Example to use to add one or multiple specific derivative types in Omeka, for example mini square thumbnails (see https://github.com/omeka/Omeka/pull/638).

SpecificDerivativePlugin.php

<?php
/**
 * Specific Derivative
 *
 * Example to use to add one or multiple specific derivative types in Omeka, for
 * example mini square thumbnails.
 *
 * It creates the directory automatically. If the path exists, this plugin is
 * useless, except the form to set constraints of the specific types.
 *
 * To use it, you need to install the patch presented in the page below and to
 * add in "config.ini":
 *
 * fileDerivatives.paths.square_mini_thumbnail = "square_mini_thumbnails"
 *
 * @see https://github.com/omeka/Omeka/pull/638
 *
 * @copyright Daniel Berthereau, 2014
 * @license http://www.cecill.info/licences/Licence_CeCILL_V2.1-en.txt
 * @package SpecificDerivative
 */

/**
 * The Specific Derivative plugin.
 * @package Omeka\Plugins\SpecificDerivative
 */
class SpecificDerivativePlugin extends Omeka_Plugin_AbstractPlugin
{
    /**
     * @var array Hooks for the plugin.
     */
    protected $_hooks = array(
        'install',
        'uninstall',
        'appearance_settings_form',
    );

    /**
     * @var array Options and their default values.
     */
    protected $_options = array(
        'square_mini_thumbnail_constraint' => 32,
    );

    /**
     * @var array Paths for specific types.
     */
    protected $_pathsByType = array(
        'square_mini_thumbnail' => 'square_mini_thumbnails',
    );

    /**
     * Installs the plugin.
     */
    public function hookInstall()
    {
        try {
            $this->_installPaths();
        }
        catch (Exception $e) {
            throw new Omeka_Plugin_Installer_Exception($e->getMessage());
        }

        $this->_installOptions();
    }

    /**
     * Uninstalls the plugin.
     */
    public function hookUninstall()
    {
        $this->_uninstallPaths();

        $this->_uninstallOptions();
    }

    /**
     * Add forms for apparence.
     *
     * To add a derivative type, it should be set in `config.ini` in option `fileDerivatives.paths.xxx`.
     * The constraint is not obligatory and it can be anything.
     *
     * @return void
     */
    public function hookAppearanceSettingsForm($args)
    {
        $form = $args['form'];
        $form->addElement('text', 'square_mini_thumbnail_constraint', array(
            'label' => __('My Square Mini Thumbnail Size'),
            'description' => __('Maximum square mini thumbnail size constraint (in pixels).'),
            'validators' => array('Digits'),
            'required' => true,
            'order' => 2,
        ));

        $form->setDefaults(array(
            'square_mini_thumbnail_constraint' => get_option('square_mini_thumbnail_constraint'),
        ));
    }

    /**
     * Create paths for specific derivative types.
     *
     * @return void Exception if an error occurs.
     */
    protected function _installPaths()
    {
        $fileDerivatives = Zend_Registry::get('bootstrap')->getResource('Config')->fileDerivatives;
        if (empty($fileDerivatives) || empty($fileDerivatives->paths)) {
            throw new Exception(__('"config.ini" does not contain paths for derivative files.'));
        }
        $configPathsByType = $fileDerivatives->paths->toArray();

        foreach ($this->_pathsByType as $type => $path) {
            if (!isset($configPathsByType[$type])) {
                throw new Exception(__('"config.ini" does not contain path for the derivative type "%s".', $type));
            }
            $dirpath = FILES_DIR . DIRECTORY_SEPARATOR . $path;
            if (!realpath($dirpath)) {
                try {
                    $this->_createFolder($dirpath);
                    // For security reason, an index.html is added.
                    copy(
                        FILES_DIR . DIRECTORY_SEPARATOR . $configPathsByType['original'] . DIRECTORY_SEPARATOR . 'index.html',
                        $dirpath . DIRECTORY_SEPARATOR . 'index.html'
                    );
                } catch (Omeka_Storage_Exception $e) {
                    throw new Exception($e->getMessage());
                } catch (Exception $e) {
                   throw new Exception(__('Error when creating folder "%s" for type "%s".', $path, $type));
                }
            }
        }
    }

    /**
     * Remove paths for specific derivative types.
     *
     * @return void
     */
    protected function _uninstallPaths()
    {
        foreach ($this->_pathsByType as $type => $path) {
            $dirpath = FILES_DIR . DIRECTORY_SEPARATOR . $path;
            $this->_removeFolder($dirpath);
        }
    }

    /**
     * Checks and creates a folder.
     *
     * @param string $path Full path of the folder to create.
     *
     * @return void Exception if an error occurs.
     */
    private function _createFolder($path)
    {
        if ($path != '') {
            if (file_exists($path)) {
                if (is_dir($path)) {
                    if (is_writable($path)) {
                        return;
                    }
                    throw new Omeka_Storage_Exception(__('Error directory non writable: "%s".', $path));
                }
                throw new Omeka_Storage_Exception(__('Failed to create folder "%s": a file with the same name exists...', $path));
            }

            if (!@mkdir($path, 0755, true)) {
                throw new Omeka_Storage_Exception(__('Error making directory: "%s".', $path));
            }
        }
    }

    /**
     * Checks and removes a folder.
     *
     * @param string $path Full path of the folder to remove.
     *
     * @return void.
     */
    private function _removeFolder($path)
    {
        foreach(glob("{$path}/*") as $file) {
            if (is_dir($file)) {
                $this->_removeFolder($file);
            }
            else {
                @unlink($file);
            }
        }
        @rmdir($path);
    }
}