Getting the difference between two frames in opencv - c++

I'm trying to get the the difference between two cv::Mat frames in OpenCv. So here is what I tried,
#include<opencv2\opencv.hpp>
#include<opencv2\calib3d\calib3d.hpp>
#include<opencv2\core\core.hpp>
#include <opencv2\highgui\highgui.hpp>
int main ()
{
cv::VideoCapture cap(0);
cv::Mat frame, frame1,frame2;
int key=0;
while(key!=27){
cap >> frame;
if(key=='c'){
frame1 = frame;
key = 0;
}
if(key =='x'){
cv::absdiff(frame, frame1, frame2); // I also tried frame2= (frame -frame1)*255;
cv::imshow("difference ",frame2);
key =0;
}
cv::imshow("stream",frame);
key = cv::waitKey(10);
}
}
the result is always the same a 0 Matrix, any idea what I'm doing wrong here?
thanks in advance for your help.

Mat objects are pointer typed. After setting frame1 to frame directly using frame1 = frame, both matrices show the same point and same frame also. You have to copy frame value using "copyTo" method of Mat.

OpenCV Matrixes use pointers internally
The documentation of the Mat type states:
Mat is basically a class with two data parts: the matrix header and a pointer to the matrix containing the pixel values.
[...]
Whenever somebody copies a header of a Mat object, a counter is increased for the matrix. Whenever a header is cleaned this counter is decreased. When the counter reaches zero the matrix too is freed. Sometimes you will want to copy the matrix itself too, so OpenCV provides the clone() and copyTo() functions.
cv::Mat F = A.clone();
cv::Mat G;
A.copyTo(G);

OpenCV overloads the affectation operator on cv::Mat objects so that the line mat1 = mat2 only affects the pointer to the data in mat1 (that points to the same data as mat2). This avoids time consuming copies of all the image data.
If you want to save the data of a matrix, you have to write mat1 = mat2.clone() or mat2.copyTo(mat1).

I was looking for a similar program and I came across your post, here is a sample I have written for frameDifferencing, hope this helps, the below function will give you the difference between two frames
/** #function differenceFrame */
Mat differenceFrame( Mat prev_frame, Mat curr_frame )
{
Mat image = prev_frame.clone();
printf("frame rows %d Cols %d\n" , image.rows, image.cols);
for (int rows = 0; rows < image.rows; rows++)
{
for (int cols = 0; cols < image.cols; cols++)
{
/* printf("BGR value %lf %lf %lf\n" , abs(prev_frame.at<cv::Vec3b>(rows,cols)[0] -
curr_frame.at<cv::Vec3b>(rows,cols)[0]),
abs(prev_frame.at<cv::Vec3b>(rows,cols)[1] -
curr_frame.at<cv::Vec3b>(rows,cols)[0]),
abs(prev_frame.at<cv::Vec3b>(rows,cols)[2] -
curr_frame.at<cv::Vec3b>(rows,cols)[0]));
*/
image.at<cv::Vec3b>(rows,cols)[0] = abs(prev_frame.at<cv::Vec3b>(rows,cols)[0] -
curr_frame.at<cv::Vec3b>(rows,cols)[0]);
image.at<cv::Vec3b>(rows,cols)[1] = abs(prev_frame.at<cv::Vec3b>(rows,cols)[1] -
curr_frame.at<cv::Vec3b>(rows,cols)[1]);
image.at<cv::Vec3b>(rows,cols)[2] = abs(prev_frame.at<cv::Vec3b>(rows,cols)[2] -
curr_frame.at<cv::Vec3b>(rows,cols)[2]);
}
}
return image;
}

