added link

morphological_closing_example_scikit-image.ipynb

@@ -29,7 +29,8 @@
     "\n",
     "Morphological erosion is used to remove pixels on the foreground objects’ boundaries. How ‘deeply’ the boundaries are eroded depends on the size of the structuring element (and shape, but in this discussion I will ignore the effect of changing the shape). This operation is in my mind analogous to peeling off a layer from an onion; the thickness of the layer is related to the structuring element size.\n",
     "\n",
-    "Twan Maintz in his book Digital and medical image processing describes the interaction of image and structuring element during erosion this way: place the structuring element anywhere in the image: if it is fully contained in the foreground object (or in one of the objects) then the origin (central) pixel of the structuring element (and only that one) is part of the eroded output. The book has a great example on page 129.\n",
+    "Twan Maintz in his book Digital and medical image processing (http://www.cs.uu.nl/docs/vakken/ibv/reader/readerINFOIBV.pdf)\n",
+    "describes the interaction of image and structuring element during erosion this way: place the structuring element anywhere in the image: if it is fully contained in the foreground object (or in one of the objects) then the origin (central) pixel of the structuring element (and only that one) is part of the eroded output. The book has a great example on page 129.\n",
     "\n",
     "Dilation does the opposite of erosion: it expands the object boundaries (adding pixels) by an amount that is again related to the size of the structuring element. This is analogous to me to adding back a layer to the onion.\n",
     "\n",