C++ : Create 3D array out of stacking 2D arrays - c++

Question

In Python I normally use functions like vstack, stack, etc to easily create a 3D array by stacking 2D arrays one onto another.
Is there any way to do this in C++?
In particular, I have loaded a image into a Mat variable with OpenCV like:
cv::Mat im = cv::imread("image.png", 0);
I would like to make a 3D array/Mat of N layers by stacking copies of that Mat variable.
EDIT: This new 3D matrix has to be "travellable" by adding an integer to any of its components, such that if I am in the position (x1,y1,1) and I add +1 to the last component, I arrive to (x1,y1,2). Similarly for any of the coordinates/components of the 3D matrix.
SOLVED: Both answers from #Aram and #Nejc do exactly what expected. I set #Nejc 's answer as the correct one for his shorter code.

Answer 1

The Numpy function vstack returns a contiguous array. Any C++ solution that produces vectors or arrays of cv::Mat objects does not reflect the behaviour of vstack in this regard, becase separate "layers" belonging to individual cv::Mat objects will not be stored in contiguous buffer (unless a careful allocation of underlying buffers is done in advance of course).
I present the solution that copies all arrays into a three-dimensional cv::Mat object with a contiguous buffer. As far as the idea goes, this answer is similar to Aram's answer. But instead of assigning pixel values one by one, I take advantage of OpenCV functions. At the beginning I allocate the matrix which has a size N X ROWS X COLS, where N is the number of 2D images I want to "stack" and ROWS x COLS are dimensions of each of these images.
Then I make N steps. On every step, I obtain the pointer to the location of the first element along the "outer" dimension. I pass that pointer to the constructor of temporary Mat object that acts as a kind of wrapper around the memory chunk of size ROWS x COLS (but no copies are made) that begins at the address that is pointed-at by pointer. I then use copyTo method to copy i-th image into that memory chunk. Code for N = 2:
cv::Mat img0 = cv::imread("image0.png", CV_IMREAD_GRAYSCALE);
cv::Mat img1 = cv::imread("image1.png", CV_IMREAD_GRAYSCALE);
cv::Mat images[2] = {img0, img1}; // you can also use vector or some other container
int dims[3] = { 2, img0.rows, img0.cols }; // dimensions of new image
cv::Mat joined(3, dims, CV_8U); // same element type (CV_8U) as input images
for(int i = 0; i < 2; ++i)
{
uint8_t* ptr = &joined.at<uint8_t>(i, 0, 0); // pointer to first element of slice i
cv::Mat destination(img0.rows, img0.cols, CV_8U, (void*)ptr); // no data copy, see documentation
images[i].copyTo(destination);
}

Answer 2

This answer is in response to the question above of:
In Python I normally use functions like vstack, stack, etc to easily create a 3D array by stacking 2D arrays one onto another.
This is certainly possible, you can add matrices into a vector which would be your "stack"
For instance you could use a
std::vector<cv::Mat>>
This would give you a vector of mats, which would be one slice, and then you could "layer" those by adding more slices vector
If you then want to have multiple stacks you can add that vector into another vector:
std::vector<std::vector<cv::Mat>>
To add matrix to an array you do:
myVector.push_back(matrix);
Edit for question below
In such case, could I travel from one position (x1, y1, z1) to an immediately upper position doing (x1,y1,z1+1), such that my new position in the matrix would be (x1,y1,z2)?
You'll end up with something that looks a lot like this. If you have a matrix at element 1 in your vector, it doesn't really have any relationship to the element[2] except for the fact that you have added it into that point. If you want to build relationships then you will need to code that in yourself.

Answer 3

You can actually create a 3D or ND mat with opencv, you need to use the constructor that takes the dimensions as input. Then copy each matrix into (this case) the 3D array
#include <opencv2/opencv.hpp>
using namespace cv;
using namespace std;
int main() {
// Dimensions for the constructor... set dims[0..2] to what you want
int dims[] = {5, 5, 5}; // 5x5x5 3d mat
Mat m = Mat::zeros(5, 5, CV_8UC1);
for (size_t i = 0; i < 5; i++) {
for (size_t k = 0; k < 5; k++) {
m.at<uchar>(i, k) = i + k;
}
}
// Mat with constructor specifying 3 dimensions with dimensions sizes in dims.
Mat 3DMat = Mat(3, dims, CV_8UC1);
// We fill our 3d mat.
for (size_t i = 0; i < m2.size[0]; i++) {
for (size_t k = 0; k < m2.size[1]; k++) {
for (size_t j = 0; j < m2.size[2]; j++) {
3DMat.at<uchar>(i, k, j) = m.at<uchar>(k, j);
}
}
}
// We print it to show the 5x5x5 array.
for (size_t i = 0; i < m2.size[0]; i++) {
for (size_t k = 0; k < m2.size[1]; k++) {
for (size_t j = 0; j < m2.size[2]; j++) {
std::cout << (int) 3DMat.at<uchar>(i, k, j) << " ";
}
std::cout << endl;
}
std::cout << endl;
}
return 0;
}

Answer 4

Based on the question and comments, I think you are looking for something like this:
std::vector<cv::Mat> vec_im;
//In side for loop:
vec_im.push_back(im);
Then, you can access it by:
Scalar intensity_1 = vec_im[z1].at<uchar>(y, x);
Scalar intensity_2 = vec_im[z2].at<uchar>(y, x);
This assumes that the image is single channel.