Related

OpenCV edit single band and remerge

I have an image which I have split into its three separate channels (b,g,r). I want to manipulate just the red band and then remerge to blue and green band to recompose image. I keep getting a sig abort in my function however. the RBandSlider refers to a global int used for a trackbar which is defaulted to 1. Almost positive the issue is within the ImageEnhancement function.
Do I need to define redBandsAdjsuted as something else or am I not grabbing the pixel local and rewriting it correctly?
Mat ImageEnhancement(Mat band){
Mat adjustedBand;
Scalar mean, std;
meanStdDev(band, mean , std);
int pixel,temp;
for(int i = 0; i < band.rows;i++){
for(int j = 0; j < band.cols;j++){
//extract pixel
pixel = band.at<Vec3b>(i,j)[0];
//pixel greater than mean
if ( pixel > mean[0]){
temp = (255);
adjustedBand.at<Vec3b>(i,j) = temp;
}
else{
temp = 0;
adjustedBand.at<Vec3b>(i,j) = temp ;
}
}
}
return adjustedBand;
}
Mat Bands[3],merged,redBandsAdjusted(image.cols,image.rows,CV_8UC1),result;
split(image, Bands);
//loop the echancement adjustment
while(true){
//adjust red band and merge
redBandsAdjusted = ImageEnhancement(Bands[2]);
vector<Mat> channels = {Bands[0],Bands[1],redBandsAdjusted};
merge(channels,merged);
}
When you do:
split(image, Bands);
You will get from a CV_8UC3 image (image) 3 CV_8U images (Bands). Everything is good until this point. Then you go to your adjusting and do 2 mistakes:
Mat adjustedBand; is never initialized... You can do Mat adjustedBand(band.rows, band.cols, CV_8UC1); or intialized in a later stage.
pixel = band.at<Vec3b>(i,j)[0]; and adjustedBand.at<Vec3b>(i,j) = temp; are for manipulating 3 channels not a 1 channel image. You need to use ucharinstead, like: adjustedBand.at<uchar>(i,j) = temp;
Those are the errors I see... fix them and try using a debugger, that way you know if something is initialize correctly or if it does the correct operation

Split OpenCV Mat without copying the data

I have a RGB image and I trying to do some modification on R channel. So I do similar to the following:
Mat img;
vector<Mat> chs;
//....
split(img, chs);
//some modification on chs[2]
imshow("Result", img);
But it seems that OpenCV copy data to chs by value (not by reference). As a result the img matrix not changed. But due to memory limitations I don't prefer to use merge function.
Is there any alternative to split the matrix in-place?
split will always copy the data, since it's creating new matrices.
The simplest way to work on, say, red channel will be using split and merge:
Mat3b img(10,10,Vec3b(1,2,3));
vector<Mat1b> planes;
split(img, planes);
// Work on red plane
planes[2](2,3) = 5;
merge(planes, img);
Note that merge doesn't allocate any new memory, so if you're ok with split, there isn't any good reason not to call also merge.
You can always work on the R channel directly:
Mat3b img(10,10,Vec3b(1,2,3));
// Work on red channel, [2]
img(2,3)[2] = 5;
If you want to save the memory used by split, you can work directly on the red channel, but it's more cumbersome:
#include <opencv2\opencv.hpp>
using namespace cv;
int main()
{
Mat3b img(10,10,Vec3b(1,2,3));
// Create a column matrix header with red plane unwound
// No copies here
Mat1b R = img.reshape(1, img.rows*img.cols).colRange(2, 3);
// Work on red plane
int r = 2;
int c = 3;
// You need to access by index, not by (row, col).
// This will also modify img
R(img.rows * r + c) = 5;
return 0;
}
You can probably find a good compromise by copying the red channel only in a new matrix (avoiding to allocate space also for other channels), and then by copying the result back into original image:
#include <opencv2\opencv.hpp>
using namespace cv;
int main()
{
Mat3b img(10,10,Vec3b(1,2,3));
// Allocate space only for red channel
Mat1b R(img.rows, img.cols);
for (int r=0; r<img.rows; ++r)
for(int c=0; c<img.cols; ++c)
R(r, c) = img(r, c)[2];
// Work on red plane
R(2,3) = 5;
// Copy back into img
for (int r = 0; r<img.rows; ++r)
for (int c = 0; c<img.cols; ++c)
img(r, c)[2] = R(r,c);
return 0;
}
Thanks to #sturkmen for reviewing the answer

