 2010. Date of publication May 6, 2010; date of current versionOctober 13, 2010. This work was supported in part by the INTERREG EUproject M.I.A.R.I.A. (an adaptive hydrogeological monitoring supporting thealpine integrated risk plan) Italy–Switzerland action 2007–2013 and in part bythe FIRB Project In.Sy.Eme (Integrated Systems for Emergencies) 2008–2010.The Associate Editor coordinating the review process for this paper wasDr. Emil Petriu.

The authors are with the Dipartimento di Elettronica e Informazione, Po-litecnico di Milano, Milan, Italy (e-mail: [email protected]; [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIM.2010.2047129

are rain and sprinkle drops, humidity, dust, and tamperingattacks, which introduce blur or dim effects in the acquiredimages. Fig. 1 shows some images taken with water dropsinsisting on the camera lens. As one would expect, there maybe a significant loss in the image quality, which also dependson the type and entity of the disturbance. Since WMSNs areenergy-constrained embedded systems, we should not wasteenergy in processing or, worse, sending images to the remotestation when the blur intensity impairs its information content.Furthermore, when such a disturbance is detected in a node,different actions can be accomplished to tackle its presence.For instance, all units in a neighborhood of the disturbance-affected node can increase their field of view, and the nodecan activate different sensors (such as microphones and pas-sive infrared/microwave motion detectors) by using intelligentcontrol actions, e.g., see [11], or, in the worst case, requesthuman intervention. In any case, any image-based algorithmin execution on the node requires a preliminary procedure todetermine whether the acquired image is blurred/obfuscated ornot to give a confidence to the available information and driveconsequent actions. The same holds for algorithms acting atthe cluster level. As a consequence, a mechanism for automati-cally diagnosing the status of the image acquisition system isrequested in WMSN applications operating in outdoor envi-ronments. In particular, in monitoring applications that performsporadic image acquisition, blur detection should not be basedon image comparisons (e.g., comparison between the currentimage and an estimated background image) since the scenebetween two consecutive images can significantly change.

The detection of external disturbances insisting on the cam-era lens remains an open research issue. For instance, themethod proposed in [12] aims at detecting tamper attacks thatobscure the camera field of view. Nevertheless, the method hasbeen designed for systems performing continuous acquisition,and it compares the acquired images with a learned backgroundof the monitored scene. As previously discussed, this could beimpractical in several WMNS applications.

In principle, any algorithm for automatic blur detectioncan be used to determine when disturbances insisting on thecamera lens are corrupting the image acquisition system. Thealgorithms presented in [13] and [14] move in this directionby determining if an image is blurred or contains blurredareas and by classifying each image/area as out of focus ormotion blurred. Unfortunately, these algorithms are based oncomputationally demanding feature extraction steps, which arehardly executable on low-performance WMSN nodes.

The effects of atmospheric elements such as rain, fog,and haze on acquired images have been studied under the

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ALIPPI et al.: DETECTING EXTERNAL DISTURBANCES ON THE CAMERA LENS IN WMSNs 2983

Fig. 1. Example of images acquired with some drops of water affecting the camera lens. Drops introduce blur and dim areas in the resulting images. In turn, theymay affect the whole image as in (a) and (b) or only some areas of the image as in (c) and (d).

assumption that these elements are present between the cameraand the observed scene, i.e., they are not insisting on the cameralens. In particular, [15] considers the case where water dropsappear in the depth of field of the camera with the consequencethat drops can be assumed as optical lenses that reflect and re-fract the light, producing a wide-angle view of the environment.An algorithm to detect and remove falling raindrops as time-varying fluctuations in video sequences is presented in [16].Other ad hoc image enhancement algorithms have been pro-posed for compensating the effects of bad weather conditionson images or videos (e.g., [17] and [18] introduce methods toremove fog and haze from a single image). These algorithmsconsider the light scattering produced by small droplets ofdust or mist in the atmosphere to perform effective contrastenhancement. However, images acquired with drops or otherexternal disturbances insisting on the camera lens are signifi-cantly different from all the aforementioned cases, as they aresignificantly blurred and the suggested solutions are not meantfor blur removal; rather, they perform contrast enhancement.The related literature suggests to jointly estimate and removeblur in a single blurred image (blind deblurring) to address theblur removal problem. Unfortunately, these solutions cannot beused to enhance the considered images, as they assume the blurto be spatially invariant [19], [20] in contrast with the spatiallyvariant nature of the blur introduced by disturbances on thecamera lens (particularly when the external disturbances do notuniformly cover the camera lens). In addition, blur removalalgorithms assume the input image to be blurred and do notprovide hints for deciding whether that is the case or not.

