Thumbnail Generation
Also known as: thumbnail creation, how thumbnails are made
Thumbnail generation is the process of decoding a photo or video frame, downscaling it to a small preview, and saving that preview so galleries can scroll smoothly without re-reading full-resolution originals. The cached previews are extra files that accumulate and consume storage over time.
- A thumbnail is a downscaled preview, not a copy of the full-resolution original.
- Video thumbnails are usually a single decoded frame from near the clip's start.
- Thumbnail caches are regenerable, so clearing them frees space without losing photos.
How thumbnails are made
When a gallery shows a grid, it cannot afford to decode every full-resolution original on the fly. Instead the system decodes each image once, downscales it to one or more small target sizes, and re-encodes the result as a compact JPEG or HEIC preview.
For videos, the system decodes one representative frame, usually near the start, and turns that single frame into a still thumbnail. Some platforms generate several sizes such as grid, list, and full-screen previews so any view can pick the closest match.
These previews are written to a thumbnail cache on disk. They are derived data: the originals stay untouched, and the cache can be rebuilt at any time from the source files.
Why the cache grows
Each original can spawn multiple cached previews at different resolutions, and every new photo or video adds more entries. Over a large library this cache can reach hundreds of megabytes or several gigabytes.
Because the cache is regenerable, clearing it is safe: previews are recreated automatically the next time you open the gallery. A storage cleaner can reclaim space by removing stale thumbnail caches without ever deleting an original photo or video.