materio-base-d7/sites/all/modules/gui/images_styles_gen/images_styles_gen.module

<?php

/**
 * Implements hook_menu().
 */
function images_styles_gen_menu() {
  
  $items['admin/config/media/image-styles/generate'] = array(
    'title' => 'pre-generate images styles',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('images_styles_gen_settings'),
    'access arguments' => array('administer image styles'),
    'type' => MENU_LOCAL_ACTION,
  );

  return $items;
}

function images_styles_gen_settings(){

  foreach(image_styles() as $style) {
    $styles_options[$style['name']] = $style['name'];
  }

  $form['images_styles_gen_styles'] = array(
    '#type'=>'select',
    '#options'=>$styles_options,
    '#default_value' => variable_get('images_styles_gen_styles', array()),
    '#title' => t('Images styles to pre generate'),
    '#multiple' => true,
  );

  return system_settings_form($form);
}


/**
 * Implements hook_cron().
 */
function images_styles_gen_cron() {
  $result = db_query('SELECT fid, uri FROM {file_managed} WHERE filemime like :mime AND status = :status', array('mime' => 'image/%', 'status' => FILE_STATUS_PERMANENT));
  $queue = DrupalQueue::get('generate_images_styles');
  $n = 0;

  $images_styles_gen_styles = variable_get('images_styles_gen_styles', array());

  watchdog('Images Styles Generate', 'active styles variable : '. print_r($images_styles_gen_styles, true), null, WATCHDOG_INFO);

  foreach(image_styles() as $style) {
    if( in_array($style['name'], $images_styles_gen_styles) ){
      $active_styles[] = $style;
    }
  }

  watchdog('Images Styles Generate', 'active styles : '. print_r($active_styles, true), null, WATCHDOG_INFO);

  foreach ($result as $img_info) {
    foreach($active_styles as $style) {
      $derivative_uri = image_style_path($style['name'], $img_info->uri);
      if (file_exists($derivative_uri)) continue; // skip existing files
      $data = (object) array(
              'style' => $style,
              'img_info' => $img_info,
              'derivative_uri' => $derivative_uri
               );
      $queue->createItem($data);
      $n ++;
    }
  }
  watchdog('Images Styles Generate', $n .' images added on cron to queue for generation.', null, WATCHDOG_INFO);
}

/**
 * Implements hook_cron_queue_info().
 */
function images_styles_gen_cron_queue_info() {
  $queues['generate_images_styles'] = array(
    'worker callback' => '_images_styles_gen_generate',
    'time' => 30
  );
	return $queues;
}

function _images_styles_gen_generate($data){
	if (!file_exists($data->derivative_uri)) {
    try {
      image_style_create_derivative($data->style, $data->img_info->uri, $data->derivative_uri);  
      watchdog('Images Styles Generate', 'Generated : '. $data->style['name'] .' | '.$data->img_info->uri, null, WATCHDOG_INFO);
    } catch (Exception $e) {
      watchdog('Images Styles Generate', 'Generation error on queue : '. $data->style['name'] .' | '. $data->img_info->uri.' | '.$e->getMessage(), null, WATCHDOG_WARNING);
    }
	}
  return true;
}