This paper proposes a novel method for monitoring the statusof the image acquisition system to detect in advance a possiblestructural information loss due to perturbations insisting on thecamera lens (e.g., drops, tampering, and dust). As such, we donot address the image enhancement issue but signal the pres-ence of an external disturbance when it arises. To accomplishsuch a task in a time-variant context without assuming stronghypotheses about the observed scene and its dynamics, werequire two independent steps: 1) measuring the blur intensityin the acquired images and 2) detecting the change in blurintensity, i.e., detecting the presence of the disturbance on thecamera lens. Two solutions providing different detection capa-bilities, computational complexities, and power consumptionare proposed here to meet the requirements of a typical WMSNscenario. The solutions act at either

1) the node level, where both the blur measure and thechange detection test are executed on a WMSN node; or

2) the network level, with the blur measure computed at thenode level and the outcoming estimate sent to a basestation to undergo the change detection phase.

In other words, the two suggested philosophies use the samefigure of merit to quantify the blur in the acquired images butimplement different change detection tests based on the avail-able hardware resources to detect a—possible—blur presence.

This paper extends the work presented in [21] as it introducesthe node-level solution, provides the analysis of its algorithmiccomplexity, and strengthens the experimental section.

The structure of this paper is described as follows. Section IIintroduces the observation model, and Section III describesthe blur change detection solutions in detail. The experimentalcampaign, which includes both real and synthetic testbeds, isfinally presented in Section IV; experiments will be tailored tothe drop case, but the methodology can easily be extended tocover similar types of external disturbances.

II. OBSERVATION MODEL

We have already seen in Section I that disturbing elementsinsisting on the camera lens are typically out of focus andinduce blur or dim effects on the acquired image z. Thisphenomenon can be modeled as the result of a degradationoperator D applied to the error-free and unknown image y (theoriginal image), i.e.,

z = D[y]. (1)

The squared brackets are used to indicate the argument of anoperator. Here, D takes into account blur and noise accordingto the widely accepted additive model [22], i.e.,

z(x) = D[y](x) = B[y](x) + η(x), x ∈ X (2)

where x indicates the pixel coordinates, X is the discreteimage grid, B is the blurring operator, and η is the noise term.Typically, the blurring operator B is assumed to be linear (e.g.,see [23]), leading to the final model, i.e.,

B[y](x) =∫X

y(x)h(x, s)ds. (3)

h(x, ·) represents the blur point spread function (PSF) at x,which is assumed to be a nonnegative function, as it performslocal smoothing on y. The model described in (3) is very


general and hosts different behaviors induced by the presenceof drops/dust on the camera lens.

We then consider the general case where each sensor node ofthe network acquires a sequence of N observations {zi}, i =1, . . . , N , i.e.,

zi(x) = Bi[yi](x) + η(x), i = 1, . . . , N (4)

with η(x) being a stationary noise. The sequence of the originalimages {yi}, i = 1, . . . , N , may significantly change in theircontent, depending on the monitored scene. As a consequence,a naive approach exploiting direct comparisons among twoconsecutive observations zi and zi+1 may easily fail, being dif-ficult to distinguish if different observations are due to differentoriginal images (i.e., yi �= yi+1) or to a change in the blurringoperator (Bi �= Bi+1).

