OpenCV: .ptr<uchar> vs .ptr<Vec3b> in grayscale images

Question

My main aim is to access the pointers properly in a single channel grayscale image and passing them to Cuda kernel functions (such as for convolution, filtering etc). But I can't figure out why I can't see memory adresses by using .ptr<uchar> for grayscale images . I have prepared a small example code for you to check.

In the code below, I'm using 2 methods to convert a color image (liquidmoon.jpeg) to single channel 8bit gray-scale image.

#include <cuda.h>
#include "opencv2/highgui/highgui.hpp"
#include <iostream>

using namespace std;
using namespace cv;

int main(void)
{
    //Method 1
    Mat img1 = imread("liquidmoon.jpeg",CV_LOAD_IMAGE_GRAYSCALE);
    cout <<"Number of channels in the first converted image : " << img1.channels() << "\n";
    cout << "ptr with uchar : "<< img1.ptr<uchar>(0)<<"\n";
    cout << "ptr with Vec3b : "<< img1.ptr<Vec3b>(0)<<"\n";

    //Method 2
    Mat img2 = imread("liquidmoon.jpeg");
    Mat gimg;
    img2.convertTo(gimg,CV_8UC1);
    cout <<"Number of channels in the second converted image : " << gimg.channels() << "\n";
    cout << "ptr with uchar : "<< gimg.ptr<uchar>(0)<<"\n";
    cout << "ptr with Vec3b : "<< gimg.ptr<Vec3b>(0)<<"\n";

} 

Using:

nvcc -o testCode testCode.cu `pkg-config opencv --cflags --libs`

And the program output is:

Number of channels in the first converted image : 1
ptr with uchar : 
ptr with Vec3b : 0x1093000
Number of channels in the second converted image : 3
ptr with uchar : 
ptr with Vec3b : 0x10eaea0

What I expect was to obtain memory addresses with .ptr<uchar>(0) (at least for the first method, since it has 1 channel), but interestingly .ptr<Vec3b>(0) gives results for the both conditions. Are those images not grayscale yet? What may be the problem?

Answer 1

It's just that gimg.ptr<uchar> returns an uchar* which cout interprets as a pointer to a C-string and attempts to display as such. Cast to a void* pointer first.

cout << "ptr with uchar : "<< static_cast<void const*>(img1.ptr<uchar>(0)) <<"\n";