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How to Regenerate Thumbnails in WordPress

Posted in PHP

You may occasionally want to regenerate image thumbnails in WordPress (such as after changing image quality settings). Here is a handy function that regenerates all thumbnails for a given attachment post

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function regenerate_attachment_images( $attachment_id )
{
	// We only want to look at image attachments
	if ( !wp_attachment_is_image($attachment_id) )
		return;
 
	$filepath = get_attached_file( $attachment_id, true );
	$metadata = wp_generate_attachment_metadata( $attachment_id, $filepath );
 
	// Was there an error?
	if ( is_wp_error( $metadata ) )
		$this->die_json_error_msg( $attachment_id, $metadata->get_error_message() );
	if ( empty( $metadata ) )
		$this->die_json_error_msg( $attachment_id, 'Unknown failure reason.' );
 
	// If this fails, then it just means that nothing was changed (old value == new value)
	wp_update_attachment_metadata( $attachment_id, $metadata );
}

The above was taken mostly from the Regenerate Thumbnails plugin with a few modifications of my own.

Example Usage

To regenerate all thumbnails for all images use the following (Warning: This could take a while if you have alot of images and/or image sizes):

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$images = new WP_Query(array(
	'post_type' => 'attachment',
	'posts_per_page' => -1,
	'post_mime_type' => 'image',
	'suppress_filters' => false,
 
	'offset' => 0,
	'post_status' => 'inherit',
	'ignore_sticky_posts' => true,
	'no_found_rows' => true,
));
 
while ( $images->have_posts() ): 
	$images->next_post();
	regenerate_attachment_images( $images->post->ID );
endwhile;