III. DETECTING CHANGES IN THE DEGRADATION PROCESS

The proposed method requires analyzing the observationsin {zi}, i = 1, . . . , N , to determine a possible change in thedegradation operator D (change associated with an externalpresence on the camera lens). Since the noise is assumed to bestationary, a structural change in the image acquisition system,such as the presence of drops/dust, reflects a change in theblur operator B: detecting a change in the blur operator impliesdetection of a structural change affecting the image acquisitionsystem.

A. Measuring the Blur

As one could imagine, it is hard to devise an index or figure ofmerit able to measure the actual blur of an image given a genericblurring operator B. What the related literature suggests insteadis to indirectly measure the blur, by relying on some detailsor frequency information present in the observed image zi, asdone when identifying the optimal camera parameters (e.g.,focal length, aperture, and exposure) before taking a shot [24]–[27]. The underlying philosophy onto which these measuresrely reflects the intuitive idea that the blur suppresses thehigh-frequency components of an image by local smoothing.Based on such observation, most of blur measures are actuallyestimates of the energy content of the image in high frequency.In the same direction, here, we consider the blur measure

mi = M [zi] =∫X

‖∇zi(x)‖1 dx (5)

where ‖ · ‖1 refers to the L1 norm. In the discrete domain,the image derivatives are computed by means of differentiatingfilters (here, the Sobel ones [22]). Note that M is indirectlya measure of the total energy of the image details; as such, itis particularly sensitive to the image content (M is low whencomputed on blur-free images having few details, as well as inimages heavily corrupted by blur). However, this measure canalso be used on partially blurred images, and it guarantees avery low computational complexity.

Let us discuss how the noise term η influences a sequenceof blur measures {mi}, i = 1, . . . , N . Typically, noisy images

have larger blur measures compared to the corresponding noise-free images: the larger the σ is, the larger the blur measure is.In addition, since the noise is stationary, it does not introduceanomalies in the sequence of blur measures. We expect thatwhen the blur measure is dominated by the noise, detecting adecrease in the blur measure according to B becomes muchmore challenging.

B. Detecting the Change

Change detection tests are statistical techniques that, by mon-itoring the behavior of a process over time, detect a possiblechange in its behavior. In the considered case, the processunder monitoring is the degradation operator D that corruptsthe sequence of unknown original images {yi}, i = 1, . . . , N ,and gives the observation sequence {zi}, i = 1, . . . , N ; see[28] for another application of change detection tests in WSNs.Among the large range of solutions (data driven, analytical, orknowledge based) present in the literature to assess a change inprocesses [29], we focus on data-driven techniques since theydo not require any a priori information about the process underinvestigation. The most common data-driven techniques forchange detection generally require a design-time configurationphase to configure the test parameters either by exploitinga priori information or through a trial-and-error approach [29]–[31]. In our problem, we suggest to use two adaptive self-configuring statistical tests, i.e., the adaptive cumulative sum(CUSUM) and the computational intelligence-based CUSUM(CI-CUSUM) [32], for their effectiveness in detecting abruptchanges and smooth drifts. Both tests are general, do notrequire any information about the process under monitoring,and exploit an initial sequence {mi}, i = 1, . . . , T , of blurmeasures computed from T external disturbance-free imagesfor the automatic configuration of their parameters. Such asequence allows the tests for both estimating the probabilitydensity function (pdf) of mi in the absence of external dis-turbance (i.e., the null hypothesis Θ0) and defining alternativehypotheses Θ1s representing the “not being in Θ0” to addressany type of nonstationary change (the alternative hypotheses areautomatically defined during the training phase).

To guarantee an accurate estimate of test parameters, theauthors suggest to consider a reasonable large training se-quence, e.g., T > 400. Both tests work on subsequences of blurmeasures (in our experiments, we considered subsequences of20 blur measures) and estimate the transition from Θ0 to Θ1 bymeasuring the log-likelihoods between the pdf in the absenceof drop/dust and the pdf’s of all the alternative hypotheses atsubsequence τ (one at a time), i.e.,

r(τ) = lnNΘ1(φτ )NΘ0(φτ )

(6)

where φτ is the average value of the blur measures of the τ thsubsequence, and NΘ is a multivariate Gaussian distributionparameterized in Θ.

