Understanding why it doesn't copy correctly using memcpy - c++

Question

I have some misunderstanding about OpenCV 4.1.0 and memcpy in C++. The question is why the image is zoomed in a lot?
I read an image like this:
Mat img = imread("lena512.bmp", 1); // Black and White Image
namedWindow("Display window", WINDOW_AUTOSIZE);
imshow("Display window", img);
After this I have 2 byte array:
int inputSize = width * height * channels;
byte* pixels = new byte[width * height * channels];
byte* out = new byte[width * height * channels];
I copy the img to pixels array:
memcpy(pixels, img.data, inputSize * sizeof(byte));
And then I want to check if retrieving image is the same as input:
Mat image = Mat(width, height , CV_8U);
memcpy(image.data, out, inputSize * sizeof(byte));

Answer 1

Mat img = imread("lena512.bmp", 1); // Black and White Image
That's the problem, the comment is a lie, and by using a magic number instead of a named constant, you can't easily tell that's the case. 1 in this context means IMREAD_COLOR -- i.e. the image is always read as a 3 channel BGR image.
However, after the shenanigans with memcpy and raw pointers, you create new Mat in the following manner:
Mat image = Mat(width, height , CV_8U);
Note that CV_8U is equivalent to CV_8UC1. Hence, you create a single channel (grayscale) Mat, but give it 3-channel data.
Getting garbage as a result is the lesser issue. The much more serious issue is that you copy 3x as much data as the target pixel buffer can hold -- basically you clobber half a megabyte of memory that doesn't belong to the Mat. That can either end with a segfault, or some really hard to find bugs (in case you overwrite some memory used by other data structures).
Update: There's another issue that I've missed (thanks to #Micka for catching that). The order of parameters of the cv::Mat constructor is rows, columns, datatype. It appears you switched width and height, although since your input image appears to be square (i.e. width == height) it didn't matter.
The correct way to allocate the second Mat would be
Mat image = Mat(height, width, CV_8UC3);