- Conference Article
14
- 10.5244/c.12.40
Building Shape Models from Image Sequences using Piecewise Linear Approximation
- Jan 01, 1998
- D R Magee + 1 more +1
A method of extracting, classifying and modelling non-rigid shapes froman image sequence ispresented. Shapes areapproximated by polygonswherethe number of sides is related to the physical features of a shape class ratherthan any particular shape. A method of ‘seeding’ the polygonal approxima-tionisgivenwhere‘seeds’ areautomaticallyextracted fromasetofdata. Mul-tiple models are built using polygons with different numbers of sides to al-low for feature occlusion. Principal component analysis (PCA) is performedon vector representations of the sides of the polygons which are normalisedby the total perimeter. This removes the need for normalisation of scale andtranslationas required in thePointDistributionModel [16]. A ‘fit score’ met-ricisdefinedwhich givesan indicationof howwellagivenshapefitsamodel. 1 Introduction There has been much work carried out on the extraction and classification of non-rigidshapes from image sequences. In this paper a new method of classifying images automat-ically from an image sequence is proposed. This method is applied in the monitoring ofcattle movement to identifya particularnon-rigidobject in a scene (a cow in ourexample)and gain knowledge about the way in which this object changes with time.Kass et al [11] use an active shape model (snake) in which an energy function is min-imisedtoextractanobjectfromascene. TerzopoulosandSzeliski[15]havemadesomere-finementstothistechniquewiththeinclusionofKalmanfiltertechniques(Kalmansnakes).Blake et al [3] have further refined the technique by adding ‘templates’to this process toincorporate a search for a specific shape. In recent years theadvantage of such model-based approaches has been shown where prior informationabout a class of shapes is usedin extraction and classification. The point distributionmodel (PDM) described by Cooteset al [16] is a useful way of describing a class of non-rigid shapeswhere a shape class isdescribed by a fixed number of points. A model is built from a group of shapes fitted withthisfixed number of ‘reference points’ and principalcomponent analysis (PCA) is used toextract ‘modesof variation’fromthedataset. Members ofashapeclass aredescribed withreference to an eigenspace where the axes are the eigenvectors produced from the princi-pal componentanalysis. A ‘valid’shape isconstructedby takingan area inthiseigenspacecentred around the mean shape (origin) and bounded by maximum deviations along eachprincipalaxes whichare related totheeigenvaluesproduced by PCA and thusthevariancewithinthe data set. PDMs rely on accurate positioningof pointsand can only describe ac-curately shape classes which deform in a linear fashion which can result in invalid shapesbeing included in the model.There havebeen a numberof methodsproposedforpointfittingaroundapre-extractedcontour [1, 2] which involve finding a number of reference points (points of high curva-ture,extreme pointsetc) and addingextrapointsspaced evenlyroundthecontoursuch thatthe contour may be approximated by a spline. Hill et al [8] have proposed a new schemewhereby a contour is approximated by an arbitrary number of straight lines as defined bythe critical point detection algorithm described by Zhu and Chirlian [19]. In the compari-sonoftwoshapestheaveragenumberofpointsrequiredtorepresent theshapes isusedand
Read more