Convert Mat to Array/Vector in OpenCV

I am novice in OpenCV. Recently, I have troubles finding OpenCV functions to convert from Mat to Array. I researched with .ptr and .at methods available in OpenCV APIs, but I could not get proper data. I would like to have direct conversion from Mat to Array(if available, if not to Vector). I need OpenCV functions because the code has to be undergo high level synthesis in Vivado HLS. Please help.
If the memory of the Mat mat is continuous (all its data is continuous), you can directly get its data to a 1D array:
std::vector<uchar> array(mat.rows*mat.cols*mat.channels());
if (mat.isContinuous())
array = mat.data;
Otherwise, you have to get its data row by row, e.g. to a 2D array:
uchar **array = new uchar*[mat.rows];
for (int i=0; i<mat.rows; ++i)
array[i] = new uchar[mat.cols*mat.channels()];
for (int i=0; i<mat.rows; ++i)
array[i] = mat.ptr<uchar>(i);
UPDATE: It will be easier if you're using std::vector, where you can do like this:
std::vector<uchar> array;
if (mat.isContinuous()) {
// array.assign(mat.datastart, mat.dataend); // <- has problems for sub-matrix like mat = big_mat.row(i)
array.assign(mat.data, mat.data + mat.total()*mat.channels());
} else {
for (int i = 0; i < mat.rows; ++i) {
array.insert(array.end(), mat.ptr<uchar>(i), mat.ptr<uchar>(i)+mat.cols*mat.channels());
}
}
p.s.: For cv::Mats of other types, like CV_32F, you should do like this:
std::vector<float> array;
if (mat.isContinuous()) {
// array.assign((float*)mat.datastart, (float*)mat.dataend); // <- has problems for sub-matrix like mat = big_mat.row(i)
array.assign((float*)mat.data, (float*)mat.data + mat.total()*mat.channels());
} else {
for (int i = 0; i < mat.rows; ++i) {
array.insert(array.end(), mat.ptr<float>(i), mat.ptr<float>(i)+mat.cols*mat.channels());
}
}
UPDATE2: For OpenCV Mat data continuity, it can be summarized as follows:
Matrices created by imread(), clone(), or a constructor will always be continuous.
The only time a matrix will not be continuous is when it borrows data (except the data borrowed is continuous in the big matrix, e.g. 1. single row; 2. multiple rows with full original width) from an existing matrix (i.e. created out of an ROI of a big mat).
Please check out this code snippet for demonstration.
Can be done in two lines :)
Mat to array
uchar * arr = image.isContinuous()? image.data: image.clone().data;
uint length = image.total()*image.channels();
Mat to vector
cv::Mat flat = image.reshape(1, image.total()*image.channels());
std::vector<uchar> vec = image.isContinuous()? flat : flat.clone();
Both work for any general cv::Mat.
Explanation with a working example
cv::Mat image;
image = cv::imread(argv[1], cv::IMREAD_UNCHANGED); // Read the file
cv::namedWindow("cvmat", cv::WINDOW_AUTOSIZE );// Create a window for display.
cv::imshow("cvmat", image ); // Show our image inside it.
// flatten the mat.
uint totalElements = image.total()*image.channels(); // Note: image.total() == rows*cols.
cv::Mat flat = image.reshape(1, totalElements); // 1xN mat of 1 channel, O(1) operation
if(!image.isContinuous()) {
flat = flat.clone(); // O(N),
}
// flat.data is your array pointer
auto * ptr = flat.data; // usually, its uchar*
// You have your array, its length is flat.total() [rows=1, cols=totalElements]
// Converting to vector
std::vector<uchar> vec(flat.data, flat.data + flat.total());
// Testing by reconstruction of cvMat
cv::Mat restored = cv::Mat(image.rows, image.cols, image.type(), ptr); // OR vec.data() instead of ptr
cv::namedWindow("reconstructed", cv::WINDOW_AUTOSIZE);
cv::imshow("reconstructed", restored);
cv::waitKey(0);
Extended explanation:
