OpenCV + visualStudio，尝试访问像素的C ++代码错误

Question

I'm Joe and I'm a new member of this site. Let's go to the point...I start programming in c++ with openCV libraries on visual studio but I have a question about image's pixels.. I wrote a code to calculate the average of the alpha channel (the foruth component of a pixel, in addition to B,G,R intensity's values).. enter image description here I fall into an error and I don't know how to fix it. Here's my code

      #include "stdafx.h"
#include "opencv2\highgui\highgui.hpp"
#include "opencv2\core\core.hpp"
#include "opencv2\opencv.hpp"

using namespace cv;
using namespace std;

int main() {
    Mat image;
    image = imread("C:/Users/Joe/Desktop/prova/paesaggioLuminoso.jpg", 1);
    namedWindow("Picture", WINDOW_AUTOSIZE); 
    imshow("Picture", image);
    waitKey(0);

    int averageAlpha = 0;
    int partial = 0;



    for (int i = 0; i < image.rows; i++) {
        for (int j = 0; j < image.cols; j++) {



            partial = partial +image.at<Vec4b>(i, j)[3];



            if (i == image.rows - 1 && j == image.cols - 1) {
                averageAlpha = partial / (image.rows * image.cols);

                cout << "Average of alpha channel is " << averageAlpha << endl;

            }

        }
    }

    return 0;
}

Please note that if I try to calculate average of B intensity (or Green or red) , my program runs correctly .

But with this fourth channel I have this error message: Unhandled exception at 0x00007FFC47603FB8 in Secondo Progetto OpenCV.exe: Microsoft C++ exception: cv::Exception at memory location 0x000000162E2FF3E0.

Assertion failed ((unsigned)(i1 * DataType<_Tp>::channels) < (unsigned)(size.p 1 * channels())) in cv::Mat::at, file c:\\users\\joe\\desktop\\joe\\opencv\\opencv\\build\\include\\opencv2\\core\\mat.inl.hpp, line 1095

Thanks for the help ! Have a good day !

EDIT: here's the error I mentioned in the comment

Errors in visual studio with a correct code

Other errors

enter image description here

Answer 1

You're passing as flag '1' which equals IMREAD_COLOR see this: https://github.com/opencv/opencv/blob/master/modules/imgcodecs/include/opencv2/imgcodecs.hpp

And IMREAD_COLOR drops the alpha channel, see this https://docs.opencv.org/3.1.0/d4/da8/group__imgcodecs.html#ga61d9b0126a3e57d9277ac48327799c80

so you should be doing

image = imread("C:/Users/Joe/Desktop/prova/paesaggioLuminoso.jpg", IMREAD_UNCHANGED);