WordPress - Hidden custom meta field for images

I'm trying to add a custom field to the media uploader in WordPress. I ahve it working, but I'd like to make the custom field key a hidden one.

If you are familiar with the way WordPress handles custom fields, you'll know that setting the key to "_something" will hide that key from the drop down lists visible to the user.

 * Add Video URL fields to media uploader
 * http://www.billerickson.net/wordpress-add-custom-fields-media-gallery/
 * @param $form_fields array, fields to include in attachment form
 * @param $post object, attachment record in database
 * @return $form_fields, modified form fields

    function capgun2012_attachment_fields( $form_fields, $post ) {

        $form_fields['capgun2012_video_url'] = array(
            'label' => 'Vimeo URL',
            'input' => 'text',
            'value' => get_post_meta( $post->ID, 'capgun2012_video_url', true ),
            'helps' => 'If provided, photo will be displayed as a video',

        return $form_fields;

    add_filter( 'attachment_fields_to_edit', 'capgun2012_attachment_fields', 10, 2 );

 * Save values of Photographer Name and URL in media uploader
 * @param $post array, the post data for database
 * @param $attachment array, attachment fields from $_POST form
 * @return $post array, modified post data

    function capgun2012_attachment_fields_save( $post, $attachment ) {

        if( isset( $attachment['capgun2012_video_url'] ) ) {
            update_post_meta( $post['ID'], 'capgun2012_video_url', $attachment['capgun2012_video_url'] );

        return $post;

    add_filter( 'attachment_fields_to_save', 'capgun2012_attachment_fields_save', 10, 2 );  

If I simply replace all the occurrences of "capgun2012_video_url" with "_capgun2012_video_url" then it doesn't work. I'm starting to think that the media uploader doesn't play well with hidden custom fields.

Please see attached screenshot of what I don't want to happen (the custom key showing in the custom fields drop down).

Thanks for the help.


This was answered here: http://devilsworkshop.org/adding-custom-fields-to-wordpress-media-gallery-upload/

Setup field:

   /* For adding custom field to gallery popup */
function rt_image_attachment_fields_to_edit($form_fields, $post) {
    // $form_fields is a an array of fields to include in the attachment form
    // $post is nothing but attachment record in the database
    //     $post->post_type == 'attachment'
    // attachments are considered as posts in WordPress. So value of post_type in wp_posts table will be attachment
    // now add our custom field to the $form_fields array
    // input type="text" name/id="attachments[$attachment->ID][custom1]"
    $form_fields["rt-image-link"] = array(
        "label" => __("Custom Link"),
        "input" => "text", // this is default if "input" is omitted
        "value" => get_post_meta($post->ID, "_rt-image-link", true),
                "helps" => __("To be used with special slider added via [rt_carousel] shortcode."),
   return $form_fields;
// now attach our function to the hook
add_filter("attachment_fields_to_edit", "rt_image_attachment_fields_to_edit", null, 2);

And this to save it:

function rt_image_attachment_fields_to_save($post, $attachment) {
    // $attachment part of the form $_POST ($_POST[attachments][postID])
        // $post['post_type'] == 'attachment'
    if( isset($attachment['rt-image-link']) ){
        // update_post_meta(postID, meta_key, meta_value);
        update_post_meta($post['ID'], '_rt-image-link', $attachment['rt-image-link']);
    return $post;
// now attach our function to the hook.
add_filter("attachment_fields_to_save", "rt_image_attachment_fields_to_save", null , 2);

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