Mat is stored as a contiguous block of memory, if created using one of its constructors or when copied to another Mat using clone() or similar methods. To convert to an array or vector we need the address of its first block and array/vector length.
Pointer to internal memory block
Mat::data is a public uchar pointer to its memory.
But this memory may not be contiguous. As explained in other answers, we can check if mat.data is pointing to contiguous memory or not using mat.isContinous(). Unless you need extreme efficiency, you can obtain a continuous version of the mat using mat.clone() in O(N) time. (N = number of elements from all channels). However, when dealing images read by cv::imread() we will rarely ever encounter a non-continous mat.
Length of array/vector
Q: Should be row*cols*channels right?
A: Not always. It can be rows*cols*x*y*channels.
Q: Should be equal to mat.total()?
A: True for single channel mat. But not for multi-channel mat
Length of the array/vector is slightly tricky because of poor documentation of OpenCV. We have Mat::size public member which stores only the dimensions of single Mat without channels. For RGB image, Mat.size = [rows, cols] and not [rows, cols, channels]. Mat.total() returns total elements in a single channel of the mat which is equal to product of values in mat.size. For RGB image, total() = rows*cols. Thus, for any general Mat, length of continuous memory block would be mat.total()*mat.channels().
Reconstructing Mat from array/vector
Apart from array/vector we also need the original Mat's mat.size [array like] and mat.type() [int]. Then using one of the constructors that take data's pointer, we can obtain original Mat. The optional step argument is not required because our data pointer points to continuous memory. I used this method to pass Mat as Uint8Array between nodejs and C++. This avoided writing C++ bindings for cv::Mat with node-addon-api.
References:
Create memory continuous Mat
OpenCV Mat data layout
Mat from array
Here is another possible solution assuming matrix have one column( you can reshape original Mat to one column Mat via reshape):
Mat matrix= Mat::zeros(20, 1, CV_32FC1);
vector<float> vec;
matrix.col(0).copyTo(vec);
None of the provided examples here work for the generic case, which are N dimensional matrices. Anything using "rows" assumes theres columns and rows only, a 4 dimensional matrix might have more.
Here is some example code copying a non-continuous N-dimensional matrix into a continuous memory stream - then converts it back into a Cv::Mat
#include <iostream>
#include <cstdint>
#include <cstring>
#include <opencv2/opencv.hpp>
int main(int argc, char**argv)
{
if ( argc != 2 )
{
std::cerr << "Usage: " << argv[0] << " <Image_Path>\n";
return -1;
}
cv::Mat origSource = cv::imread(argv[1],1);
if (!origSource.data) {
std::cerr << "Can't read image";
return -1;
}
// this will select a subsection of the original source image - WITHOUT copying the data
// (the header will point to a region of interest, adjusting data pointers and row step sizes)
cv::Mat sourceMat = origSource(cv::Range(origSource.size[0]/4,(3*origSource.size[0])/4),cv::Range(origSource.size[1]/4,(3*origSource.size[1])/4));
// correctly copy the contents of an N dimensional cv::Mat
// works just as fast as copying a 2D mat, but has much more difficult to read code :)
// see http://stackoverflow.com/questions/18882242/how-do-i-get-the-size-of-a-multi-dimensional-cvmat-mat-or-matnd
// copy this code in your own cvMat_To_Char_Array() function which really OpenCV should provide somehow...
// keep in mind that even Mat::clone() aligns each row at a 4 byte boundary, so uneven sized images always have stepgaps
size_t totalsize = sourceMat.step[sourceMat.dims-1];
const size_t rowsize = sourceMat.step[sourceMat.dims-1] * sourceMat.size[sourceMat.dims-1];
