Image gallery captions in alt

Question

I modified the gallery shortcode to implement in a slideshow and I need to add the captions in the alt element of images.

e.g.

<img width="110" height="110" title="My Title" alt="Caption here" class="attachment-thumbnail" src="...">

Below is the new shortcode in my functions.php file. I don't know where to change alt or how to retrieve the captions for my gallery images.

remove_shortcode('gallery', 'gallery_shortcode');
add_shortcode('gallery', 'gallery_shortcode_slide');


function gallery_shortcode_slide($attr) {
    global $post, $wp_locale;

    static $instance = 0;
    $instance++;

    // Allow plugins/themes to override the default gallery template.
    $output = apply_filters('post_gallery', '', $attr);
    if ( $output != '' )
        return $output;

    // We're trusting author input, so let's at least make sure it looks like a valid orderby statement
    if ( isset( $attr['orderby'] ) ) {
        $attr['orderby'] = sanitize_sql_orderby( $attr['orderby'] );
        if ( !$attr['orderby'] )
            unset( $attr['orderby'] );
    }

    extract(shortcode_atts(array(
        'order'      => 'ASC',
        'orderby'    => 'menu_order ID',
        'id'         => $post->ID,
        'itemtag'    => 'ul',
        'icontag'    => 'li',
        'captiontag' => 'p',
        'columns'    => 3,
        'size'       => 'slidethumb',
        'include'    => '',
        'exclude'    => ''
    ), $attr));

    $id = intval($id);
    if ( 'RAND' == $order )
        $orderby = 'none';

    if ( !empty($include) ) {
        $include = preg_replace( '/[^0-9,]+/', '', $include );
        $_attachments = get_posts( array('include' => $include, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby) );

        $attachments = array();
        foreach ( $_attachments as $key => $val ) {
            $attachments[$val->ID] = $_attachments[$key];
        }
    } elseif ( !empty($exclude) ) {
        $exclude = preg_replace( '/[^0-9,]+/', '', $exclude );
        $attachments = get_children( array('post_parent' => $id, 'exclude' => $exclude, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby) );
    } else {
        $attachments = get_children( array('post_parent' => $id, 'post_status' => 'inherit', 'post_type' => 'attachment', 'post_mime_type' => 'image', 'order' => $order, 'orderby' => $orderby) );
    }

    if ( empty($attachments) )
        return '';

    if ( is_feed() ) {
        $output = "\n";
        foreach ( $attachments as $att_id => $attachment )
            $output .= wp_get_attachment_link($att_id, $size, true) . "\n";
        return $output;
    }

    $itemtag = tag_escape($itemtag);
    $captiontag = tag_escape($captiontag);
    $columns = intval($columns);
    $itemwidth = $columns > 0 ? floor(100/$columns) : 100;
    $float = is_rtl() ? 'right' : 'left';

    $selector = "gallery-{$instance}";

    $size_class = sanitize_html_class( $size );
    $output .= "<div id='gallery' class='ad-gallery'>";
    $output .= "<div class='ad-image-wrapper'></div><div class='ad-controls'></div><div class='ad-nav'><div class='ad-thumbs'>";    

    $output .= "<{$itemtag} class='ad-thumb-list'>";

    $i = 0;
    foreach ( $attachments as $id => $attachment ) {
        $link = isset($attr['link']) && 'file' == $attr['link'] ? wp_get_attachment_link($id, $size, false, false) : wp_get_attachment_link($id, $size, true, false);

        $output .= "
            <{$icontag}>
                $link               
            </{$icontag}>";
        if ( $captiontag && trim($attachment->post_excerpt) ) {
            $output .= "
                <{$captiontag} class='ad-image-description'>
                " . wptexturize($attachment->post_excerpt) . "
                </{$captiontag}>";
        }       
    }


    $output .= "</{$itemtag}>";

    $output .= "</div></div></div>";

    return $output;
}