Use phpqrcode library

OK.

I'll try to find some time this week end.

1. I use libraries module in order to centralize external libraries : http://drupal.org/project/libraries
   This is not mandatory.
2. Download phpqrcode : http://phpqrcode.sourceforge.net/
3. De-archive phpqrcode and install it in desired location. In my case : /sites/all/libraries/.
4. Now you have all the phpqrcode code in : /sites/all/libraries/phpqrcode
5. Replace mobile_codes_generate function in mobile_codes/mobile_codes.module by the code below.
   The call to Nokia webservice is commented and replaced by phpqrcode generation.

In mobile_codes/mobile_codes.module

/**
 * Generate a mobile code
 */
function mobile_codes_generate($data, &$arguments = array()) {
  $values = mobile_codes_process_arguments($arguments);
  if (($file = mobile_codes_load_mobile_code($data, $arguments)) === FALSE) {

    $dir = '/mobile_codes/'. $arguments['preset'];
    $file = file_directory_path() . $dir .'/'. time() . rand(1000, 9999) .'.png';

    if ($arguments['data'] == 'link' && ($arguments['tinyurl'] == 1 || drupal_strlen($data) > 60)) {
      $headers = array('Content-type' => 'application/x-www-form-urlencoded');
      $tinyurl = drupal_http_request('http://tinyurl.com/api-create.php?url='. urlencode($data), $headers, 'POST', NULL);
      if ($tinyurl->code == 200) {
        $data = $tinyurl->data;
      }
    }


      
/*
    $url = 'http://mobilecodes.nokia.com/'. $arguments['type'] .'?'. $values['text']['data'] .'='.
      urlencode($data) .'&'. $values['text']['size'] .'='. $values['size'][$arguments['type']]
      .'&name='. $arguments['name'] . $values['text']['margin_encoding'] .'&type='. $arguments['data']
      .'&MODE='. $values['mode'] .'&a=create';
    mobile_codes_directory_check($dir);
    $image = drupal_http_request($url);
    //file_save_data($image->data, $file, FILE_EXISTS_REPLACE);
 */
 
 
   // QR Code Configuration
   $QRCodeConfig = array();
   // Level of error correction
   // (one form: L,M,Q,H)
   $QRCodeConfig['ECC'] = 'L';
   
   // Code square size in pixels.ie : 4, 4x4 (4x zoom)
   $QRCodeConfig['Square size'] = 8;
   
   // Code outerFrame in pixels (white boundary around) 
   $QRCodeConfig['Outer Frame'] = 2;
   
   
   mobile_codes_directory_check($dir);
   
   // Insert phpqrcode using libraries module
   include_once (libraries_get_path('phpqrcode/qrlib.php'));
   
   
   // Create QRCode
   QRcode::png( $data,
                $file,
                $QRCodeConfig['ECC'],
                $QRCodeConfig['Square size'],
                $QRCodeConfig['Outer Frame']);

    mobile_codes_save_mobile_code($data, $arguments, $file);
  }

  return $file;
}

Hope this work for you.

I've done the same stuff you need !

Here is the source code added on a module especially created for the project.
The project is called tazas which is spanish word for english word cups. So the module is called tazas.

I add 3 functions :

1. tazas_nodeapi, to add data to the node
2. tazas_theme, to apply theme
3. theme_tazas_qrcode, to create the output

Here is the code :

/**
 * Implementation of hook_nodeapi().
 */

function tazas_nodeapi(&$node, $op,$teaser, $page) {

  global $user;
  
  switch($op):
    case 'view':
      // Abort if the user is an anonymous user (not logged in) or
      // if the node is not being displayed on a page by itself
      // (for example, it could be in a node listing or search result).
      if ($user->uid == 0 || !$page):
        break;
      endif;
      
      // Find out which node types we should generate QR code for.     
      $types_to_generate_QR_code = variable_get('tazas_node_types', array());

      // Abort if this node is not one of the types we should annotate.
      if (!in_array($node->type,$types_to_generate_QR_code,true)):
        break;
      else:
      
      $node->content['QR_Code'] = array('#value' => theme('tazas_qrcode', $node),
                                        '#weight' => 2 );
      endif;
  endswitch;
  
}

/**
 * Implementation of hook_theme().
 */
function tazas_theme() {
  return array('tazas_qrcode' => array( 'arguments' => array('node'),), );
}

/**
 * Create HTML for QR code.
 */
function theme_tazas_qrcode($node) {
  global $base_url;
  $node_alias = $base_url.url("node/$node->nid");
  $output = '<div class="tazas-qrcode">'."\n"
            .'<p>'.$node_alias.'</p>'."\n"
            .theme('mobilecode', $node_alias, array('preset' => 'default'))."\n"
            .'</div>';
            
  return $output;
}

Guess you can find a better and thinner solution ! This is a quick answer I wrote with lack of knowledges on certain Drupal area.
Feel free to feedback remarks !

You have to create a new module.
Do not create a content type for testing

You could in a first try comment the node type evaluation code. The function tazas_nodeapi is then :

/**
* Implementation of hook_nodeapi().
*/

function tazas_nodeapi(&$node, $op,$teaser, $page) {

  global $user;
  
  switch($op):
    case 'view':
      // Abort if the user is an anonymous user (not logged in) or
      // if the node is not being displayed on a page by itself
      // (for example, it could be in a node listing or search result).
      if ($user->uid == 0 || !$page):
        break;
      endif;
      
      $node->content['QR_Code'] = array('#value' => theme('tazas_qrcode', $node),
                                        '#weight' => 2 );
  endswitch;
  
}

or even thinner with no display condition :

function tazas_nodeapi(&$node, $op,$teaser, $page) {
  
  switch($op):
    case 'view':

      $node->content['QR_Code'] = array('#value' => theme('tazas_qrcode', $node),
                                        '#weight' => 2 );
  endswitch;
  
}

The Mobile Codes module is well build so you can read the code and the comments to learn more about how to realize specific things.

Mobile Codes module store QR_Code pictures in files/mobile_codes directory : when asking to display QR_Code it determines whether it have to generate a new file or display an existing one.

To retrieve this picture you only have to use theme function for Mobile Codes module with the current node alias :

   global $base_url;

   # compute node alias
   $node_alias = $base_url.url("node/$node->nid");  

   # render QR Code
   print theme('mobilecode', $node_alias, array('preset' => 'default'));

If I well understand your needs, I think module Link is useless as QR_code is generated using node alias data. If you want to have some automatic URL alias creation I then recommend using Path Auto module.
If you want to retrieve QR_Code image from node, you can use Mobile Codes API to do this.

You can also create a specific content type for QR_Code representing a specific URL. In this case you can use CCK field to reference a node (and then it's alias you need to build QR Code).

Or maybe I do not understand your need ! In my case it was the following : generate QR_CODE for content node linking. Let's say I create a book page : the QR Code representing the page URL is generated and displayed to specific user which then can copy it and add it to whatever print media desired (flyer, poster, CV...).

I realize we are out of the scope of this issue : use phpqrcode library.
So if you want to continue this discussion I suggest you open a support request ticket.

Please, open a support ticket in Mobiles Codes and add the link to this ticket in this thread.
Since #12 we are beyond the scope !