PNG: Druckgröße (DPI) auslesen mit PHP

Mit PHP kann man direkt eine PNG-Bild-Datei einlesen, aber man bekommt nur die Pixel und die Größe, aber keine Information über die Druckauflösung (DPI, dots per inch). Man muss noch einmal selbst die PNG-Datei öffnen und die Information auslesen.

Die folgendne PHP-Klasse liest die PNG-Datei, bis sie die entsprechende Information gefunden hat.

<?php

/**
  * Class to read PNG image with dpi / resolution information.
  *
  * Siehe auch http://www.fileformat.info/format/png/corion.htm
  *
  * @copyright Ingmar Decker 2010, www.webdecker.de
  * @author    Ingmar Decker 
  */
class PngImageReader {

    public function PngImageReader() {
    }
    
    /** Returns type: 'png'. */
    public function getType() {
        return 'png';
    }
    
    /** Returns true iff this reader can read given file (at least file extension should be
      * checked.
      * @return bool True if reader can read file, false otherwise
      */
    public function canRead($filename) {
        $pi = pathinfo($filename);
        $ext = strtolower($pi['extension']);
        return ($ext == 'png');
    }
    
    /** Reads file and returns array with 6 elements: type name, image resource, with, height, dpi for x, dpi for y.
      * Dpi values might be 0 if file format does not provide dpi or other resolution information.
      */
    public function read($filename) {
        $type = $this->getType();
        $r = array($type, null, 0, 0, 0, 0);
        $image = imagecreatefrompng($filename);
        if ($image) {
            $w = imagesx($image);
            $h = imagesy($image);
            list($xdpi, $ydpi) = $this->getDpi($filename);            
            $r = array($type, $image, $w, $h, $xdpi, $ydpi);
        }
        return $r;
    }
    
    /** Get dpi / print resolution of given png file. Returns array with dpiX, dpiY as values. */
    private function getDpi($filename) {
        $dpi = array(0, 0);
        $file = fopen($filename, 'rb');
        $header = fread($file, 8);
        if (substr($header, 1,3) != 'PNG') throw new Exception("$filename is not PNG file."); 
        while (!feof($file)) {
            list($len, $type, $data, $crc) = $this->readChunk($file);
            if ($type == 'pHYs') {
                $dx = unpack('N', substr($data, 0, 4));
                $dy = unpack('N', substr($data, 4, 4));               
                $unit = $data[8];              
                if (ord($unit) == 1) {
                    // 1 == meter, 0 == unknown
                    // $dx, $dy are dots or pixels per meter, 1 inch = 0.0254 meter
                    $dpi = array($dx[1] * 0.0254, $dy[1] * 0.0254);
                    break;
                }
            }            
        }
        fclose($file);       
        return $dpi;
    }
    
    /** Reads a png chunk and returns array with length in bytes, type string, data bytes, crc code.
      */
    private function readChunk($file) {
        $data = '';
        $type = '';
        $crc = '';
        $len = fread($file, 4);
        if ($len) {
            $len = unpack('N', $len); // 'N' = big endian unsigned long, 'V' little endian
            $len = $len[1];
            $type = fread($file, 4);
            if ($len) $data = ($len ? fread($file, $len) : '');
            $crc = fread($file, 4);
        }
        return array($len, $type, $data, $crc);
    }

}

?>