Ordering pixels to median Filter

Question

I have a question on how do I calculate the value of a pixel.

For example I take 3x3 pixels and need to order and select the pixel[4] to apply the median filter.

How do I calculate the value of those pixels to order then?

Do I order each channel (R G B) and then select the middle term of each and put the value into to the pixel?

Or I have to sum the 3 channels?

I found online a code with PIL on python, but I don't understand how the "members.sort()" function works

def FiltroMediana(path):

img = Image.open(path)
members = [(0, 0)] * 9
width, height = img.size
newimg = Image.new("RGB", (width, height), "white")
for i in range(1, width - 1):
    for j in range(1, height - 1):
        members[0] = img.getpixel((i - 1, j - 1))
        members[1] = img.getpixel((i - 1, j))
        members[2] = img.getpixel((i - 1, j + 1))
        members[3] = img.getpixel((i, j - 1))
        members[4] = img.getpixel((i, j))
        members[5] = img.getpixel((i, j + 1))
        members[6] = img.getpixel((i + 1, j - 1))
        members[7] = img.getpixel((i + 1, j))
        members[8] = img.getpixel((i + 1, j + 1))
        members.sort()
        newimg.putpixel((i, j), (members[4]))

newimg.save("MedianFilterPIL.png", "PNG")
newimg.show()

The sort function call is [pythons list.sort()](https://www.geeksforgeeks.org/python-list-sort/) command. It simply orders the list after adding in 8 entries — GPPK, Feb 26 '19 at 13:20
In case anyone is feeling really keen... https://www.comm.utoronto.ca/~kostas/Publications2008/pub/proceed/38.pdf — Mark Setchell, Feb 26 '19 at 16:08
There is a generic filter module by the way that gives you all the framework of the loops and passes you all the surrounding pixels by the way. I used it here https://stackoverflow.com/a/54440152/2836621 for a modal filter, but you only need to change one line to make it a median filter. — Mark Setchell, Feb 26 '19 at 16:11

score 2 · Answer 1 · answered Feb 26 '19 at 13:31

There is no "nice" way to apply a median filter to a color image.

filtering the three planes separately creates new colors that weren't in the image,
true color filtering is achieved by finding the color that minimizes the total distance to the others; but this is time consuming.

Ordering pixels to median Filter

1 Answers1