Source

File: wp-admin/includes/image.php

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.
		$image_meta = _wp_make_subsizes( $missing_sizes, $image_file, $image_meta, $attachment_id );
	}

	/** This filter is documented in wp-admin/includes/image.php */
	$image_meta = apply_filters( 'wp_generate_attachment_metadata', $image_meta, $attachment_id, 'update' );

	// Save the updated metadata.
	wp_update_attachment_metadata( $attachment_id, $image_meta );

	return $image_meta;
}