Error when trying to access a pixel value in OpenCV - c++

Question

I just started learning c++ and OpenCV. I've tried to write a program that allows me to click on a pixel and print out the pixel value in BGR. So far, when I click, my program returns the coordinate of the clicked pixel but when I want to access the BGR-value my program crashes. I'm streaming video from my webcam as input and I don't get any errors when i compile.
I've been reading a lot of similar questions in here but I haven't found any solution.
My program crashes when I include the line:
"cv::Vec3f BGR = matOriginal.at(click[0][0],click[0][1]);"
The rest of my program is as follows:
int click[1][2];
void mouseEvent(int Event, int x, int y, int flags, void* param)
{
if(Event == CV_EVENT_LBUTTONDOWN)
{
printf("Valgt pixel: [%d,%d] \n ",x,y);
click[0][0] = x;
click[0][1] = y;
}
}
int main()
{
VideoCapture capWebcam(0);
if(capWebcam.isOpened() == false)
{
printf("Error: capWebcam not accessed successfully \n");
return(1);
}
Mat matOriginal;
char charCheckForEscKey=0;
while(charCheckForEscKey !=27)
{
if(capWebcam.read(matOriginal) == NULL)
{
printf("Error: Frame not read\n \n");
break;
}
cv::imshow("Original",matOriginal);
setMouseCallback("Original",mouseEvent,0);
// Print BGR-value when mouseclick
if(click[0][0]>0 && click[0][1]>0)
{
cv::Vec3f BGR = matOriginal.at<cv::Vec3f>(click[0][0],click[0][1]); // When this statement is included, I get an ERROR
double B=BGR[0]; // B
double G=BGR[1]; // G
double R=BGR[2]; // R
click[0][0] = 0; // Resetting the click-matrix
click[0][1] = 0;
}
charCheckForEscKey = cv::waitKey(10);
}
return(0);
}
I'm using Microsoft Visual C++ 2013 Express
Thanks in advance for your help

Answer 1

Ok, two different things could be going on here:
The crash happens because click is not initialized:
When the execution of your program reaches the while loop, and the user has not pressed the mouse button, there's no way to tell what values are stored by click since they are most probably garbage from the memory.
This means that when you program first runs, click could be storing values larger than the dimensions of matOriginal, thus rendering you a crash for trying to access indexes that doesn't exist.
Always initialize your variables!
int main()
{
click[0][0] = -1;
click[0][1] = -1;
// ...
}
What I just said could be tested by printing these values to the screen before accessing them:
std::cout << "X: " << click[0][0] << " Y: " << click[0][1] << std::endl;
cv::Vec3f BGR = matOriginal.at<cv::Vec3f>(click[0][0], click[0][1]);
The crash happens because cv::Mat is expecting the X,Y (row,column) values in the reverse order (column,row):
I think that's most likely the cause of the problem. Try:
cv::Vec3f BGR = matOriginal.at<cv::Vec3f>(click[0][1], click[0][0]);