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Maybe use stl::vector instead of array since different images will be different sizes. Also, data in grayscale images are normally unsigned char (uchar). Something lke:

Mat img = imread("C:/OpenCV4_0_1/sources/samples/data/lena.jpg");
Mat gimg;

cvtColor(img, gimg, COLOR_RGB2GRAY);

vector<unsigned short> pixelList(gimg.size().area());
pixelList.assign(gimg.begin<uchar>(), gimg.end<uchar>());