JPG: Druckgröße (DPI) auslesen mit PHP

Im Web kursieren Code-Teile und Klassen in PHP, mit denen man die Auflösung (Druckauflösung, DPI, dots per inch) aus einer JPEG-Datei lesen kann. Hat leider nicht oft nicht funktioniert. Habe es noch etwas optimiert.

Die Lösung basiert auf der, die in der PHP-Online-Dokumentation zu finden ist unter imagejpeg(), dazu Infos von www.fileformat.info  über den JFIF-Header.

    /** Reads JPG-file and returns array with 6 elements: type name, image resource, width, height, dpi for x, dpi for y.
      * Dpi values might be 0 if file format does not provide dpi or other resolution information.
      * Usage: 
      * list($type, $image, $w, $h, $dpix, $dpiy) = readJpeg($filename);
      */
    function readJpeg($filename) {
        $type = 'jpeg';
        $r = array($type, null, 0, 0, 0, 0);
        $image = @imagecreatefromjpeg($filename);
        if ($image) {
            $w = imagesx($image);
            $h = imagesy($image);
            $dpix = 0;
            $dpiy = 0;
            
            // from: http://php.net/manual/de/function.imagejpeg.php
            // soapergem at gmail dot com
            // 06-Nov-2008 09:48            
            $exif = null;
            if (function_exists('exif_read_data')) exif_read_data($filename, 'IFD0');
            $x = $y = 0;
            if (isset($exif['XResolution']) && isset($exif['YResolution'])) {
                #$x = intval(preg_replace('@^(\\d+)/(\\d+)$@e', '$1/$2', $exif['XResolution']));
                #$y = intval(preg_replace('@^(\\d+)/(\\d+)$@e', '$1/$2', $exif['YResolution']));
                $m = array();
                if (ereg('^([0-9]+)/([0-9]+)$', $exif['XResolution'], $m)) {               
                    if ($m[2] != 0) {
                        $x = $m[1] / $m[2];
                    } else $x = 0;
                } else {
                    $x = floatval($exif['XResolution']);
                }
                if (ereg('^([0-9]+)/([0-9]+)$', $exif['YResolution'], $m)) {
                    if ($m[2] != 0) {
                        $y = $m[1] / $m[2];
                    } else $y = 0;
                } else {
                    $y = floatval($exif['YResolution']);
                }                
            }
            if (!$x && !$y && $fp = fopen($filename, 'r')) {
                // from. http://www.fileformat.info/format/jpeg/egff.htm
                // last visited 29.3.2010
                // typedef struct _JFIFHeader
                // {
                //   BYTE SOI[2];          /* 00h  Start of Image Marker     */
                //   BYTE APP0[2];         /* 02h  Application Use Marker    */
                //   BYTE Length[2];       /* 04h  Length of APP0 Field      */
                //   BYTE Identifier[5];   /* 06h  "JFIF" (zero terminated) Id String */
                //   BYTE Version[2];      /* 07h  JFIF Format Revision      */
                //   BYTE Units;           /* 09h  Units used for Resolution */
                //   BYTE Xdensity[2];     /* 0Ah  Horizontal Resolution     */
                //   BYTE Ydensity[2];     /* 0Ch  Vertical Resolution       */
                //   BYTE XThumbnail;      /* 0Eh  Horizontal Pixel Count    */
                //   BYTE YThumbnail;      /* 0Fh  Vertical Pixel Count      */
                // } JFIFHEAD;
                // Units, Xdensity, and Ydensity identify the unit of measurement used to describe the image resolution. 
                // Units may be 01h for dots per inch, 02h for dots per centimeter, or 00h for none (use measurement as 
                // pixel aspect ratio). Xdensity and Ydensity are the horizontal and vertical resolution of the image 
                // data, respectively. If the Units field value is 00h, the Xdensity and Ydensity fields will contain 
                // the pixel aspect ratio (Xdensity : Ydensity) rather than the image resolution. Because non-square 
                // pixels are discouraged for portability reasons, the Xdensity and Ydensity values normally equal 1 
                // when the Units value is 0. 
                
                $string = fread($fp, 20);               
                fclose($fp);                
                $units = hexdec(bin2hex(substr($string, 13, 1)));
                $jx = hexdec(bin2hex(substr($string, 14, 2)));
                $jy = hexdec(bin2hex(substr($string, 16, 2)));              

                if ($units == 1) {
                    // resolution in dots per inch
                    $x = $jx;
                    $y = $jy;
                } else if ($units == 2) {
                    $x = $jx * 2.54;
                    $y = $jy * 2.54;
                }
            }
            if ($x && $y) {
                $dpix = $x;
                $dpiy = $y;
            }   
            $r = array($type, $image, $w, $h, $dpix, $dpiy);
        }       
        return $r;
    }

Das hat bei mir jetzt für die Varianten JPEG mit EXIF, JPEG ohne EXIF und JPEG mit beiden funktioniert.