The log-likelihood ratio has an important property: a changein the pdf of the process under monitoring can be detectedby analyzing the sign of the log-likelihood ratios. Both tests


are able to detect the presence of drops in the images bysequentially checking whether the φτ s have been generatedaccording to a pdf associated with Θ0 or one of the alternativehypotheses. When one of the cumulative sums of the r(τ)overcomes an automatically defined threshold, the test detectsa change in the statistical behavior of the φτ s (a detaileddescription of both tests is given in [32]). Since the log-likelihood ratio compares couples of pdf’s, both tests have anumber of running log-likelihood ratios that are equal to thenumber of alternative hypotheses defined by the test (i.e., eachlog-likelihood ratio compares the null hypothesis with one ofthe alternative hypotheses). The main difference between theadaptive CUSUM and the CI-CUSUM test consists of the setof considered features. More specifically, the adaptive CUSUMassesses changes in the mean and the variance of the processunder monitoring, while the CI-CUSUM exploits a larger set offeatures (i.e., not only the mean and variance but also featuresderived from the pdf and the cumulative density function, aswell as features inspired by change detection tests present inthe literature), and it is more accurate at the expense of a signif-icant increase in computational complexity. Obviously, a highernumber of alternative hypotheses guarantee a more effectiveexploration of the hypotheses space and, hence, a larger changedetection ability. The selection between the adaptive CUSUMand the CI-CUSUM test is thus strictly related to the availablecomputational resources and the desired detection accuracy.We suggest considering the adaptive CUSUM test for the nodesolution (i.e., the change detection test is directly executed onnodes), while the CI-CUSUM test is the suitable choice forthe network solution at either the cluster heads or the remotecontrol station.

IV. EXPERIMENTS

The proposed methods have been tested on two applications.The first benchmark refers to a sequence of synthetically gener-ated observations (Application D1); the second refers to a realsequence of images (Application D2). In both cases, our goalis to detect the presence of water drops. Four figures of merithave been suggested to assess the performance of the proposedsolutions.

DL Detection latency. It represents the number of imagesrequired to detect a change in the blurring process afterthe drop arrival.

FP False positives. It measures the number of blur changeserroneously detected by the test.

FN False negatives. It measures the number of blur changesnot detected by the test.

ET Execution time (in seconds) of the adaptive CUSUM andthe CI-CUSUM test estimated with Matlab.1

Execution times have separately been evaluated for the train-ing phase needed to configure the test parameters (ET training)and for the operational phase (ET operational). The configura-tion set accounts for 500 blur measures computed from blur-free observations; the validation set accounts for 1500 ones.

1Reference platform: Intel Core 2 Duo 2.53-GHz CPU, no parallel threads.

Fig. 2. Example of synthetically generated observations. First row: the bluraffects the whole image, σ = 0.08, and ν = 1, 4, 8, respectively. Second row:the blur affects only some part of the image, σ = 0.02, and ν = 1, 4, 8,respectively.

A. Application D1

A set of sequences of observations has been generatedaccording to (4). Each sequence contains 2000 observationsobtained from 75 gray-scale 512 × 512 pixel original images,scaled in the [0, 1] value interval. In each sequence, the first1000 observations are blur-free, i.e., Bi = I, i = 1, . . . , 1000,where I stands for the identity operator; the others have beenaffected by a blurring operator Bi, i = 1001, . . . , 2000, havingthe PSFs of (3) defined as

h(x, s) ={

δ(x − s), x ∈ X0

g(x − s), x ∈ X1, X0 ∪ X1 = X (7)

where X0 ∩ X1 = ∅, δ is the Dirac’s delta function, and ga Gaussian kernel of standard deviation ν. Sets X0 and X1

denote blur-free and blurred areas, respectively. Therefore, theconsidered blur may affect only some parts of the originalimage, and within the blurred areas, the blurring operator isspace invariant. We considered two different cases: in the first,the blurring operator affects the whole image (i.e., X1 = X ,FULL blur); in the second, the blurring operator affects onlysome parts of the image (i.e., X1 ⊂ X , PART blur). In this lattercase, the sets X0 and X1 are the same for all sequences. In eachsequence, the noise term is Gaussian η ∼ N(0, σ2) added toboth the blur-free and blurred images.

