Instafilter\Image::_resize PHP Method

    protected function _resize($width, $height = null, $keepar = true, $pad = true)
    {
        $sizes = $this->sizes();
        if ($height == null or $width == null) {
            if ($height == null and substr($width, -1) == '%') {
                $height = $width;
            } elseif (substr($height, -1) == '%' and $width == null) {
                $width = $height;
            } else {
                if ($height == null and $width != null) {
                    $height = $width * ($sizes->height / $sizes->width);
                } elseif ($height != null and $width == null) {
                    $width = $height * ($sizes->width / $sizes->height);
                } else {
                    throw new \InvalidArgumentException("Width and height cannot be null.");
                }
            }
        }
        $origwidth = $this->convert_number($width, true);
        $origheight = $this->convert_number($height, false);
        $width = $origwidth;
        $height = $origheight;
        $x = 0;
        $y = 0;
        if ($keepar) {
            // See which is the biggest ratio
            if (function_exists('bcdiv')) {
                $width_ratio = bcdiv((double) $width, $sizes->width, 10);
                $height_ratio = bcdiv((double) $height, $sizes->height, 10);
                $compare = bccomp($width_ratio, $height_ratio, 10);
                if ($compare > -1) {
                    $height = ceil((double) bcmul($sizes->height, $height_ratio, 10));
                    $width = ceil((double) bcmul($sizes->width, $height_ratio, 10));
                } else {
                    $height = ceil((double) bcmul($sizes->height, $width_ratio, 10));
                    $width = ceil((double) bcmul($sizes->width, $width_ratio, 10));
                }
            } else {
                $width_ratio = $width / $sizes->width;
                $height_ratio = $height / $sizes->height;
                if ($width_ratio >= $height_ratio) {
                    $height = ceil($sizes->height * $height_ratio);
                    $width = ceil($sizes->width * $height_ratio);
                } else {
                    $height = ceil($sizes->height * $width_ratio);
                    $width = ceil($sizes->width * $width_ratio);
                }
            }
        }
        if ($pad) {
            $x = floor(($origwidth - $width) / 2);
            $y = floor(($origheight - $height) / 2);
        } else {
            $origwidth = $width;
            $origheight = $height;
        }
        //it's more efficient to save it to the temp file, and do our changes there
        //$this->imagick()->setImageGravity(\Imagick::GRAVITY_CENTER);
        //$this->imagick()->resizeImage($width, $height);
        $image = '"' . $this->image() . '"';
        $this->exec('convert', "-define png:size=" . $origwidth . "x" . $origheight . " " . $image . " " . "-background none " . "-resize \"" . ($pad ? $width : $origwidth) . "x" . ($pad ? $height : $origheight) . "!\" " . "-gravity center " . "-extent " . $origwidth . "x" . $origheight . " " . $image);
    }