How to make a slideshow shortcode based on the WordPress gallery shortcode?

This is the wordpress function for the gallery shortcode, i would like to understand how this works so i can implement a slideshow shortcode and adding the img id only as gallery shortcode does.

    function gallery_shortcode($attr) {
$post = get_post();

static $instance = 0;
$instance++;

if ( ! empty( $attr['ids'] ) ) {
    // 'ids' is explicitly ordered, unless you specify otherwise.
    if ( empty( $attr['orderby'] ) )
        $attr['orderby'] = 'post__in';
    $attr['include'] = $attr['ids'];
}

// 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 ? $post->ID : 0,
    'itemtag'    => 'dl',
    'icontag'    => 'dt',
    'captiontag' => 'dd',
    'columns'    => 3,
    'size'       => 'thumbnail',
    'include'    => '',
    'exclude'    => '',
    'link'       => ''
), $attr, 'gallery'));

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

if ( !empty($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) ) {
    $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);
$icontag = tag_escape($icontag);
$valid_tags = wp_kses_allowed_html( 'post' );
if ( ! isset( $valid_tags[ $itemtag ] ) )
    $itemtag = 'dl';
if ( ! isset( $valid_tags[ $captiontag ] ) )
    $captiontag = 'dd';
if ( ! isset( $valid_tags[ $icontag ] ) )
    $icontag = 'dt';

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

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

$gallery_style = $gallery_div = '';
if ( apply_filters( 'use_default_gallery_style', true ) )
    $gallery_style = "
    <style type='text/css'>
        #{$selector} {
            margin: auto;
        }
        #{$selector} .gallery-item {
            float: {$float};
            margin-top: 10px;
            text-align: center;
            width: {$itemwidth}%;
        }
        #{$selector} img {
            border: 2px solid #cfcfcf;
        }
        #{$selector} .gallery-caption {
            margin-left: 0;
        }
        /* see gallery_shortcode() in wp-includes/media.php */
    </style>";
$size_class = sanitize_html_class( $size );
$gallery_div = "<div id='$selector' class='gallery galleryid-{$id} gallery-columns-{$columns} gallery-size-{$size_class}'>";
$output = apply_filters( 'gallery_style', $gallery_style . "ntt" . $gallery_div );

$i = 0;
foreach ( $attachments as $id => $attachment ) {
    if ( ! empty( $link ) && 'file' === $link )
        $image_output = wp_get_attachment_link( $id, $size, false, false );
    elseif ( ! empty( $link ) && 'none' === $link )
        $image_output = wp_get_attachment_image( $id, $size, false );
    else
        $image_output = wp_get_attachment_link( $id, $size, true, false );

    $image_meta  = wp_get_attachment_metadata( $id );

    $orientation = '';
    if ( isset( $image_meta['height'], $image_meta['width'] ) )
        $orientation = ( $image_meta['height'] > $image_meta['width'] ) ? 'portrait' : 'landscape';

    $output .= "<{$itemtag} class='gallery-item'>";
    $output .= "
        <{$icontag} class='gallery-icon {$orientation}'>
            $image_output
        </{$icontag}>";
    if ( $captiontag && trim($attachment->post_excerpt) ) {
        $output .= "
            <{$captiontag} class='wp-caption-text gallery-caption'>
            " . wptexturize($attachment->post_excerpt) . "
            </{$captiontag}>";
    }
    $output .= "</{$itemtag}>";
    if ( $columns > 0 && ++$i % $columns == 0 )
        $output .= '<br style="clear: both" />';
}

$output .= "
        <br style='clear: both;' />
    </div>n";

return $output;