    function __construct(Frame $frame, Dompdf $dompdf)
    {
        parent::__construct($frame, $dompdf);
        $url = $frame->get_node()->getAttribute("src");

        $debug_png = $dompdf->getOptions()->getDebugPng();
        if ($debug_png) {
            print '[__construct ' . $url . ']';
        }

        list($this->_image_url, /*$type*/, $this->_image_msg) = Cache::resolve_url(
            $url,
            $dompdf->getProtocol(),
            $dompdf->getBaseHost(),
            $dompdf->getBasePath(),
            $dompdf->getOptions()
        );

        if (Cache::is_broken($this->_image_url) &&
            $alt = $frame->get_node()->getAttribute("alt")
        ) {
            $fontMetrics = $dompdf->getFontMetrics();
            $style = $frame->get_style();
            $font = $style->font_family;
            $size = $style->font_size;
            $word_spacing = $style->word_spacing;
            $letter_spacing = $style->letter_spacing;

            $style->width = (4 / 3) * $fontMetrics->getTextWidth($alt, $font, $size, $word_spacing, $letter_spacing);
            $style->height = $fontMetrics->getFontHeight($font, $size);
        }
    }
The software allows user input to control or influence paths or file names that are used in filesystem operations.