wp_create_image_subsizes example

function wp_update_image_subsizes( $attachment_id ) {
    $image_meta = wp_get_attachment_metadata( $attachment_id );
    $image_file = wp_get_original_image_path( $attachment_id );

    if ( empty( $image_meta ) || ! is_array( $image_meta ) ) {
        /* * Previously failed upload? * If there is an uploaded file, make all sub-sizes and generate all of the attachment meta. */
        if ( ! empty( $image_file ) ) {
            $image_meta = wp_create_image_subsizes( $image_file, $attachment_id );
        } else {
            return new WP_Error( 'invalid_attachment', __( 'The attached file cannot be found.' ) );
        }
    } else {
        $missing_sizes = wp_get_missing_image_subsizes( $attachment_id );

        if ( empty( $missing_sizes ) ) {
            return $image_meta;
        }

        // This also updates the image meta.