How to copy single image from vector of images to a temporary cv::Mat?

Question

I call a function rgbTOgray and trying to copy each image in a "vector of images" (which is a vector of colored images) to a temporary cv::Mat using a for loop. And, convert it to grayscale using cvtColor. Here is the code:

std::vector<cv::Mat> rgbTOgray(std::vector<cv::Mat>& rgb_image)
{
  cv::Mat temp_gray, temp_rgb;
  std::vector<cv::Mat> gray_image;
  std::vector<cv::Mat>::iterator itr;

  for ( itr = rgb_image.begin() ; itr < rgb_image.end() ; itr++) 
  {
    temp_rgb = *itr;
    cvtColor(temp_rgb, temp_gray, CV_RGB2GRAY);
    gray_image.push_back(temp_gray);
  }

  return gray_image;
}

EDIT: I am returning a vector of Gray-scale images "gray_image". But this doesn't seem to work.

EDIT_2: OpenCV Error: Assertion failed (scn == 3 || scn == 4) in cvtColor

Could anybody please help me out? I am new and learning all this stuff. Thanks!

Answer 1

You are not storing in vector gray_image the gray images, but cv::Mat headers with pointers to a single block of memory data which is firstly allocated by temp_gray . You must clone those data not to overwrite your images:

gray_image.push_back(temp_gray.clone());

By the way, I would add gray_image.reserve(rgb_image.size()); before your loop.

Answer 2

One problem is that you are forgetting that cv::Mat is reference counted with a "copy" not really copying anything, just a pointer.
The push_back(temp_grey) keeps pushing the same image into your array.
You should declare cv::Mat temp_gray inside the loop to make sure you get fresh copies for each element.

Also, check the number of channels of your "rgb" images ( temp_rgb.channels() ). The assert basically says that they are not 3/4 channels.

Answer 3

I do't know exactly why (I'm a learner too) but in my case it works only with .clone() when you push back the temp image just gray-convert.

Anyway, the iterator line in your code is correct.

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <iostream>

using namespace cv;
using namespace std;

std::vector<cv::Mat> rgb2gray(std::vector<cv::Mat>& rgb_image)
{
  cv::Mat temp_gray;
  std::vector<cv::Mat> gray_image;
  std::vector<cv::Mat>::iterator itr;

  gray_image.reserve(rgb_image.size());
  for ( itr = rgb_image.begin() ; itr < rgb_image.end() ; itr++)
  {
    cvtColor(*itr, temp_gray, CV_RGB2GRAY);
    gray_image.push_back(temp_gray.clone());
  }
  return gray_image;
}

int main(int /*argc*/, char** /*argv*/) {
    Mat imgA, imgB;   
    imgA = imread("test1.jpg");
    imgB = imread("test2.jpg");

    vector<Mat> input;
    input.push_back(imgA);
    input.push_back(imgB);

    vector<Mat> output;
    output = rgb2gray(input);

    stringstream ss;
    for(size_t i = 0; i<input.size(); i++) {
        ss.str(""); ss << "in" << i;
        imshow( ss.str(), input[i] );

        ss.str(""); ss << "out" << i;
        imshow( ss.str(), output[i] );
    }

    while( true ) {
        char c = (char)waitKey(10);
        if( c == 27 )
            { break; }
    }
    return 0;
}