Grafika\Gd\Filter\Sharpen::apply PHP Method

    public function apply($image)
    {
        $amount = $this->amount;
        // build matrix
        $min = $amount >= 10 ? $amount * -0.01 : 0;
        $max = $amount * -0.025;
        $abs = (4 * $min + 4 * $max) * -1 + 1;
        $div = 1;
        $matrix = array(array($min, $max, $min), array($max, $abs, $max), array($min, $max, $min));
        // apply the matrix
        imageconvolution($image->getCore(), $matrix, $div, 0);
        return $image;
    }