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Demo using WP's admin_post_thumbnail_html filter to insert content in featured image metabox
* Adds a detailed UI to WP's thumbnail HTML markup used in the Feature Image metabox.
* Adds a description of featured images, a listing of the formats used by GV and
* previews of how the images will appear in features slider and thumbnails.
* Note that $thumbnail_id is only saved when the post is saved. Immediately after the featured image is changed
* this value is updated, but not the postmeta field in the database (nor OUR fields which are tied to it)
* SO:
* - empty $thumbnail_id means EITHER the post didn't have a thumbnail when it was saved OR that the "remove featured image" button was just pressed.
* - non-empty $thumbnail_id means EITHER the post was saved with a featured image OR an unsaved featured image was added and this is the preview.
* @param string $output The thumbnail HTML markup
* @param int $post_id The post ID of the post being edited
* @param int $post_id The post ID for the attachment post
function gv_filter_admin_post_thumbnail_html($output, $post_id, $thumbnail_id = ""){
// ...
* If this post has a featured image set
* Note: Could be current (saved in DB) value, or temporary (used for generating preview) thumbnail_id
if ($thumbnail_id) :
* Get the post id of the attachment that was chosen as featured image
$featured_image_post_id = $thumbnail_id;
* Get the data arrays ['url', 'width', 'height', 'is_intermediate' ] about our CUSTOM sizes
* Note: WP will return the largest size available even if it's too small, so these dimensions
* are likely to describe "featured_image_small" (CUSTOM) or "full size" (original upload).
$featured_image_large_data = wp_get_attachment_image_src($featured_image_post_id, 'featured_image_large');
- // For the record: No stated size string gives you the old small square 'thumbnail' size
- //$featured_image_thumbnail_data = wp_get_attachment_image_src($featured_image_post_id);
* Determine the width and height of the file chosen for featured_image_large
$featured_image_large_width = $featured_image_large_data[1];
$featured_image_large_height = $featured_image_large_data[2];
// ...
return $output;
add_filter('admin_post_thumbnail_html', 'gv_filter_admin_post_thumbnail_html', 10, 3);
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The comments say that the code does more than it actually does. This filter is not doing anything, and does not affect the variable being filtered.

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