size_t coordinates[sourceMat.dims-1] = {0};
std::cout << "Image dimensions: ";
for (int t=0;t<sourceMat.dims;t++)
{
// calculate total size of multi dimensional matrix by multiplying dimensions
totalsize*=sourceMat.size[t];
std::cout << (t>0?" X ":"") << sourceMat.size[t];
}
// Allocate destination image buffer
uint8_t * imagebuffer = new uint8_t[totalsize];
size_t srcptr=0,dptr=0;
std::cout << std::endl;
std::cout << "One pixel in image has " << sourceMat.step[sourceMat.dims-1] << " bytes" <<std::endl;
std::cout << "Copying data in blocks of " << rowsize << " bytes" << std::endl ;
std::cout << "Total size is " << totalsize << " bytes" << std::endl;
while (dptr<totalsize) {
// we copy entire rows at once, so lowest iterator is always [dims-2]
// this is legal since OpenCV does not use 1 dimensional matrices internally (a 1D matrix is a 2d matrix with only 1 row)
std::memcpy(&imagebuffer[dptr],&(((uint8_t*)sourceMat.data)[srcptr]),rowsize);
// destination matrix has no gaps so rows follow each other directly
dptr += rowsize;
// src matrix can have gaps so we need to calculate the address of the start of the next row the hard way
// see *brief* text in opencv2/core/mat.hpp for address calculation
coordinates[sourceMat.dims-2]++;
srcptr = 0;
for (int t=sourceMat.dims-2;t>=0;t--) {
if (coordinates[t]>=sourceMat.size[t]) {
if (t==0) break;
coordinates[t]=0;
coordinates[t-1]++;
}
srcptr += sourceMat.step[t]*coordinates[t];
}
}
// this constructor assumes that imagebuffer is gap-less (if not, a complete array of step sizes must be given, too)
cv::Mat destination=cv::Mat(sourceMat.dims, sourceMat.size, sourceMat.type(), (void*)imagebuffer);
// and just to proof that sourceImage points to the same memory as origSource, we strike it through
cv::line(sourceMat,cv::Point(0,0),cv::Point(sourceMat.size[1],sourceMat.size[0]),CV_RGB(255,0,0),3);
cv::imshow("original image",origSource);
cv::imshow("partial image",sourceMat);
cv::imshow("copied image",destination);
while (cv::waitKey(60)!='q');
}
Instead of getting image row by row, you can put it directly to an array. For CV_8U type image, you can use byte array, for other types check here.
Mat img; // Should be CV_8U for using byte[]
int size = (int)img.total() * img.channels();
byte[] data = new byte[size];
img.get(0, 0, data); // Gets all pixels
byte * matToBytes(Mat image)
{
int size = image.total() * image.elemSize();
byte * bytes = new byte[size]; //delete[] later
std::memcpy(bytes,image.data,size * sizeof(byte));
}
You can use iterators:
Mat matrix = ...;
std::vector<float> vec(matrix.begin<float>(), matrix.end<float>());
cv::Mat m;
m.create(10, 10, CV_32FC3);
float *array = (float *)malloc( 3*sizeof(float)*10*10 );
cv::MatConstIterator_<cv::Vec3f> it = m.begin<cv::Vec3f>();
for (unsigned i = 0; it != m.end<cv::Vec3f>(); it++ ) {
for ( unsigned j = 0; j < 3; j++ ) {
*(array + i ) = (*it)[j];
i++;
}
}
Now you have a float array. In case of 8 bit, simply change float to uchar, Vec3f to Vec3b and CV_32FC3 to CV_8UC3.
If you know that your img is 3 channel, than you can try this code
Vec3b* dados = new Vec3b[img.rows*img.cols];
for (int i = 0; i < img.rows; i++)
for(int j=0;j<img.cols; j++)
dados[3*i*img.cols+j] =img.at<Vec3b>(i,j);
If you wanna check the (i,j) vec3b you can write
std::cout << (Vec3b)img.at<Vec3b>(i,j) << std::endl;
std::cout << (Vec3b)dados[3*i*img.cols+j] << std::endl;
Since answer above is not very accurate as mentioned in its comments but its "edit queue is full", I have to add correct one-liners.
Mat(uchar, 1 channel) to vector(uchar):
std::vector<uchar> vec = (image.isContinuous() ? image : image.clone()).reshape(1, 1); // data copy here
vector(any type) to Mat(the same type):
Mat m(vec, false); // false(by default) -- do not copy data

