DOC : Add comment for the use of strided output array for generic_fil…

…ter in RAG

skimage/future/graph/rag.py

@@ -143,6 +143,12 @@ def __init__(self, label_image=None, connectivity=1, data=None, **attr):
 
         if label_image is not None:
             fp = ndi.generate_binary_structure(label_image.ndim, connectivity)
+            # In the next ``ndi.generic_filter`` function, the kwarg
+            # ``output`` is used to provide a strided array with a single
+            # 64-bit floating point number, to which the function repeatedly
+            # writes. This is done because even if we don't care about the
+            # output, without this, a float array of the same shape as the
+            # input image will be created and that could be cost expensive.
             ndi.generic_filter(
                 label_image,
                 function=_add_edge_filter,