rgbtohsv

M.rgb_to_hsv = function(r, g, b)
    r, g, b = (r / 255), (g / 255), (b / 255)
    local mx = math.max(r, g, b)
    local mn = math.min(r, g, b)
    local h, s

    if mx == mn then
        h = 0
    d = 0
    else
    local d = mx - mn
        if     mx == r then h = (g - b) / d + (g < b and 6 or 0)
        elseif mx == g then h = (b - r) / d + 2
        elseif mx == b then h = (r - g) / d + 4 end
        h = h / 6
    end
    s = (mx == 0) and 0 or (d / mx)

    return h, s, mx
end