Hi @Amit:
The answer is “Yes, it is possible.” The follow up question is “Do you really want to?”
I thought it would be fun to see if I could build a plugin to do what you are asking for so I decided to see if it was possible. Of course I was able to make it work but I fear that the plugin code I wrote needs to be so tightly coupled with the existing code in WordPress that it might easily break on a core update.
This code handles all the admin console side of things by creating a new menu item in the Appearance section called “Special Background.” What it does not do is provide any way to actually use the background; that’s phase two of the plugin and will require hooking many of the functions in /wp-includes/themes.php and frankly I don’t know if I’ll get a chance to write that code.
The reason I stopped without doing it is I didn’t know the requirements for where the special background should show and where the regular one would show. I assume maybe for selected pages and/or URL paths?
Nonetheless, here if the code for the plugin (which you can also download from gist):
<?php
/*
Plugin Name: Special Background
Plugin URI: http://wordpress.stackexchange.com/questions/972/
Description: Example to show how to add a special background using exiting background admin page in core.
Version: 0.1
Author: Mike Schinkel
Author URI: http://mikeschinkel.com/custom-wordpress-plugins/
*/
add_filter('admin_menu','add_special_background_menu_item');
function add_special_background_menu_item() {
add_theme_page(__('Special Background'), __('Special Background'),'edit_theme_options','special-background','special_background_admin_page');
}
add_filter('admin_init','add_js_for_special_background');
function add_js_for_special_background() {
global $custom_background;
if (is_special_background_page()) {
wp_enqueue_script('custom-background');
wp_enqueue_style('farbtastic');
}
$hook = 'load-appearance_page_special-background';
add_action($hook, array(&$custom_background, 'admin_load'));
add_action($hook, array(&$custom_background, 'take_action'), 49);
add_action($hook, array(&$custom_background, 'handle_upload'), 49);
}
add_filter('theme_mod_background_image', 'theme_mod_special_background_image');
add_filter('theme_mod_background_image_thumb','theme_mod_special_background_image_thumb');
add_filter('theme_mod_background_repeat', 'theme_mod_special_background_repeat');
add_filter('theme_mod_background_position_x', 'theme_mod_special_background_position_x');
add_filter('theme_mod_background_attachment', 'theme_mod_special_background_attachment');
add_filter('theme_mod_background_color', 'theme_mod_special_background_color');
function theme_mod_special_background_image($defaults) {
return theme_mod_special_background_image_attrs('image',$defaults);
}
function theme_mod_special_background_image_thumb($defaults) {
return theme_mod_special_background_image_attrs('image_thumb',$defaults);
}
function theme_mod_special_background_repeat($defaults) {
return theme_mod_special_background_image_attrs('repeat',$defaults);
}
function theme_mod_special_background_position_x($defaults) {
return theme_mod_special_background_image_attrs('position_x',$defaults);
}
function theme_mod_special_background_attachment($defaults) {
return theme_mod_special_background_image_attrs('attachment',$defaults);
}
function theme_mod_special_background_color($defaults) {
return theme_mod_special_background_image_attrs('color',$defaults);
}
function theme_mod_special_background_image_attrs($attr,$defaults) {
if (is_special_background_page()) {
$mods = get_option( 'mods_' . get_current_theme() );
$defaults = (!empty($mods["special_background_{$attr}"]) ? $mods["special_background_{$attr}"] : '');
}
return $defaults;
}
add_filter('pre_update_option_mods_' . get_current_theme(),'pre_update_option_special_background_image',10,2);
function pre_update_option_special_background_image($newvalue, $oldvalue) {
static $times_called = 0;
if (!empty($_POST) && is_special_background_page()) {
if ((isset($_POST['action']) && $_POST['action']=='save') || isset($_POST['reset-background']) || isset($_POST['remove-background'])) {
switch ($times_called) {
case 0:
$newvalue = special_background_image_value_swap('image',$newvalue,$oldvalue);
break;
case 1:
$newvalue = special_background_image_value_swap('image_thumb',$newvalue,$oldvalue);
break;
}
} else {
if ($times_called==0 && isset($_POST['background-repeat'])) {
$newvalue = special_background_image_value_swap('repeat',$newvalue,$oldvalue);
}
if ($times_called==1 && isset($_POST['background-position-x'])) {
$newvalue = special_background_image_value_swap('position_x',$newvalue,$oldvalue);
}
if ($times_called==2 && isset($_POST['background-attachment'])) {
$newvalue = special_background_image_value_swap('attachment',$newvalue,$oldvalue);
}
if ($times_called==3 && isset($_POST['background-color'])) {
$newvalue = special_background_image_value_swap('color',$newvalue,$oldvalue);
}
}
$times_called++;
}
return $newvalue;
}
function special_background_image_value_swap($swap_what,$newvalue,$oldvalue) {
$newvalue["special_background_{$swap_what}"] = $newvalue["background_{$swap_what}"];
$newvalue["background_{$swap_what}"] = $oldvalue["background_{$swap_what}"];
return $newvalue;
}
function special_background_admin_page() {
global $custom_background;
if (is_special_background_page()) {
global $parent_file,$submenu_file,$title;
$parent_file="themes.php";
$submenu_file="themes.php?page=special-background";
$title="Special Background";
require_once(ABSPATH . 'wp-admin/admin-header.php');
ob_start();
$custom_background->admin_page();
$html = ob_get_clean();
$html = preg_replace('#<h2>([^<]+)</h2>#','<h2>Special Background</h2>',$html);
echo $html;
include(ABSPATH . 'wp-admin/admin-footer.php');
exit;
}
}
function is_special_background_page() {
global $pagenow;
return ($pagenow=='themes.php' &&
isset($_GET['page']) && $_GET['page']== 'special-background');
}
Frankly it is too much code to explain proactively but I’ll be happy to answer specific questions.