For both the FULL and the PART blur, we considered eightvalues of the standard deviation of the Gaussian kernel g of(7) ν = 1, 2, . . . , 8 and four values of σ; the standard deviationof η ranges from 0.02 to 0.08 (with a step of 0.02). For eachparameter pair (ν, σ), we generated 100 different sequences tocompute the figures of merit for the two solutions. Fig. 2 showsthat observations generated with such parameters, at least forhigh values of ν, are very similar to images acquired with adrop on the camera lens, such as those in Figs. 1 and 5.

Tables I and II show the change detection ability of thenetwork and the node-level solutions, respectively. On onehand, the network-level solution guarantees less FP s than thenode-level one, thanks to the superior detection ability of the


TABLE INETWORK-LEVEL SOLUTION: FP AND FN EVALUATED ON DATA

SYNTHETICALLY GENERATED IN APPLICATION D1

TABLE IINODE-LEVEL SOLUTION: FP AND FN EVALUATED ON DATA

SYNTHETICALLY GENERATED IN APPLICATION D1

CI-CUSUM test. This is particularly evident in the “PART”case at low values of ν, where the node-level solution is not ableto detect the presence of blur (FNs in the last rows of Table II).On the other hand, the node-level solution guarantees very lowdetection delays (the test is very quick in detecting changes)and a reduced computational complexity. Figs. 3 and 4 showthe detection latency on the considered data set for both thenode- and network-level solutions. In both cases, the amountof images required to detect the presence of blur decreases asν increases. We observe that the FP values are independentfrom the values of ν (the standard deviation of the GaussianPSF), while FNs decrease as ν increases: the higher the blur,the easier its detection. In particular, when the blur corruptsonly part of the image (the “PART” case), the values of FNare higher than when the blur corrupts the whole image (the“FULL” case). Moreover, at low values of ν, the node-level

solution is not able to reliably detect the presence of drops(see the FN in the last rows of Table II). Both the network-and node-level solutions are able to cope with the considerednoise levels: the values of FP s, FNs, and DLs show that thedetection performance is not altered. We comment that it isextremely important to provide a reduced number of FP s, asthese false alarms are sent over the network and they may resultin a waste of resources. Policies at the unit and cluster levelscould be implemented to reduce FP s by exploiting informationcoming from neighboring units. The execution time averagedover the algorithm runs is given in Table III and shows that thenode solution is considerably faster than the network one duringboth the training and operational phases.

B. Application D2

The second application refers to a set of 25 uncompressedvideo sequences acquired in five different dynamic scenarios(three outdoor and two indoor). Each sequence is composedof 2000 frames (320 × 240 pixels) recorded by an integratedwebcam of a laptop computer. Each frame has been convertedinto gray scale by averaging the red–green–blue values of eachpixel. The first 1000 frames are drop-free, while the next 1000have been acquired with some water drops on the cameralens. Fig. 5 shows, along rows, six frames taken from a videosequence (one per each scenario). Similar to Application D1,the training phase of both tests exploits the first 500 drop-free images of each sequence. The figures of merit have beenevaluated by averaging the results of the 25 video sequences;the comparison between the performance of the two solutionsis presented in Table IV.