OpenCV Mat object - get data length

In OpenCV, I'm able to capture frames using VideoCapture in C++, however, when I try to get the data from a frame and calculate length, it just returns me 0.
Below is my sample code:
VideoCapture cap(0);
for(;;) {
Mat frame;
cap >> frame;
int length = strlen((char*) frame.data); // returns 0
}
As I mentioned above that if I save the frame in a PNG file, I can actually see the image so I'm not able to understand why the data length is coming out to be zero.
Any clue?
You can also do:
Mat mat;
int len = mat.total() * mat.elemSize(); // or mat.elemSize1()
The strlen method only works on strings, which are arrays of chars terminated by a special character:
http://www.cplusplus.com/reference/cstring/strlen/
You have cast a Mat type as a char*, so it is not a string.
Building on the solution here, try:
Mat mat;
int rows = mat.rows;
int cols = mat.cols;
int num_el = rows*cols;
int len = num_el*mat.elemSize1();
to get the size of one channel in bytes. Also, use elemSize() if you want all the channels (i.e. you'll get 3 times the value of elemSize1() if the Mat is a 3 channel image).
Take a look here for discussion of the various types Mat can contain:
http://docs.opencv.org/modules/core/doc/basic_structures.html#mat-type

PCA + SVM using C++ Syntax in OpenCV 2.2

I'm having problems getting PCA and Eigenfaces working using the latest C++ syntax with the Mat and PCA classes. The older C syntax took an array of IplImage* as a parameter to perform its processing and the current API only takes a Mat that is formatted by Column or Row. I took the Row approach using the reshape function to fit my image's matrix to fit in a single row. I eventually want to take this data and then use the SVM algorithm to perform detection, but when I do that all my data is just a stream of 0s. Can someone please help me out? What am I doing wrong? Thanks!
I saw this question and it's somewhat related, but I'm not sure what the solution is.
This is basically what I have:
vector<Mat> images; //This variable will be loaded with a set of images to perform PCA on.
Mat values(images.size(), 1, CV_32SC1); //Values are the corresponding values to each of my images.
int nEigens = images.size() - 1; //Number of Eigen Vectors.
//Load the images into a Matrix
Mat desc_mat(images.size(), images[0].rows * images[0].cols, CV_32FC1);
for (int i=0; i<images.size(); i++) {
desc_mat.row(i) = images[i].reshape(1, 1);
}
Mat average;
PCA pca(desc_mat, average, CV_PCA_DATA_AS_ROW, nEigens);
Mat data(desc_mat.rows, nEigens, CV_32FC1); //This Mat will contain all the Eigenfaces that will be used later with SVM for detection
//Project the images onto the PCA subspace
for(int i=0; i<images.size(); i++) {
Mat projectedMat(1, nEigens, CV_32FC1);
pca.project(desc_mat.row(i), projectedMat);
data.row(i) = projectedMat.row(0);
}
CvMat d1 = (CvMat)data;
CvMat d2 = (CvMat)values;
CvSVM svm;
svm.train(&d1, &d2);
svm.save("svmdata.xml");
What etarion said is correct.
To copy a column or row you always have to write:
Mat B = mat.col(i);
A.copyTo(B);
The following program shows how to perform a PCA in OpenCV. It'll show the mean image and the first three Eigenfaces. The images I used in there are available from http://www.cl.cam.ac.uk/research/dtg/attarchive/facedatabase.html:
#include "cv.h"
#include "highgui.h"
using namespace std;
using namespace cv;
Mat normalize(const Mat& src) {
Mat srcnorm;
normalize(src, srcnorm, 0, 255, NORM_MINMAX, CV_8UC1);
return srcnorm;
}
int main(int argc, char *argv[]) {
vector<Mat> db;
// load greyscale images (these are from http://www.cl.cam.ac.uk/research/dtg/attarchive/facedatabase.html)
db.push_back(imread("s1/1.pgm",0));
db.push_back(imread("s1/2.pgm",0));
db.push_back(imread("s1/3.pgm",0));
db.push_back(imread("s2/1.pgm",0));
db.push_back(imread("s2/2.pgm",0));
db.push_back(imread("s2/3.pgm",0));
db.push_back(imread("s3/1.pgm",0));
db.push_back(imread("s3/2.pgm",0));
db.push_back(imread("s3/3.pgm",0));
db.push_back(imread("s4/1.pgm",0));
db.push_back(imread("s4/2.pgm",0));
db.push_back(imread("s4/3.pgm",0));
int total = db[0].rows * db[0].cols;
// build matrix (column)
Mat mat(total, db.size(), CV_32FC1);
for(int i = 0; i < db.size(); i++) {
Mat X = mat.col(i);
db[i].reshape(1, total).col(0).convertTo(X, CV_32FC1, 1/255.);
}
// Change to the number of principal components you want:
int numPrincipalComponents = 12;
// Do the PCA:
PCA pca(mat, Mat(), CV_PCA_DATA_AS_COL, numPrincipalComponents);
// Create the Windows:
namedWindow("avg", 1);
namedWindow("pc1", 1);
namedWindow("pc2", 1);
namedWindow("pc3", 1);
// Mean face:
imshow("avg", pca.mean.reshape(1, db[0].rows));
// First three eigenfaces:
imshow("pc1", normalize(pca.eigenvectors.row(0)).reshape(1, db[0].rows));
imshow("pc2", normalize(pca.eigenvectors.row(1)).reshape(1, db[0].rows));
imshow("pc3", normalize(pca.eigenvectors.row(2)).reshape(1, db[0].rows));
// Show the windows:
waitKey(0);
}
and if you want to build the matrix by row (like in your original question above) use this instead:
// build matrix
Mat mat(db.size(), total, CV_32FC1);
for(int i = 0; i < db.size(); i++) {
Mat X = mat.row(i);
db[i].reshape(1, 1).row(0).convertTo(X, CV_32FC1, 1/255.);
}
and set the flag in the PCA to:
CV_PCA_DATA_AS_ROW
Regarding machine learning. I wrote a document on machine learning with the OpenCV C++ API that has examples for most of the classifiers, including Support Vector Machines. Maybe you can get some inspiration there: http://www.bytefish.de/pdf/machinelearning.pdf.
data.row(i) = projectedMat.row(0);
This will not work. operator= is a shallow copy, meaning no data is actually copied. Use
cv::Mat sample = data.row(i); // also a shallow copy, points to old data!
projectedMat.row(0).copyTo(sample);
The same also for:
desc_mat.row(i) = images[i].reshape(1, 1);
I would suggest looking at the newly checked in tests in svn head
modules/core/test/test_mat.cpp
online here : https://code.ros.org/svn/opencv/trunk/opencv/modules/core/test/test_mat.cpp
has examples for PCA in the old c and new c++
Hope that helps!