public function calculate(ColorJizz $foreground, ColorJizz $background)
    {
        $fgLuma  = $this->relativeLuminosity($foreground->toRGB());
        $bgLuma  = $this->relativeLuminosity($background->toRGB());

        if ($fgLuma > $bgLuma) {
            return ($fgLuma + 0.05) / ($bgLuma + 0.05);
        } else {
            return ($bgLuma + 0.05) / ($fgLuma + 0.05);
        }
    }