Comment 0 for bug 1251770

Suppose
img
is an image with both negative and positive pixels.

Then
hp.show(hp.propagate(img,0.01)
gives a distinctly different result than
hp.show(hp.propagate(img - img.min(),0.01)

The second result looks more similar to the un-propagated image, as desired for a propagation of a small finite distance.