opencv slicing of a vector Mat - c++

Question

I am new with OpenCV. I am working on Visual Studio 2017 and use the plugin Image Watch to see Mat file of openCV.
What I've done:
I have to read a binary file to get 1000 images (256*320 pixels uint16 so 2 octets by pixel) in an array of double. After this, I wanted to see with Image Watch my data to be sure all is okay. So I convert the first image into a uchar on 8 bit to visualise it. I add my code (most part don't read it, just go to the end) :
#include "stdafx.h"
#include <iostream>
#include "stdio.h"
#include <fstream>
#include <stdint.h>
#include "windows.h"
#include <opencv2/core/core.hpp> // cv::Mat
#include <math.h>
#include <vector>
using namespace std;
using namespace cv;
template<class T>
T my_ntoh_little(unsigned char* buf) {
const auto s = sizeof(T);
T value = 0;
for (unsigned i = 0; i < s; i++)
value |= buf[i] << CHAR_BIT * i;
return value;
}
int main()
{
ifstream is("Filename", ifstream::binary);
if (is) {
// Reading size of the file and initialising variables
is.seekg(0, is.end);
int length = is.tellg();
int main_header_size = 3000;
int frame_header_size = 1000;
int width = 320, height = 256, count_frames = 1000;
int buffer_image = width * height * 2;
unsigned char *data_char = new unsigned char[length]; // Variable which will contains all the data
// Initializing 3D array for stocking all images
double ***data;
data = new double**[count_frames];
for (unsigned i = 0; i < count_frames; i++) {
data[i] = new double*[height];
for (unsigned j = 0; j < height; j++)
data[i][j] = new double[width];
}
// Reading the file once
is.seekg(0, is.beg);
is.read(reinterpret_cast<char*>(data_char), length);
// Convert pixel by pixel uchar into uint16 (using pointer on data_char)
int indice, minid = 65536.0, maxid = 0.0;
for (unsigned count = 0; count < count_frames; count++) {
// Initialize pointer address
indice = main_header_size + count * (frame_header_size + buffer_image) + frame_header_size;
for (unsigned i = 0; i < height; i++) {
for (unsigned j = 0; j < width; j++) {
data[count][i][j] = my_ntoh_little<uint16_t>(data_char + indice);
// Search for min/max for normalize after
if (data[count][i][j] < minid and count == 0)
minid = data[count][i][j];
if (data[count][i][j] > maxid and count == 0)
maxid = data[count][i][j];
// Updating pointer to next pixel
indice += 2;
}
}
}
// Get back first image, normalize between 0-255, cast into uchar to the future Mat object
uchar *dataImRGB = new uchar[width * height * 3];
int image_display = 900;
int pixel_norm;
for (unsigned i = 0; i < height; i++) {
for (unsigned j = 0; j < width; j++) {
pixel_norm = round((data[image_display][i][j] - double(minid)) / double(maxid - minid) * 255);
dataImRGB[i * 320 * 3 + 3 * j] = static_cast<uchar>(pixel_norm);
dataImRGB[i * 320 * 3 + 3 * j + 1] = static_cast<uchar>(pixel_norm);
dataImRGB[i * 320 * 3 + 3 * j + 2] = static_cast<uchar>(pixel_norm);
}
}
// Create Mat object (it is imageRGB8 I can see on Image watch)
Mat imageRGB8 = Mat(width, height, CV_8UC3, dataImRGB);
// Creating a list of Map and add first Mat
vector<Mat> listImages;
listImages.push_back(imageRGB8);
// -----------------------------------------------------------------------------------------
// -----------------------------------------------------------------------------------------
// Future : directly keep the uchar read in the original file and import it on a Mat object
// But how to get the pixel at (0,0) of the first Mat on the vector ?
// -----------------------------------------------------------------------------------------
// -----------------------------------------------------------------------------------------
// De-Allocate memory to prevent memory leak
for (int i = 0; i < count_frames; ++i) {
for (int j = 0; j < height; ++j)
delete[] data[i][j];
delete[] data[i];
}
delete[] data;
}
return 0;
}
Where I am stuck:
I don't know how to work with this vector, how to manipulate the data. For example, if i want to do the mean of all images, so the mean of all Mat objects in the vector, how to do this ? Or just how to get the first pixel of the third image in the vector ? These examples have for aim to explain me the slicing with such type of data because I know how it works with vector of double, but not with openCv object.
Thank you in advance for any help/advice.

Answer 1

Assuming that you have got all of your images properly packed into your image list you can do the following:
This will get the mean of all images in your list:
cv::Scalar meansum(0.0f,0.0f,0.0f);
size_t length = listImages.size();
for (size_t i = 0; i < length; i++){
//mu == mean of current image
cv::Scalar mu = cv::mean(listImages[i]);
meansum += mu;
}
float means[3] = { meansum[0] / length, meansum[1] / length, meansum[2] / length };
std::cout << "Means " << means[0] << " " << means[1] << " " << means[2] << std::endl;
To get the first pixel in your third image you can use the at() method or a row pointer. (Row pointers are faster, but don't have any guards against accessing out of bounds memory locations.)
Mat third_image = list_images[2];
//using at()
uchar first_pixel_blue_value = third_image.at<uchar>(0,0,0);
std::cout<<(int)first_pixel_blue_value<<std::endl;
//using row pointer
uchar* row = third_image.ptr<uchar>(0); //pointer to row 0
std::cout<<"blue: " <<(int)row[0];
std::cout<<" green: "<<(int)row[1];
std::cout<<" red: " <<(int)row[2];
More info can be found here:
https://docs.opencv.org/3.1.0/d2/de8/group__core__array.html (under functions)
and here:
https://docs.opencv.org/trunk/d3/d63/classcv_1_1Mat.html