Magic.lua

local imlib2 = require "imlib2"

math.randomseed(os.time())

local white, black =
    imlib2.image.load("first.png"),
    imlib2.image.load("second.jpg")

local width, height =
    math.min(white:get_width(), black:get_width()),
    math.min(white:get_height(), black:get_height())

for _, image in ipairs{white, black} do
    local cwidth, cheight =
        image:get_width(),
        image:get_height()
    image:crop(
        (cwidth  - width ) / 2,
        (cheight - height) / 2,
        width,
        height
    )
end

local _color = imlib2.color.new(0, 0, 0)
local function color(val, alpha)
    _color.red = val
    _color.green = val
    _color.blue = val
    _color.alpha = alpha
    return _color
end

local target = imlib2.image.new(width, height)


target:set_has_alpha(true)
for y=1,height do
    for x=1,width do
        local px_white, px_black =
            white:get_pixel(x, y),
            black:get_pixel(x, y)

        local l_w = (px_white.red + px_white.green + px_white.blue) / 3 + 1
        local l_b = (px_black.red + px_black.green + px_black.blue) / 3 + 1

        -- Normalize
        l_w = (l_w) / 2 + 128
        l_b = (l_b)

        local a_l = (l_w + l_b) / 2
        local d_l = l_w - l_b

        target:draw_pixel(x, y, color(a_l - 1, (255 - d_l) / 2 - 1))
    end
end
target:save('result.png')