That's very cool. LIBCVD does something similar in that updates are only made to the histogram for pixels that enter or leave the structuring element as it's shifted across the image. Any plans to replace erode and dilate with rank.minimum and rank.maximum?

I was planning to add another PR which addresses this. So, I would leave the current dilate and erode functions and only "redirect" images of certain dtype to the rank filters, which are only implemented for uint8 and uint16.