Missing thumbnails, maybe digikam4.db involved

[Remco Viëtor]

On vendredi 1 décembre 2017 22:43:14 CET ShutterBug wrote:
> I recently encountered this problem, as well, using DigiKam 4.12 on Ubuntu
> 16.04.3.  Just for fun, I tried this (and it worked!):
> 
> 1. Close DigiKam.
> 2. Open the file browser (Nautilus) and navigate to the folder that contains
> the image files that DigiKam no longer displays thumbnails for.  Example:
> /Pictures/1973Pix
> 3. Open another file browser window and navigate to the folder above the
> problem folder. Example:  /Pictures
> 4. Create another folder and move the files from the problem folder to the
> new folder.
> Example:
> $mkdir 1973Pix-a
> $cd 1973Pix-a
> $mv /Pictures/1973Pix/* .
> (Of course, you can do this all in Nautilus)
> 5. Launch DigiKam and let it scan the target Collection for new or deleted
> files.  Look (in DigiKam) under the new Album folder.  You should now see
> the missing thumbnail images, complete with any tag info you've added
> previously!
> Optional:  I wanted to retain the original folder names.  So, I reversed the
> process:
> 6. Close DigiKam.
> 7. Move the files from the new folder back to the original folder (Example:
> 1973Pix-a  > 1973Pix)
> 8. Delete the empty new folder (Example: $rm 1973Pix-a)
> 9. Launch DigiKam and wait for the rescan to complete.
> 
> I haven't tried this yet, but suspect that the entire target Collection root
> folder could be renamed, the target Collection reset in the DigiKam
> configuration settings, scanned by DigiKam, exit DigiKam, folder renamed
> back, again reset the Collection target, launch DigiKam and allow it to
> complete its scan.  I suspect this would catch any and all missing
> thumbnails.
> 
> Caveat:
> **Always keep a backup of your image files, just in case something goes
> wrong!**
> 
> 
> Hope this helps someone...DigiKam's a really great application!

Iirc, already for that version of Digikam (>2 years old), you could force 
Digikam to refresh all thumbnails by deleting the thumbnails-digikam.db
(or safer, rename it/move it). Perhaps not worthwhile for a few missing 
thumbs, but probably safer/faster than moving the whole collection around 
(which implies *two* scans of the whole collection, tags included).

Also, if you go the way of moving files around outside Digikam, be careful with 
sidecars: forget one, and the tagging for the corresponding image could be 
lost (esp. for raw files, depending on config settings, other formats could also 
be affected).

Remco

P.S. Is it known why some thumbnails disappear, or are not generated?