get("#356") -- num2hex
get("#348") -- rgb functions

function roughHashImage(img)
  local hash = 0
  local w, h = img.width, img.height
  
  for y = 0, h-1 do
    for x = 0, w-1 do
      local pix = img.getInt(x, y)
      local b = math.floor(bright(rgb(pix))*25+0.5)
      local p = b
      local scale = bit32.band(y*w+x, 0xFFFF)+1
      local arg = ((p+1)*scale*7) % 1234567
      hash = bit32.bxor(hash, arg)
      --[[if x < 10 then
        print("scale", scale, "pix", pix, "p", p, "arg", arg, "hash", hash)
      end]]
    end
    --print("y", y, "hash", hash)
  end
  
  return num2hex(hash, 8)
end

Began life as a copy of #380