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Get pixel value of sobel Mat with C++

asked 2019-05-10 01:52:11 -0500

PaulV gravatar image


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:

image description

I tried for example

        Scalar intensity1 =<uchar>(100, 100);
    // or this:
    //Scalar intensity =<uchar>(Point(100, 100));
    // or this:
    //Mat cropedImage = sobel_x(Rect(100, 100, 1, 1));
    //Scalar intensity1 =<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


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answered 2019-05-10 03:45:34 -0500

berak gravatar image

updated 2019-05-10 03:50:57 -0500

please AVOID to use Mat::at as good as you can. in 95% of all cases it's the wrong thing to do.

to print out a 1 pixel Mat use:

cout << sobel_x(Rect(100, 100, 1, 1)) << endl;

this solves 3 problems:

  • you don't have to mess with the type. (what is ddepth, and why do you assume uchar here ?)
  • you don't have to mess with indexing (you did it wrong above, in a 1x1 Matrix, (0,0) is the only valid index, (1,1) is out of bounds))
  • you don't have to care, if cout treats your uchar values as ascii
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Thank you for your fast answer. Your code works fine but how can I assign that value to a variable (for example integer)?

PaulV gravatar imagePaulV ( 2019-05-10 08:06:52 -0500 )edit

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Asked: 2019-05-10 01:52:11 -0500

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Last updated: May 10 '19