opencv, accessing element for downsampling but white window appear - c++

Question

i'm learning c++ api of opencv, and for a simple approach i've started with try to downsample image (ok i know that there is pyrDown with gaussian resampling but it's for learning how to access element in Mat class)
this is my code:
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <iostream>
#define original_window "original"
#define manual_window "manual"
using namespace cv;
using namespace std;
Mat img, manual;
void downsample(Mat src, Mat &dst, const Size& s) {
float factor = src.rows/(float)s.width;
Mat_<Vec3f> _dst = Mat(s, src.type());
Mat_<Vec3f> _src = src;
for(int i=0; i<src.cols; i+=factor) {
int _i = i/factor;
for(int j=0; j<src.rows; j+=factor) {
int _j = j/factor;
_dst (_j, _i) = _src(j,i);
}
}
cout << "downsample image size: " << _dst.rows << " " << _dst.cols << endl;
dst = Mat(_dst);
}
int main(int /*argc*/, char** /*argv*/) {
img = imread("lena.jpg");
cout << "original image size: " << img.rows << " " << img.cols << endl;
downsample(img, manual, Size(img.cols/2, img.rows/2));
namedWindow(original_window, CV_WINDOW_AUTOSIZE);
namedWindow(manual_window, CV_WINDOW_AUTOSIZE);
while( true )
{
char c = (char)waitKey(10);
if( c == 27 )
{ break; }
imshow( original_window, img );
imshow( manual_window, manual );
}
return 0;
}
now, i'm doing a downsampling in a fool way: i'm just deleting elements. and i'm try to use c++ api with Mat_.
in manual window i get a white window, and i don't understand why. event if i try to cout manual i'seeing different values.. what's wrong with this piece of code?
EDIT 1
i've found a solution:
dst.convertTo(dst, src.type()); // in this particular case: src.type() == CV_8UC3
at the end of downsample()
now my question is: why that? i declare Mat(s, src.type()); why it is modified?
EDIT 2
if i use #go4sri answer with this line
_dst (_j, _i) = src.at<Vec3f>(j, i);
i get this output:
i really does not understand why..

Answer 1

The way to access an element in OpenCV's Mat is as follows:
for a single channel matrix(tmp)
Matrix_Name.at<dataType>(row, col)
For a three channel matrix( as is the case for a color image), you will need to use the Vec3b/Vec3f type depending upon if yours is a unsigned char/float matrix.
As yours is a unsigned char 3Dimensional matrix:
you will have to access it as src.at<Vec3b>(i, j)
Your downsample should have been:
void downsample(const Mat& src, Mat &dst, const Size& s) {
float factor = src.rows/(float)s.height;
Mat _dst = Mat(s, src.type());
for(int i=0; i < src.cols; i += factor) {
int _i = i/factor;
for(int j=0; j<src.rows; j+=factor) {
int _j = j/factor;
_dst.at<Vec3b> (_j, _i) = src.at<Vec3b>(j, i);
}
}
cout << "downsample image size: " << _dst.rows << " " << _dst.cols << endl;
dst = Mat(_dst);
}