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Face Recognition Sumitha Balasuriya

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Computer Vision Image processing is a precursor to Computer Vision – making a computer understand and interpret whats in an image or video. Recognition 3D shape Tracking Segmentation Categorisation / Retrieval Robotics

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Innate Face Recognition Ability Face recognition almost instantaneous Highly invariant to pose, scale, rotation, lighting changes Can handle partial occasions and changes in age And we can do all this for faces of several thousand individuals Who is this?

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Fully Automated Face Recognition Face Detection Colour Motion Template Fiduciary points Face Detection Recognition Image / Video Identity

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Face Detection Detect face and facial landmarks Register (align) face using an image transform Australian National University

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Face Recognition approaches Geometric features Recognition possible at low resolution and at high noise levels because geometrical features such as nose width and eye separation are used Automated extraction of facial geometric features is very hard Template matching Extract facial regions (matrix of pixels) and compare with that of known individuals Need templates for different face poses/views Very high dimensional data

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High Dimensional Correlated Data Images as a high dimensional vector A typical image used for image processing will be 512x512= dimension vector! (Registered) face images are highly correlated Image space (high dimensional space of all possible images) Face images I 1,1 I 1,2 I ….

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Transform Face images to a Face Space The basis vectors (U i ) of face space are the Principal Components of face images. These Principal Components (PCs) correspond to the directions of greatest variation of the face dataset. The first PC corresponds to the direction (and scale) of greatest variation, the 2 nd PC corresponds to the orthogonal direction with the second greatest variation and so on … Embedding a face image vector into face space is done by calculating the vectors projection onto the Principal Components. Face space (low dimensional space of face images) U1U1 U2U2 U3U3 Face images f = U * (I - A) Embedded face Eigenfaces Mean face image Face image Change dimensionality of f by changing that of U

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Principal Component Analysis PCA – find the Principal Components of a dataset Finding the Eigenvectors and eigenvalues of the covariance matrix (Eigen decomposition) The covariance matrix contains the relationships (correlations) between the variables of the vectors. Variances along the diagonals and covariance between each pair of variables in other positions (Karhunen-Loeve transform)

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PCA (continued) For images will run out of memory because for a dataset of say 512x512 images X*X T will be a x matrix. If it has double precision (64bit) values C will be 512GB!!! Covariance matrix for images Therefore for image we instead compute the reduced covariance matrix If we have n datapoints c will be nxn. As n is normally much smaller than the dimensionality of the images the matrix c is computable Find the Eigenvectors and eigenvalues of c, solve the following linear equations Eigenvalues (degree of variation) Eigenvectors (Principal Compoments!!) Reduced covariance matrix >> [V,lamda]=eig(c); You can get Matlab to solve equations to give the eigenvectors and eigenvalues

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PCA (continued) Eigenvectors (Eigenfaces) of covariance matirx Another way to compute PCA – Singular Value Decomposition method SVD decomposes an mxn matrix X to a product X = U * S * V T Dont need to compute covariance matrix Diagonal matrix with eigenvalues along diagonal Right singular vectors (eigenvectors of X T *X) Left singular vectors (eigenvectors of X*X T ) You can get Matlab to solve these equations as well to get U, S and V >> [U,S,V]=svd(X); Use eigen decomposition with the reduced covariance matrix if you run out of memory with svd (seems to work upto 100x100 images)

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High eigenvalue eigenfaces seem to capture lighting and other global intensity changes Eigenfaces 1 to 9 Average face (data not mean subtracted!)

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Eigenfaces 10 to 18 Lower eigenvalue eigenfaces are better for recognition

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Transform into Face Space Projection f = U * (I - A) U1U1 U2U2 U3U3 Transform known faces to face space Face space

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Reconstruction F = U T * U * (I - A) Mean subtracted image Transform to face space Transform back to image space Can know whether image contains a face by calculating reconstruction error Face detection method! Very high compression! Face space with 28 eigenvectors 100x100x8bits = bytes compress to 28*32 bits=112 bytes Only compress faces

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Classifiers Classify an unknown face into one of the known face classes Important concepts Inter-class (between-class) variance Intra-class (within-class) variance Need a metric to measure distance for class membership ?

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Metric A non-negative function g(x,y) describing the "distance" between neighbouring points for a given set and satisfies (1)g(x,y)+g(y,z)g(x,z)(triangle inequality) (2)g(x,y)=g(y,x)(symmetry) (3)g(x,x)=0, as well as g(x,y)=0 x=y

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Euclidean distance (~300BC) Mahalanobis distance (1936) Superior to Euclidean distance because it takes distribution of the points (correlations) into account The distance between two N dimensional points scaled by the statistical variation in each component of the point. Decision surface

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Mahalanobis classifier Need many representative examples from each class to compute covariance matrix

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Recap Image processing precursor to Computer Vision Face Detection Images are high dimensional correlated data Principal Component Analysis –Eigen decomposition –Singular Value Decomposition Eigenfaces for recognition, compression, face detection Classifiers and Metrics

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Questions Will performance improve if we used orientated edge responses of the face image for recognition? Why? A popular approach uses orientated responses only from certain specific points on the face image (say edge of nose, sides of eyes). What are the advantages/disadvantages of this approach? PCA maximises the variance between vectors embedded in the eigenspace. What does LDA (Linear Discriminant Analysis) do?

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