PCA in OpenCV using the new C++ interface

Question

As an aside: Apologies if I'm flooding SO with OpenCV questions :p

I'm currently trying to port over my old C code to use the new C++ interface and I've got to the point where I'm rebuilidng my Eigenfaces face recogniser class.

 Mat img = imread("1.jpg");
 Mat img2 = imread("2.jpg");
 FaceDetector* detect = new HaarDetector("haarcascade_frontalface_alt2.xml");

 // convert to grey scale
 Mat g_img, g_img2;
 cvtColor(img, g_img, CV_BGR2GRAY);
 cvtColor(img2, g_img2, CV_BGR2GRAY);

 // find the faces in the images
 Rect r = detect->getFace(g_img);
 Mat img_roi = g_img(r);

 r = detect->getFace(g_img2);
 Mat img2_roi = g_img2(r);

 // create the data matrix for PCA
 Mat data;
 data.create(2,1, img2_roi.type());
 data.row(0) = img_roi;
 data.row(1) = img2_roi;

 // perform PCA
 Mat averageFace;
 PCA pca(data, averageFace, CV_PCA_DATA_AS_ROW, 2);

 //namedWindow("avg",1); imshow("avg", averageFace); - causes segfault
 //namedWindow("avg",1); imshow("avg", Mat(pca.mean)); - doesn't work

I'm trying to create the PCA space, and then see if it's working by displaying the computed average image. Are there any other steps to this?

Perhaps I need to project the images onto the PCA subspace first of all?

Answer 1

Your error is probably here:

Mat data;
data.create(2,1, img2_roi.type());
data.row(0) = img_roi;
data.row(1) = img2_roi;

PCA expects a matrix with the data vectors as rows. However, you never scale the images to the same size so that they have the same number of pixels (so the dimension is the same), also data.create(2,1,...) - the 1 needs to be the dimension of your vector, i.e. the number of your pixels. Then copy the pixels from the crop to your matrix.