Question

I’m trying to index a 3 channel image in opencv.

When I read in image files this code works

int Blue  = LeftCol.at<cv::Vec3b>(v,u)[0];
int Green = LeftCol.at<cv::Vec3b>(v,u)[1]; 
int Red   = LeftCol.at<cv::Vec3b>(v,u)[2]; 

But it crashes when I use a webcam input. The webcam has 3 channels and u,v starts at 0,0.
I have no idea why it won’t work.
I’ve tried all variations of Vec3b, Vec3i, Vec3s, Vec3f, Vec3d

I’m lost…. why can’t I index this webcam image?

EDIT

Right so after many hours this is where I’ve got to…here’s an outline of the program. I was having the problem I mentioned above inside a function. So I’ve gone back to basic, trying to look at the matrix before the function…

void main (int argc, char** argv) {
Mat LeftCol;
while (1==1) {
    if (ProgramMode == "Files") {
        //read in the colour images
        LeftCol  = imread(ColImLeft.c_str(),1);
        RightCol = imread(ColImRight.c_str(),1);

    } else if (ProgramMode == "Camera") {
        VideoCapture CapLeft, CapRight;
        CapLeft.open(1);
        CapRight.open(2);

        CapLeft  >> LeftCol;
        CapRight >> RightCol;

                    //THIS WORKS, THIS PIXEL VALUES ARE DISPLAYED
        cout << "uchar" << endl;
        for (int x=0;x<10;x++) {
            for (int y=0;y<10;y++) {
                int pixel = LeftCol.at<cv::Vec3b>(x,y)[0];
                cout << pixel;
            }
            cout << endl;
        }
    } //end if

            ///////ADDED THIS BIT ////////
    cout << "channels = " << LeftCol.channels() << endl;
            //^^This bit works, output shows "channels = 3"

            //vv This bit doesn't work.... so there's a problem with LeftCol.
            //I wonder if reading the data like CapLeft  >> LeftCol; is changing something
    imshow("Test",LeftCol);
            ///////ADDED THIS BIT ////////

           //THIS DOES NOT WORK WHEN USING THE CAMERA INPUT, PROGRAM CRASHES
    cout << "uchar" << endl;
    for (int x=0;x<10;x++) {
        for (int y=0;y<10;y++) {
            int pixel = LeftCol.at<cv::Vec3b>(x,y)[0];
            cout << pixel;
        } //end for
        cout << endl;
    } //end for

   } //end while
} //end main

Right I have got it working but it’s not ideal. I’m creating a temp Mat to read the files into it then cloning them.

        Mat TempLeft;
        Mat TempRight;

        VideoCapture CapLeft, CapRight;
        CapLeft.open(1);
        CapRight.open(2);

        CapLeft  >> TempLeft;
        CapRight >> TempRight;

        LeftCol = TempLeft.clone();
        RightCol = TempRight.clone();

Answer 1

OpenCV makes soft copies of images whenever possible. From the documentation:

the array assignment is an O(1) operation because it only copies the header and increases the reference counter. The Mat::clone() method can be used to get a full (deep) copy of the array when you need it.

I suspect what is happening is LeftCol uses data which still belongs with the VideoCapture object. If this is the case then when CapLeft and CapRight go out of scope at the end of the if they are closed by the destructor and the image data which LeftCol is still pointing to is destroyed.

Possible solutions would be to clone the image as you are doing, or declare VideoCapture CapLeft, CapRight; outside of the if block (you can still open them inside if needed).