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If contourArea doesn't give you expected result, you can follow this approach:

1. Copy the contour in a new empty Mat

2. Then countNonZero applied on the new submatrix will give you the number of non-black pixel

       //1. Copy the contour in a new empty Mat
       Rect cRect = boundingRect(*it);
       Mat subImg = dilatedImg(cRect);
       double cArea = countNonZero(subImg);
   

Note: if you doesn't have a binary image, you may need to threshold it before to count only the right pixels.

If contourArea doesn't give you expected result, you can follow this approach:

1. Copy the contour in a new empty Mat

2. Then countNonZero applied on the new submatrix will give you the number of non-black pixel

       //1. Copy the contour in a new empty Mat
       Rect cRect = boundingRect(*it);
       Mat subImg = dilatedImg(cRect);
       double cArea = countNonZero(subImg);
   

Note: if you doesn't have a binary image, you may need to threshold it before to count only the right pixels.

If contourArea doesn't give you expected result, you can follow this approach:

1. Copy the contour in a new empty Mat

2. Then Apply countNonZero applied on the new submatrix and you will give you get the number of non-black pixel

       //1. Copy the contour in a new empty Mat
       Rect cRect = boundingRect(*it);
   boundingRect(*it); //it is an iterator for your contours vector
       Mat subImg = dilatedImg(cRect);
       double cArea = countNonZero(subImg);
   

Note: if you doesn't have a binary image, you may need to threshold it before to count only the right pixels.