title: Gaussian blurring with OpenCV: only blurring a subregion of an image?
body: <p>Is it possible to only blur a subregion of an image, instead of the whole image with OpenCV, to save some computational cost?</p>
  
  <p><strong>EDIT</strong>: One important point is that when blurring the boundary of the subregion, one should use the existing image content as much as possible; only when the convolution exceeds the boundary of the original image, an extrapolation or other artificial border conditions can be used.</p>