Get pixel value of sobel Mat with C++

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Hi,

I need to read the value of single pixels in an Mat. Normally thats no problem but I created the Mat with the Sobel command and always get an Error if I try to read the value of an Pixel. I created the Sobel Mat like that:

        // ### Sobel ###
    Mat sobel_x, sobel_y;
    /// Sobel X
    Sobel(src, sobel_x, ddepth, 1, 0, 5, scale, delta, BORDER_DEFAULT);
    /// Sobel Y
    Sobel(src, sobel_y, ddepth, 0, 1, 5, scale, delta, BORDER_DEFAULT);
    imshow("Sobel y", sobel_y);
    imshow("Sobel x", sobel_x);

If I try to read an Pixel I get this Error:

I tried for example

        Scalar intensity1 = sobel_x.at<uchar>(100, 100);
    // or this:
    //Scalar intensity = sobel_x.at<uchar>(Point(100, 100));
    // or this:
    //Mat cropedImage = sobel_x(Rect(100, 100, 1, 1));
    //Scalar intensity1 = cropedImage.at<uchar>(1, 1);
    // even if I try to just the read type of the Mat I get an error:
    //int type = sobel_x.type();

hopefully someone can help me with that.

Thank you very much

Paul