PCA人脸识别

PCA方法由于其在降维和特征提取方面的有效性，在人脸识别领域得到了广泛的应用。

其基本原理是:利用K-L变换抽取人脸的主要成分，构成特征脸空间，识别时将测试图像投影到此空间，得到一组投影系数，通过与各个人脸图像比较进行识别。

进行人脸识别的过程，主要由训练阶段和识别阶段组成：

训练阶段

第一步：写出训练样本矩阵，其中向量xi为由第i个图像的每一列向量堆叠成一列的MN维列向量,即把矩阵向量化。假设训练集有200个样本,，由灰度图组成，每个样本大小为M*N。

第二步：计算平均脸

Ψ=1200∑i=1i=200xi

第三步：计算差值脸,计算每一张人脸与平均脸的差值

di=xi?Ψ,i=1,2,...,200

第四步：构建协方差矩阵

C=1200∑i=1200didiT=1200AAT

第五步：求协方差矩阵的特征值和特征向量，构造特征脸空间

求出 ATA 的特征值 λi及其正交归一化特征向量νi，根据特征值的贡献率选取前p个最大特征值及其对应的特征向量，贡献率是指选取的特征值的和与占所有特征值的和比，即：

?=∑i=1i=pλi∑i=1i=200λi≥a

若选取前p个最大的特征值，则“特征脸”空间为：

w=(u1,u2,...up)

第六步：将每一幅人脸与平均脸的差值脸矢量投影到“特征脸”空间，即

Ωi=wTdi(i=1,2,...,200)

识别阶段

第一步：将待识别的人脸图像Γ与平均脸的差值脸投影到特征脸空间，得到其特征向量表示：

ΩΓ=wT(Γ?Ψ)

第二布：采用欧式距离来计算ΩΓ与每个人脸的距离εi

εi2=∥∥Ωi?ΩΓ∥∥2(i=1,2,...,200)

求最小值对应的训练集合中的标签号作为识别结果

需要说明的是协方差矩阵AAT的维数为MN*MN，其维数是比较较大的，而我们在这里的训练样本个数为200，ATA的维数为200*200小了许多，实际情况中，采用奇异值分解(SingularValue Decomposition ,SVD)定理，通过求解ATA的特征值和特征向量来组成特征脸空间的。必须明白的是特征脸空间是由ATA的子空间构成，我们的识别任务也是将原始ATA所构成的空间投影到我们选取前p个最大的特征值对应的特征向量组成的子空间里，进行比较，选取最近的训练样本为标号。

代码

训练过程代码如下：

function [m, A, Eigenfaces] = EigenfaceCore(T)
% Use Principle Component Analysis (PCA) to determine the most
% discriminating features between images of faces.
%
% Description: This function gets a 2D matrix, containing all training image vectors
% and returns 3 outputs which are extracted from training database.
%
% Argument:      T                      - A 2D matrix, containing all 1D image vectors.
%                                         Suppose all P images in the training database
%                                         have the same size of MxN. So the length of 1D
%                                         column vectors is M*N and ‘T‘ will be a MNxP 2D matrix.
%
% Returns:       m                      - (M*Nx1) Mean of the training database
%                Eigenfaces             - (M*Nx(P-1)) Eigen vectors of the covariance matrix of the training database
%                A                      - (M*NxP) Matrix of centered image vectors
%
% See also: EIG

% Original version by Amir Hossein Omidvarnia, October 2007
%                     Email: aomidvar@ece.ut.ac.ir                  

%%%%%%%%%%%%%%%%%%%%%%%% Calculating the mean image
m = mean(T,2); % Computing the average face image m = (1/P)*sum(Tj‘s)    (j = 1 : P)
Train_Number = size(T,2);

%%%%%%%%%%%%%%%%%%%%%%%% Calculating the deviation of each image from mean image
A = [];
for i = 1 : Train_Number
    temp = double(T(:,i)) - m; % Computing the difference image for each image in the training set Ai = Ti - m
    A = [A temp]; % Merging all centered images
end

%%%%%%%%%%%%%%%%%%%%%%%% Snapshot method of Eigenface methos
% We know from linear algebra theory that for a PxQ matrix, the maximum
% number of non-zero eigenvalues that the matrix can have is min(P-1,Q-1).
% Since the number of training images (P) is usually less than the number
% of pixels (M*N), the most non-zero eigenvalues that can be found are equal
% to P-1. So we can calculate eigenvalues of A‘*A (a PxP matrix) instead of
% A*A‘ (a M*NxM*N matrix). It is clear that the dimensions of A*A‘ is much
% larger that A‘*A. So the dimensionality will decrease.

