Some information that might makes it easier to track the issue.
--fix-cache
The --fix-cache option was included in version 2.6.2. It is not in man but found by:
nemo --help
...
--fix-cache Repair the user thumbnail cache - this can be useful if
you're having trouble with file thumbnails.
Must be run as root
From changelog:
[ Michael Webster ]
* Adapt Nemo to deal with problem situations with the user thumbnail cache:
- Added --fix-cache command line option (must be run as root)
- Added quick check at startup to detect major problems and disable
thumbnailing until fixed.
- Individual thumbnailed files are checked on the fly for permission issues.
- When a problem is detected, an infobar appears, offering to fix the problem,
or ignore it.
* thumbnails: Remove an orphan comment from previous commits,
fix wording of the --fix-cache command-line option.
ThumbnailFactory
Nemo uses the GnomeDesktopThumbnailFactory.
As such it creates a thumbnail in a cache directory. Typically ~/.thumbnails/[SIZE] or ~/.cache/thumbnails/[SIZE]. Where depends on system environment; more specifically xdg settings like XDG_CACHE_HOME. N.B! This is a shared cache that can be used by multiple applications.
The file name of a thumbnail is generated by hashing the original file's URI. The library can then check if the file is already cached by looking at the hash sum of the it's location.
If an error occurs a file is created in .../thumbnails/fail/gnome-thumbnail-factory/[HASH].png
Identify source for cached thumbnail
ThumbnailFactory add PNG tEXt properties to the thumbnails. Among these are the URI. You can use various tools to extract this information, for example with Imagemagick:
identify -format "%[Thumb::URI]\n" ~/.cache/thumbnails/normal/774b4a5ba74455xxx.png
file:///home/foo/some_image.jpg
or the more verbose:
identify -format "%[*]" ~/.cache/thumbnails/normal/774b4a5ba74455xxx.png
or simply:
identify -verbose ~/.cache/thumbnails/normal/774b4a5ba74455xxx.png
This information is also stored in the failed thumbnails.
Custom thumbnail generator
One can add custom thumbnail generators by adding configuration files in share/thumbnailers. Typically: /usr/share/thumbnailers/ or ~/.local/share/thumbnailers
Example taken from arch Linux PCManFM for creating PDF thumbnails with Imagemagick:
[Thumbnailer Entry]
TryExec=convert
Exec=convert %i[0] -thumbnail %s %o
MimeType=application/pdf;application/x-pdf;image/pdf;
You could try this as a last resort. (Believe) nemo uses GTK library for thumbnail generation of images.
You might
- Try to run the
--fix-cache as root.
- Try to remove, (or move), the thumbnails cache directories – start fresh. N.B: The cache directory is shared between applications using GnomeDesktopThumbnailFactory.
- Run nemo from command line to see if any specific errors are reported.
- ...
