Reconstruct a missing RGB channel

Python 3 + scikit-image

Simply sets the color of the missing channel to the average of the other two.

import sys
from skimage import io, color

im = io.imread(sys.argv[1])
h, w, c = im.shape

removed_channel_options = {0, 1, 2}
for y in range(h):
    for x in range(w):
        if len(removed_channel_options) == 1: break
        removed_channel_options -= {i for i, c in enumerate(im[y][x]) if c > 0}
removed_channel = removed_channel_options.pop()

for y in range(h):
    for x in range(w):
        p = [float(c) / 255 for c in im[y][x][:3]]
        p = [sum(p)/2 if i == removed_channel else p[i]
             for i in range(3)]
        im[y][x] = [int(c*255) for c in p] + [255]*(c == 4)

io.imsave(sys.argv[2], im)

And the restored images:

forest fox icy

Lua, Love2D

Just sets the missing channel to the smaller of the two remaining channels.

local inp = love.image.newImageData(arg[2])

local channels = {1, 2, 3}
local removed = nil
local removed_options = {true,true,true}

inp:mapPixel(function(x,y,r,g,b)
    local o = {r,g,b}
    for k,v in pairs(o) do
        if v > 0 then
            removed_options[k] = false
        end
    end
    return r,g,b
end)

for k,v in pairs(removed_options) do
    if v then
        removed = k
        break
    end
end
inp:mapPixel(function(x,y,r,g,b)
    local o = {r,g,b}
    o[removed] = math.min(o[removed%3+1], o[(removed+1)%3 + 1])
    return unpack(o)
end)

inp:encode('png', IMAGE:gsub("%.png", "2.png"))

Takes a filename as an argument on the command line.

Forest Fox Snow