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() command. It simply orders the list after adding in 8 entries — GPPK
– GPPK, Commented Feb 26, 2019 at 13:20
In case anyone is feeling really keen... comm.utoronto.ca/~kostas/Publications2008/pub/proceed/38.pdf — Mark Setchell
– Mark Setchell, Commented Feb 26, 2019 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 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
– Mark Setchell, Commented Feb 26, 2019 at 16:11

user1196549 · Accepted Answer · 2019-02-26 13:31:37Z

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.

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