L = A‘*A; % L is the surrogate of covariance matrix C=A*A‘.
[V D] = eig(L); % Diagonal elements of D are the eigenvalues for both L=A‘*A and C=A*A‘.

%%%%%%%%%%%%%%%%%%%%%%%% Sorting and eliminating eigenvalues
% All eigenvalues of matrix L are sorted and those who are less than a
% specified threshold, are eliminated. So the number of non-zero
% eigenvectors may be less than (P-1).

L_eig_vec = [];
for i = 1 : size(V,2)
    if( D(i,i)>4e+07)
        L_eig_vec = [L_eig_vec V(:,i)];
    end
end

%%%%%%%%%%%%%%%%%%%%%%%% Calculating the eigenvectors of covariance matrix ‘C‘
% Eigenvectors of covariance matrix C (or so-called "Eigenfaces")
% can be recovered from L‘s eiegnvectors.
Eigenfaces = A * L_eig_vec; % A: centered image vectors

识别过程代码如下：

function OutputName = Recognition(TestImage, m, A, Eigenfaces)
% Recognizing step....
%
% Description: This function compares two faces by projecting the images into facespace and
% measuring the Euclidean distance between them.
%
% Argument:      TestImage              - Path of the input test image
%
%                m                      - (M*Nx1) Mean of the training
%                                         database, which is output of ‘EigenfaceCore‘ function.
%
%                Eigenfaces             - (M*Nx(P-1)) Eigen vectors of the
%                                         covariance matrix of the training
%                                         database, which is output of ‘EigenfaceCore‘ function.
%
%                A                      - (M*NxP) Matrix of centered image
%                                         vectors, which is output of ‘EigenfaceCore‘ function.
%
% Returns:       OutputName             - Name of the recognized image in the training database.
%
% See also: RESHAPE, STRCAT

% Original version by Amir Hossein Omidvarnia, October 2007
%                     Email: aomidvar@ece.ut.ac.ir                  

%%%%%%%%%%%%%%%%%%%%%%%% Projecting centered image vectors into facespace
% All centered images are projected into facespace by multiplying in
% Eigenface basis‘s. Projected vector of each face will be its corresponding
% feature vector.

ProjectedImages = [];
Train_Number = size(Eigenfaces,2);
for i = 1 : Train_Number
    temp = Eigenfaces‘*A(:,i); % Projection of centered images into facespace
    ProjectedImages = [ProjectedImages temp];
end

%%%%%%%%%%%%%%%%%%%%%%%% Extracting the PCA features from test image
InputImage = imread(TestImage);
temp = InputImage(:,:,1);

[irow icol] = size(temp);
InImage = reshape(temp‘,irow*icol,1);
Difference = double(InImage)-m; % Centered test image
ProjectedTestImage = Eigenfaces‘*Difference; % Test image feature vector

%%%%%%%%%%%%%%%%%%%%%%%% Calculating Euclidean distances
% Euclidean distances between the projected test image and the projection
% of all centered training images are calculated. Test image is
% supposed to have minimum distance with its corresponding image in the
% training database.

Euc_dist = [];
for i = 1 : Train_Number
    q = ProjectedImages(:,i);
    temp = ( norm( ProjectedTestImage - q ) )^2;
    Euc_dist = [Euc_dist temp];
end

[Euc_dist_min , Recognized_index] = min(Euc_dist);
OutputName = strcat(int2str(Recognized_index),‘.jpg‘);

其中训练样本的最后一行代码：

Eigenfaces = A * L_eig_vec; % A: centered image vectors

和识别过程的

 temp = Eigenfaces‘*A(:,i); % Projection of centered images into facespace

写成公式也就是如下

T=(AV)TA=VT(ATA)

这里VT是新坐标系下的基，投影的结果也就是新坐标系下的系数。基于PCA的人脸识别也就是我们在新的坐标系下比较两个向量的距离。稍后上传完整代码。

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