The experimental results on this data set are in line with thoseof Application D1: the network-level solution guarantees lowerFP s than the node solution, which, on the contrary, provides aprompter detection ability (lower values of DL) and reducedexecution time. When processing these video sequences, theFP s are determined by accidental and unpredictable occludingobjects that do not appear in the training set, as, for example,the shadowed face and the transparent plastic bag appearing inthe sequence illustrated in the third and fourth rows of Fig. 5,respectively. In fact, different from the synthetically generatedsequences of Application D1, here, the training set might not befully representative of all the original images yi that the WMSNnode has to face in working conditions. Occluding objects mayalso induce FN when they are shown in the training set, as thedecay in the blur measures due to the drop arrival could. Motionblur, which frequently occurs in images of dynamic scenesacquired in low-light conditions, may also cause both FP s andFNs since it causes loss of details in the observations and sub-sequent decay of the blur measures. This challenging problemcould be (at least) partially addressed by integrating lightinginformation and exposure times in the change detection tests.The detection performance can be improved by consideringlonger training sequences and, when possible, by updating thetraining set with user-supervised and disturbance-free imagesacquired during the test execution.

Fig. 6 shows the blur measures mi associated with thesequence including the frames depicted in the second row


Fig. 3. DL of the network solution as a function of ν (the standard deviation of the Gaussian PSF), computed for different values of σ (the noise standarddeviation). (a) Plot of the DL when the blur affects the whole image. (b) Plot of the DL when the blur affects only some areas.

Fig. 4. DL of the node solution as a function of ν (the standard deviation of the Gaussian PSF), computed for different values of σ (the noise standard deviation).(a) Plot of the DL when the blur affects the whole image. (b) Plot of the DL when the blur affects only some areas.

TABLE IIIAPPLICATION D1: ET AVERAGED OVER THE 2 × 8 × 4 × 100

ALGORITHM RUNS FOR BOTH THE NETWORK-LEVEL

AND NODE-LEVEL SOLUTIONS

of Fig. 5: in this case, both solutions detect the drop arrivalwithin the subsequence ending at frame 1180 (DL = 180).Fig. 7 shows the blur measures mi computed from the sequenceillustrated in the third row of Fig. 5: in this case, the networksolution detects the drop arrival within the subsequence endingat frame 1240 (DL = 240), while the node solution bears a

false positive at frame 880, because of a sudden decay in theblur measures.

C. Computational Complexity of the Node-Level Solution

As expected, the execution times reported in Tables IIIand IV show that the adaptive CUSUM has a significantly lowercomputational complexity than the CI-CUSUM in both thetraining and operational phases. In fact, the adaptive CUSUMassesses changes by solely inspecting variations in the meanand variance of φτ s, while the CI-CUSUM considers a largerset of features to improve the detection ability.

We present a detailed analysis of the computational complex-ity on the adaptive CUSUM to justify its use at the node level.


Fig. 5. Example of observations composing the video sequences. Each row shows six frames taken from an acquired video sequence. In all sequences the dropsappear at frame 1001.

TABLE IVAPPLICATION D2: DETECTION PERFORMANCE EVALUATED FOR BOTH THE

NETWORK-LEVEL AND NODE-LEVEL SOLUTIONS. FP AND FN HAVE

BEEN COMPUTED OVER 25 VIDEO SEQUENCES, WHILE THE VALUES

OF DL AND ET HAVE BEEN AVERAGED OVER THE 25 RUNS

The evaluation of the blur measure by (5) is discretized andimplemented with (2(s − 1) + 4) integer operations per pixel,with s being the number of nonzero coefficients of the con-volutional filter used for computing image derivatives. In ourMatlab implementation, s = 6 (requiring 14 integer operationsper pixel); s can be reduced to 2 when a 1-D filter is used (sixinteger operations per pixel).

The operational phase of the adaptive CUSUM, which con-stitutes the computational load in working conditions requires,for each subsequence of 20 blur measures:

1) computing the mean of the 20 blur measures of the frame(20 floating-point operations);

2) computing two logarithms (2 × 35 floating-pointoperations);

3) evaluating a 1-D standard Gaussian function in threepoints (3 × 28 floating-point operations).

The number of floating-point operations has been estimatedwith Matlab. It follows that the execution of the adaptive

Fig. 6. Blur measures (5) computed in the sequence shown in the second rowof Fig. 5.

