title: substract region from image and keep the borders
body: <p>I have a 200x200 pixels image and I want to keep only the data for a certain region inside it.
  Check the following image:</p>
  
  <p><a href="https://i.stack.imgur.com/dxzNY.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/dxzNY.png" alt="Image"></a></p>
  
  <p>The whole out square is 200x200 pixels.I want to remove from it the smaller square(white).So ,keep only the information that is included in the blue area.But , I want to keep the 200x200 dimensions.</p>
  
  <p>I tried:</p>
  
  <pre><code>    Mat whiteArea;
      whiteArea = ImageInitial( Range(50,200) , Range(50,200) );
  
      Size size(200,200);
      Mat dst;
      resize(whiteArea,dst,size);
  
      Mat FinalImage;
      subtract(ImageInitial,dst,FinalImage); 
  </code></pre>
  
  <p>I am resizing the white area because I want to substract it from the initial image.
  My problem is that it gives me the initial image.</p>
  
  <p>Maybe the resize is the problem .but then how to substract 2 different sized images?</p>