lock thumb creation to ttshowcase

/* With this function, thumbnails for ttshowcase will only get created when getting upload inside the ttshowcase custom post type */
add_filter( 'intermediate_image_sizes', 'ttshowcase_image_sizes_lock', 999 );
function ttshowcase_image_sizes_lock( $image_sizes ){

    $ttshowcase_sizes = array( 'ttshowcase_small', 'ttshowcase_normal' );

    if( isset($_REQUEST['post_id']) && 'ttshowcase' != get_post_type( $_REQUEST['post_id'] ) ) {
        $image_sizes = array_diff($image_sizes, $ttshowcase_sizes);
    }

    return $image_sizes;
}