CUSUM test requires 175 floating-point operations per eachsubsequence of 20 blur measures. Such a reduced and com-putationally light sequence of operations can realistically beexecuted on a WMSN node having limited hardware and energyresources such as the [7]–[10] node platforms.


Fig. 7. Blur measures (5) computed in the sequence shown in the third rowof Fig. 5.

V. CONCLUSIONS

This paper has presented two different solutions for detect-ing the presence of external disturbances on the camera lensin WMSN nodes. Such aspect is particularly relevant in theWMSN community since nodes quite often operate outdoorsin harsh environments and it is important to continuouslyassess the status of the image acquisition system. The proposedsolutions have combined a simple and easy-to-compute blurmeasure and a change detection test and have been proveneffective on both synthetically generated images and videosequences acquired from a webcam.

The analysis of the computational complexity has shownthat the node-level solution can be implemented on WMSNnodes deployed in critical environments. Ongoing work re-gards the implementation of the node-level solution on anSTMicroelectronics prototype board, which represents the cur-rent state of the art in low-power smart cameras [10]. Thisboard is equipped with the ST-VS6724 2-megapixel camera[33] and the ST STR912FA microcontroller [34], running at96 MHz with 96-Kb SRAM. The board is able to processimages in real time, delivering video streams at 30 fps. Fur-thermore, we are investigating strategies at the cluster levelto improve the detection performance and reduce the numberof FP s by exploiting local knowledge; such clusters can begenerated as in [35].

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Cesare Alippi (SM’94–F’06) received the Dr.Ing.degree (summa cum laude) in electronic engineeringand the Ph.D. degree in computer engineering fromthe Politecnico di Milano, Milan, Italy, in 1990 and1995, respectively.

He has completed research work in computersciences at the University College, London, U.K.,and the Massachusetts Institute of Technology,Cambridge. He is currently a Full Professor of in-formation processing systems with the Dipartimentodi Elettronica e Informazione, Politecnico di Milano.

He is the author of more than 120 technical papers published in internationaljournals and conference proceedings. His research interests include application-level analysis and synthesis methodologies for embedded systems, computa-tional intelligence, and wireless sensor networks.

Dr. Alippi is an Associate Editor of the IEEE TRANSACTIONS ON NEURAL

NETWORKS and the IEEE TRANSACTIONS ON INSTRUMENTATION AND

MEASUREMENT.

Giacomo Boracchi received the M.S. degree inmathematics from the Universitá Statale degli Studidi Milano, Milan, Italy, in 2004 and the Ph.D. degreein computer engineering from the Politecnico diMilano, Milan, Italy, in 2008.

He was Researcher with the Tampere InternationalCenter for Signal Processing, Tampere, Finland, in2004. He is currently a Postdoctoral Researcher withthe Dipartimento di Elettronica e Informazione, Po-litecnico di Milano. His research interests includemathematical and statistical methods for signal and

image processing, and computational intelligence.

Romolo Camplani received the Dr.Ing. degree inelectronic engineering and the Ph.D. degree in elec-tronic and computer science from the Universitàdegli Studi di Cagliari, Cagliari, Italy, in 2003 and2008, respectively, and the M.Eng. degree in embed-ded system design from the Università della SvizzeraItaliana, Lugano, Switzerland.

He is currently a Postdoctoral Researcher with theDipartimento di Elettronica e Informazione, Politec-nico di Milano, Milan, Italy. His research interestsinclude wireless sensors networks, adaptive routing

algorithms, and power-aware embedded applications.

Manuel Roveri received the Dr.Eng. degree in com-puter science engineering from the Politecnico diMilano, Milan, Italy, in June 2003, the M.S. degreein computer science from the University of Illinois atChicago in December 2003, and the Ph.D. degree incomputer engineering from the Politecnico di Milanoin May 2007.

He is currently an Assistant Professor with theDipartimento di Elettronica e Informazione, Po-litecnico di Milano. His research interests includecomputational intelligence, adaptive algorithms, and

wireless sensor networks.

detecting external disturbances on the camera lens in wireless

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