Wordpress media upload define default meta values - wordpress

Question

i want to add default values to metadata of images automatically while/after they are uploaded.
// add default values to custom fields upon upload
add_action( 'add_attachment', 'as_set_image_meta_upon_image_upload' );
function as_set_image_meta_upon_image_upload( $post_ID ) {
// Check if uploaded file is an image, else do nothing
if ( wp_attachment_is_image( $post_ID )) {
if (strlen(trim(get_post_meta( $post_ID, 'photographer', true))) == 0 ) {
update_post_meta( $post_ID, 'photographer', 'unknown' );
}
if (strlen(trim(get_post_meta( $post_ID, 'copyright', true))) == 0 ) {
update_post_meta( $post_ID, 'copyright', 'not defined' );
}
}
}
it works fine for the custom fields 'photographer' and 'copyright' but i also want to do it for the image caption. In another section of the code (hooked to attachment_fields_to_save) i am successfully using this statement to fill empty captions
if (strlen(trim($post['post_excerpt'])) == 0) {
$pretext = '[Attribution: ';
$pretext .= get_post_meta( $post_ID, 'photographer', true);
$pretext .= '; Copyright: ';
$pretext .= get_post_meta( $post_ID, 'copyright', true);
$pretext .= ']';
$post['post_excerpt'] = $pretext.' '.$post['post_excerpt'];
}
but i would rather move it to the 'add_attachment' hook. But when i do try to add it the image upload fails. I think because it uses the $post variable which is not used by the hook. i tried to replace $post with get_post($post_ID) to limit myself to the variables used by the hook but that didnt work either. I also tried to put
$post = get_post($post_ID)
before the if statement but then the whole site crashes, probably its some kind of global variable i shouldnt mess with by assigning it manually.
So my question is how can i fit the if statement in the 'add_attachment' hook, or, more general, how can i assign a default caption to images upon upload.

Answer 1

i managed to automatically fill in a default caption upon upload
add_action( 'add_attachment', 'as_set_image_meta_upon_image_upload' );
function as_set_image_meta_upon_image_upload( $post_ID ) {
// Check if uploaded file is an image, else do nothing
if ( wp_attachment_is_image( $post_ID )) {
//default photographer
if (strlen(trim(get_post_meta( $post_ID, 'photographer', true))) == 0 ) {
update_post_meta( $post_ID, 'photographer', 'unknown' );
}
//default copyright
if (strlen(trim(get_post_meta( $post_ID, 'copyright', true))) == 0 ) {
update_post_meta( $post_ID, 'copyright', 'not defined' );
}
//default caption
$my_image_meta = array(
'ID' => $post_ID,
'post_excerpt' => '[Attribution: unknown; Copyright: not defined] ',
);
wp_update_post( $my_image_meta );
}
}
when i tried to load the actual values of 'photographer' and 'copyright' they returned empty, i suppose because there are just being filled in the same hook. good enough though