Understanding and Troubleshooting the 'Skip Regenerating' Option in Regenerate Thumbnails

Many users of the popular 'Regenerate Thumbnails' plugin have reported a consistent issue: the "Skip regenerating existing correctly sized thumbnails" option does not work as expected. Despite the checkbox being selected, the plugin proceeds to regenerate every single image, ignoring existing thumbnails. This guide explains the likely cause of this bug and offers potential workarounds.

What is the Problem?

When the "Skip regenerating" option is enabled, users expect the plugin to only process images that are missing thumbnails or have incorrect sizes. However, numerous reports confirm that the plugin regenerates all images, which is evident from the updated file timestamps on the server. This behavior negates the performance benefit of the feature, especially on sites with large media libraries.

Why Does This Happen?

Analysis from the community suggests this is a confirmed bug, not an isolated configuration error. The root cause appears to be a change in WordPress core. A specific core commit altered the order in which image metadata is saved. The plugin's function that checks for existing thumbnails relies on metadata that WordPress core now strips before the plugin can access it. Essentially, the plugin is looking for information that is no longer available at that point in the process, causing it to assume all thumbnails are missing and need to be regenerated.

This issue has been officially acknowledged by the 'Regenerate Thumbnails' team and is being tracked in a GitHub issue report. As of now, there is no official estimated timeframe for a fix within the plugin.

Common Solutions and Workarounds

Since a direct fix from the plugin is pending, users have explored several alternative approaches:

1. Use WP-CLI (With Caution): Some users have attempted to use the wp media regenerate command via WP-CLI. However, reports indicate this method may also be affected by the same underlying issue and might not properly skip existing images.
2. Manual Regeneration in Batches: For very large libraries, a practical workaround is to manually regenerate thumbnails in smaller batches. While the skip option may not work, you can process a few hundred images at a time to avoid server timeouts or script failures.
3. Monitor for an Update: The most definitive solution will be an update to the plugin that addresses the core compatibility issue. Users are advised to watch the official GitHub repository for progress on the issue.

A Note on the Related "Delete" Option

Users looking for the "Delete thumbnail files for old unregistered sizes" option should note that it was temporarily removed from the plugin. The 'Regenerate Thumbnails' team disabled it because the functionality that updated post content to remove references to deleted images caused problems. While the team has stated the feature will return, there is no known timeline for its reimplementation.

If you encounter this bug, you are not alone. The community is actively discussing it, and the best course of action is to employ a workaround while waiting for an official patch.